- Haskell-Cafe - Haskell.org

Seeking: 12 month internship opportunities for Haskell enthusiasts
by Jacques Carette 30 Jan '19

30 Jan '19

I'm writing on behalf of some (quite good) undergraduate students in CS who have been quite taken by Haskell, whose program includes a 12 month internship at a company. Start time could be anytime between May and September, and duration would be 12 months. [Some students who start in May end up doing 16 months]. If anyone here has leads on such opportunities, please contact me. Location is flexible, as long as the language of work is English. Jacques

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Re: [Haskell-cafe] Montreal Haskell
by Doug McIlroy 28 Jan '19

28 Jan '19

> Morgan-Stanley do their own internal version of Haskell, and they're > strongly oriented towards performance and correctness Interestingly eclectic. They also employ Bjarne Stroustup, the eminence grise of C++. Doug McIlroy

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Montreal Haskell
by Asher Klein 27 Jan '19

27 Jan '19

Hi everyone! New member here! Anyone know of Haskell jobs in Montreal for new grads? Thanks!

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Munich Haskell Meeting, 2019-01-28 @ 19:30
by Leah Neukirchen 27 Jan '19

27 Jan '19

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 28 at Cafe Puck at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jan-2019/ Everybody is welcome! cu, -- Leah Neukirchen <leah(a)vuxu.org> http://leah.zone

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generics-sop : deep flattening of nested algebraic values ?
by Marco Zocca 27 Jan '19

27 Jan '19

Hi all, I'm a bit at a loss regarding this: say we have a value from a nested algebraic type (definitions at the bottom) testT2 :: T2 testT2 = T2 B (Left 42) a generic encoding computed with `from` only goes one level deep, leaving the sub-terms as they are : λ> from testT2 SOP (Z (I B :* (I (Left 42) :* Nil))) However, is it possible to recursively encode and collect all the sub-terms, until primitive types or enumerations are encountered? The encoded sub-terms from the example above are as follows: λ> from B SOP (S (Z Nil)) λ> from (Left 32 :: Either Int Char) SOP (Z (I 32 :* Nil)) where {-# language DeriveGeneric #-} import qualified GHC.Generics as G import Generics.SOP data T1 = A | B | C deriving (G.Generic) instance Generic T1 data T2 = T2 { t21 :: T1 , t22 :: Either Int Char } deriving (G.Generic) instance Generic T2

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Is there a recursion-scheme function to push info down one level?
by Robin Palotai 25 Jan '19

25 Jan '19

I came up with this utility function so I can access some info (`n`) from the parent's level: hoistWithUpper :: forall f g s t n . (Functor g) => (forall a. f a -> n) -> n -> (forall a. n -> f a -> g a) -> (n -> s -> t) -> Free f s -> Free g t hoistWithUpper fu n0 hoistFr hoistPure = go n0 where go :: n -> Free f s -> Free g t go n fr = case fr of Pure s -> Pure (hoistPure n s) Free f -> let n2 = fu f in Free (go n2 <$> (hoistFr n f :: g (Free f s))) I wonder if there's already a generalized form of this in recursion-schemes? Admittedly I'm fine with my helper so don't loose nights on this, but a little type golfing never hurts. There's a similar function `inherit` [1] in fixplate, but that operates on Fix (Mu there), not Free. With Free I guess the complication is managing the different way of maintaining annotation at the Free and Pure ctors. Practically I pass in (\n f -> ConstProd (Pair (Const n) f)) -- for hoistFr (\n u -> (n,u)) -- for hoistPure. where newtype ConstProd c f a = ConstProd (Product (Const c) f a) Thanks! Robin [1]: http://hackage.haskell.org/package/fixplate-0.1.7/docs/src/Data-Generics-Fi…

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[Job] Senior back-end engineer at CentralApp
by Ashesh Ambasta 23 Jan '19

23 Jan '19

Hi everyone, This is my first post to Haskell-Cafe, so pardon me for any errors. I'm writing on behalf of my company, CentralApp, to pitch our job opening. <https://centralapp.workable.com/j/9AFEDD1C3C> We're a small company and I'm the sole back-end engineer so far, and we're investing heavily only in Haskell as our back-end language of choice. A large part of our codebase is in Scala, since that was the choice I made 4-5 years ago. Since then, I got introduced to Haskell and I'm only considering it as the programming language of choice for our back-end infrastructure. We have plenty of engineering challenges in the pipeline since we're undergoing good growth. We serve millions of API requests per week and we're scaling the product out further. We also don't have much in the way of technological debt going forward. Some of the recent /interesting/ Haskell projects we've introduced to our infrastructure are: _Lazy SSL engine based on LetsEncrypt _ This is a Haskell program that issues SSL certificates using LetsEncrypt by doing /on-the-fly/ and /lazy/ certificate issuance. This allows us to vastly simplify HTTPS for 100's of domains we serve. _API Gateway for request pipe-lining_ We're a service oriented architecture. This usually entails that most of our API requests need to be serviced by a combination of back-end services, instead of just one. A trivial way to solve it would be: say a request needs to be passed to service A and then to B, then the responses from each of these services need to be accumulated into one client response. A can first receive the client request, do something, and then make another request to B to do something else. A can then accumulate the response from B along with its own response, and reply to the client. As one might imagine, it becomes increasingly cumbersome and hard to debug this kind of setup. Not only do you introduce dependencies between services A and B, coupling them tighter than you'd like, but it gets really messy when you're going to deal with authentication, authorisation and rapid changes in either A or B. In the worst case, you end up with request cycles (e.g. B makes a request to A), which are requests that never terminate and can bring down large parts of your infrastructure. The other big elephant in the room is also service discovery in a distributed environment. What makes sure that service A can reach a healthy instance of service B? To address this growing pain, we wrote a Haskell API Gateway service, which we lovingly call Quasar (I'm a fan of astronomical names) which flattens request pipe-lining at the API gateway level. What it does is: 1. recognises the request (based on verb + path) 2. identifies the resources being affected in this request (based on the query params/ JSON body) 3. authenticates the user 4. authorises the user for access to the identified resources 5. performs a request based pipeline flow: which is just a traversal of a request-tree, where each node is an "operation" to be performed on an input and output. The input being the response of the previous operation, and the output being the response of this operation. We leverage Haskell's elegant abstraction over operations like this (STM in particular) to ensure the responses are accumulated and returned to the client. -- These are some of the ways we're using Haskell and how it has benefited us. Going forward, given we have a fairly large codebase in Scala, we'd embark on replacing a service with its Haskell equivalent given the amount of changes required in the Scala counterpart is larger than a threshold. All new services that go into our infrastructure, however, will be written in Haskell. So far, our staple libraries of choice are: * Squeal * Servant ... and the usual suspects (Wai/Warp, Aeson, etc.) -- Other than that, we're based in the heart of Brussels, which is a fantastic city to work in, and we will offer a competitive compensation. Free weekly Belgian beers are on us. If this sounds interesting, please feel free to apply on the link above (https://centralapp.workable.com/j/9AFEDD1C3C) Or if you have any questions, please ask! While we'd prefer colleagues based /in /Belgium, or willing to relocate, we're still open to considering all applications. Thanks! -- Best, Ashesh Ambasta Founder (Engineering) CentralApp SA

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avoid to print empty string
by Damien Mattei 21 Jan '19

21 Jan '19

i have this in a Monad IO: forM_ fltrdNamesBDs $ \(name,bdSidonie,bdWDS) -> if (bdWDS /= bdSidonie) then putStrLn $ name ++ " " ++ (show (bdSidonie :: Maybe Float)) ++ " " ++ show (bdWDS :: Maybe Float) ++ " " ++ show (bdWDS == bdSidonie) else putStr "" is there a way to remove the silly putStr "" that output an empty string, i tried with when.... but as when return Nothing in case of False it fails to compile

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ANN: network 3.0.0.0
by evan＠evan-borden.com 21 Jan '19

21 Jan '19

Announcing the release of network 3.0.0.0 http://hackage.haskell.org/package/network-3.0.0.0 Version 3.0.0.0 of network is less platform dependent, easier to build, easier to use correctly (yay no more String api), more extensible, and all around safer. Kazu Yamamoto and Tamar Christina deserve a special shout out, they put an exceptional amount of thought and work into this version and it really shows. Thanks to network's contributors for kicking the tires and continuing to keep quality high. Version 3.0.0.0 * Breaking change: the Network and Network.BSD are removed. Network.BSD is provided a new package: network-bsd. * Breaking change: the signatures are changed: ``` old fdSocket :: Socket -> CInt new fdSocket :: Socket -> IO CInt old mkSocket :: CInt -> Family -> SocketType -> ProtocolNumber -> SocketStatus -> IO Socket new mkSocket :: CInt Socket ``` * Breaking change: the deprecated APIs are removed: send, sendTo, recv, recvFrom, recvLen, htonl, ntohl, inet_addr, int_ntoa, bindSocket, sClose, SocketStatus, isConnected, isBound, isListening, isReadable, isWritable, sIsConnected, sIsBound, sIsListening, sIsReadable, sIsWritable, aNY_PORT, iNADDR_ANY, iN6ADDR_ANY, sOMAXCONN, sOL_SOCKET, sCM_RIGHTS, packSocketType, getPeerCred. * Breaking chage: SockAddrCan is removed from SockAddr. * Socket addresses are extendable with Network.Socket.Address. * "socket" is now asynchronous-exception-safe. [#336](https://github.com/haskell/network/pull/336) * "recvFrom" returns (0, addr) instead of throwing an error on EOF. [#360](https://github.com/haskell/network/pull/360) * All APIs are available on any platforms. * Build system is simplified. * Bug fixes. -- Evan Borden

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building with stack, need some older versions for one library
by Dennis Raddle 20 Jan '19

20 Jan '19

I'm assembling a large stack project from various places. I just did "stack new", using the default resolver, copied in most of my code, and got it compiled. Except I'm stuck on one thing. I need to use the 'midi-0.2.2' library (reads/writes MIDI files). Its depends on older versions of the same of the core packages. What should I do now? Do I modify the LTS resolver to something older and then type "stack build" again? How can I find an appropriate resolver without hit-and-miss? Mike

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Fragile GHC rank-2 type inference?
by Viktor Dukhovni 20 Jan '19

20 Jan '19

[ I am seeing somewhat subtle, and to me surprising, type-inference obstacles. I am presently using GHC 8.6.3. I don't know whether what I'm seeing is a feature or a bug. ] Given a function 'mkEnv' (a command-line option parser built using optparse-applicative) that returns a rank-2 type (via ApplicativeDo): {-# LANGUAGE ApplicativeDo #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE RecordWildCards #-} type Locker = forall a. IO a -> IO a data Env = Env { envLocker :: Locker, ... } mkEnv :: Locker -> Env mkEnv envLocker = do ... pure Env{..} in my complete program, the call to 'mkEnv' fails to compile, when called via: mkLockEnv :: IO Env mkLockEnv = do lock <- newMVar () let locker :: Locker locker = withMVar lock . const ... (mkEnv locker) -- Error message: * Couldn't match type ‘a’ with ‘a0’ ‘a’ is a rigid type variable bound by a type expected by the context: Locker at ... 132 Expected type: IO a -> IO a Actual type: IO a0 -> IO a0 * In the second argument of ‘mkEnv’, namely ‘locker’ * Relevant bindings include locker :: IO a0 -> IO a0 (bound at ...) | 132 | ... (mkEnv locker) | ^^^^^^ But after inlining the "locker" binding, the code compiles: mkLockEnv :: IO Env mkLockEnv = do lock <- newMVar () ... (mkEnv (withMVar lock . const)) Given "let-bound polymorphism": https://kseo.github.io/posts/2016-12-27-higher-rank-polymorphism.html I would not have expected the change to make a difference... In trying to simplify the code (attached) to understand the source of the problem, surprisingly, the simplified program compiles in either form. It is not apparent what facet of the larger program has a bearing on the construction of the rank-2 environment. The background is that some of my Haskell applications employ a shared lock to serialize writes to stdout from concurrent forkIO "threads". The lock is the usual: type Lock = MVar () withLock :: Lock -> IO a -> IO a withLock lock = withMVar lock . const I am however tempted to abstract away the concrete lock type, leaving just a polymorphic closure, which results in 'Env' having a rank-2 type: {-# LANGUAGE RankNTypes #-} data Env = Env { envLocker :: Locker, ... } type Locker = forall a. IO a -> IO a type EnvReader a = ReaderT Env IO a runLocked :: forall a. EnvReader a -> EnvReader a runLocked action = ask >>= \env@Env{..} -> liftIO $ envLocker $ runReaderT action env and this works in the attached sample program which builds and runs: $ echo "The answer to life the universe and everything is:" | ./locker --answer 42 The answer to life the universe and everything is: 42 but a larger program where a "runLocked" call is located deeper in the call chain, fails to compile except as described at the top of the message. -- Viktor.

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Re: [Haskell-cafe] determine a type by universal property
by Olaf Klinke 19 Jan '19

19 Jan '19

Am 18.01.2019 um 13:00 schrieb haskell-cafe-request(a)haskell.org: > > Am Donnerstag, den 17.01.2019, 14:33 +0100 schrieb Olaf Klinke: >> I have two type signatures where I conjecture the only functors satisfying these >> are the Identity functor and functors of the form (,)s. Can anyone give hints as >> how to tackle a proof? >> >> Signature 1: What functors t admit a function >> forall f. Functor f => t (f a) -> f (t a) > > What about the functor > > data Void1 a > > It seems I can write > > g :: forall f. Functor f => t (f a) -> f (t a) > g x = case x of {} > > but Void1 is not the identity. (I guess it is `(,) Void`, if you want…) > > So if you allow the latter, let’s try a proof. Assume we have t, and > > g :: forall f. Functor f => t (f a) -> f (t a) > > We want to prove that there is an isomorphism from t to ((,) s) for > some type s. Define > > s = t () > > (because what else could it be.) Now we need functions > > f1 :: forall a. t a -> (t (), a) > f2 :: forall a. (t (), a) -> t a > > that are isomorphisms. Here is one implementation: > > f1 :: forall a. t a -> (t (), a) > f1 = swap . g . fmap (λx → (x,())) > > {- note: > x1 :: t a > x2 :: t ((,) a ()) > x2 = fmap (λx → (x,())) x1 > x3 :: (,) a (t ()) > x3 = g x2 > x4 :: (t (), a) > x4 = swap x3 > -} > > f2 :: forall a. (t (), a) -> t a > f2 (s, x) = x <$ s > > Now, are these isomorphisms? At this point I am not sure how to > proceed… probably invoking the free theorem of g? But even that will > not work nicely. Consider > > type T = (,) Integer > g :: forall f. Functor => T (f a) -> f (T a) > g (c, fx) = (,) (c + 1) <$> fx > > here we count the number of invocations of g. Surely with this g, > f2 . f1 cannot be the identity. > > > Cheers, > Joachim Hi Joachim, thanks a lot for the clue. I had not thought about the function \x -> (x,()). Your last example of g shows that if a g exists, it is not necessarily unique. Possibly I am missing a law for the function g that entails uniqueness. The identity law in Data.Traversable comes to mind and would rule out your last example. > Am 17.01.2019 um 15:42 schrieb Oleg Grenrus <oleg.grenrus(a)iki.fi>: > > I immediately see > > forall f a b. Functor f => (a -> f b) -> t a -> f (t b) indeed my type forall a f. Functor f => t (f a) -> f (t a) is interdefinable with your type above. > > which is Lens' (t a) a, agreed. > which is an iso > > exists e. (t a) ~ (e, a) I fail to follow this step. How does the Lens above imply this isomorphism? Is this a lens-specific thing or were you thinking along the same lines as Joachim? Let me give a bit of context. I was looking at generic functions that give rise to monad transformer instances of the form MonadFoo m => MonadFoo (BarT m) e.g. MonadExcept m => MonadExcept (StateT s m) One can factor StateT s into two adjoint functors, G = (s->) and F = (,)s. For defining the instance above generically it was necessary for the functor F to aadmit functions with the signatures in my original post. Then I conjectured that I actually had not generalized anything if the signatures forced my F to be of the form (,)s. Olaf

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Re: [Haskell-cafe] ANN: LeanCheck v0.9.0 -- enumerative property testing
by Johannes Waldmann 19 Jan '19

19 Jan '19

Yes, leancheck is awesome. > When should I reach to enumerative testing? I think enumerative testing is best suited to algebraic data types. https://mail.haskell.org/pipermail/haskell-cafe/2018-September/129951.html In the past I had one standard example where random enumeration seems better than enumerative: (non)associativity of floating point ops (I like to use this in teaching) That's immediate in Quickcheck quickCheck $ \ x y z -> (x::Double) + (y+z) == (x+y)+z *** Failed! Falsifiable (after 3 tests and 3 shrinks): -0.3 -1.4266097805446862 -1.2543117889178947 You don't get to see a counterexample in Smallcheck Prelude Test.SmallCheck> smallCheck 5 $ \ x y z -> (x::Double) + (y+z) == (x+y)+z Completed 50653 tests without failure. To my surprise, we now have Prelude Test.LeanCheck> check $ \ x y z -> (x::Double) + (y+z) == (x+y)+z *** Failed! Falsifiable (after 87 tests): 0.0 Infinity (-Infinity) and I get the above (with "real" numbers) via Prelude Test.LeanCheck> checkFor 1000 $ \ x y z -> isInfinite x || isInfinite y || isInfinite z || (x::Double) + (y+z) == (x+y)+z *** Failed! Falsifiable (after 306 tests): 1.0 1.0 0.3333333333333333 Turns out the enumeration in Leancheck uses Rationals, while Smallcheck uses encodeFloat, which happens to produce only "nice" numbers (very few bits set) in the beginning. Prelude Test.LeanCheck> list 3 series :: [Double] [0.0,1.0,-1.0,2.0,0.5,-2.0,4.0,0.25,-0.5,-4.0,-0.25] Prelude Test.LeanCheck> take 12 $ list :: [Double] [0.0,1.0,-1.0,Infinity,0.5,2.0,-Infinity,-0.5,-2.0,0.3333333333333333,3.0,-0.3333333333333333] - J.W. PS: this post wouldn't be complete without me complaining that I cannot (easily) put the type annotation where it belongs - in the declaration of the name: Prelude Test.LeanCheck> check $ \ (x::Double) y z -> x + (y+z) == (x+y)+z <interactive>:22:12: error: Illegal type signature: ‘Double’ Type signatures are only allowed in patterns with ScopedTypeVariables Wat? TypeVariables? There aren't any!

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ANN: LeanCheck v0.9.0 -- enumerative property testing
by Rudy Matela 18 Jan '19

18 Jan '19

Hello Haskell Café, A new version of LeanCheck is out (v0.9.0). LeanCheck is a property testing library (like QuickCheck) that tests values enumeratively. _Example._ Here's a simple example of LeanCheck in action showing that sorting is idempotent and list union is not commutative: > import Test.LeanCheck > import Data.List (sort, union) > check $ \xs -> sort (sort xs) == sort (xs::[Int]) +++ OK, passed 200 tests. > check $ \xs ys -> xs `union` ys == ys `union` (xs::[Int]) *** Failed! Falsifiable (after 4 tests): [] [0,0] LeanCheck works on all types that are instances of the Listable typeclass and is able to derive instances automatically using either Template Haskell or GHC.Generics. See [LeanCheck's Haddock documentation] for more details. _Whats new?_ Version 0.9.0 marks the addition of Listable typeclass instances for most standard Haskell types defined in the Haskell 2010 Language Report. This means you'll be able to test more functions without needing to define Listable instances yourself. [LeanCheck's changelog] provides more details. A separate package [leancheck-instances] provides instances for other types in the Haskell Platform. _Installing._ You can find LeanCheck on [Hackage] or [GitHub]. It is also tracked on [Stackage]. As usual, you can install it with: $ cabal install leancheck -- Rudy [Hackage]: https://hackage.haskell.org/package/leancheck [GitHub]: https://github.com/rudymatela/leancheck [Stackage]: https://www.stackage.org/package/leancheck [LeanCheck's changelog]: https://hackage.haskell.org/package/leancheck/changelog [LeanCheck's Haddock documentation]: https://hackage.haskell.org/package/leancheck/docs/Test-LeanCheck.html

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determine a type by universal property
by Olaf Klinke 17 Jan '19

17 Jan '19

Dear cafe, We know how to derive free theorems from a type [1]. But there exist also examples where the existence of a certain function determines the type. I suppose this is a special case of a free theorem? For example, every functor that is right adjoint to another functor in Hask is representable in the sense of [2], that is, isomorphic to (r->) for some type r. [see * below] I have two type signatures where I conjecture the only functors satisfying these are the Identity functor and functors of the form (,)s. Can anyone give hints as how to tackle a proof? Signature 1: What functors t admit a function forall f. Functor f => t (f a) -> f (t a) Signature 2: What functors t admit a function t (t a -> b) -> a -> b Signature 1 is like Data.Traversable.sequence, only the Applicative constraint is weakened to Functor. It is somewhat dual to Distributive [3]. Signature 2 can, with some currying, for t = (,)s be transformed into Data.Function.flip. I am unable to comprehend the free theorem for signature 2. For signature 1, the Rank-2 type seems to cause trouble with free-theorems? Thanks, Olaf [1] https://hackage.haskell.org/package/free-theorems [2] https://hackage.haskell.org/package/adjunctions [3] https://hackage.haskell.org/package/distributive [*] The argument goes like this: (1) bottoms aside, every type x is isomorphic to () -> x. (2) f and g are adjoint if and only if (f a -> b) is isomorphic to (a -> g b). (3) g b is isomorphic to () -> g b by (1) with x = g b (4) () -> g b is isomorphic to f () -> b by (2) with a = (). (5) By (3) and (4), g b is isomorphic to f () -> b. We have found g = (r->) with r = f ().

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Taking over maintainership of bitvec
by Andrew Lelechenko 17 Jan '19

17 Jan '19

I would like to take over `bitvec` package (https://hackage.haskell.org/package/bitvec) Unfortunately, the source code has not been updated for years and the test suite does not build nowadays. There are also known issues. I have prepared patches and tried to reach its maintainer James Cook (cced) by email, but have not heard back. There was a takeover of another James' package in October 2018 (https://mail.haskell.org/pipermail/libraries/2018-October/029062.html) In that case James appeared to be out of reach. -- Best regards, Andrew

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HDBC database: error on accent table name
by Damien Mattei 16 Jan '19

16 Jan '19

Hi, i have this error: *** Exception: SqlError {seState = "", seNativeError = 1064, seErrorMsg = "You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'es where Nom = 'A 20'' at line 1"} when doing this : rows_coordonnees <- quickQuery' conn "select * from sidonie.Coordonnées where Nom = 'A 20'" [] it seems tha the tabel name: Coordonnées that contain an accent is causing serious problem to the parser at some point, if i use a table name without accent it works fine. i'm at the point to rename the table which have great impact on all the project build with many other languages (Scheme) that deal correctly the table name with accent. any idea? to make accent works with haskell. Regards, Damien -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

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[Job] Soostone is hiring Haskell developers (NYC or remote)
by Ozgun Ataman 16 Jan '19

16 Jan '19

Dear Haskell Cafe, Soostone (http://www.soostone.com) is looking to hire for engineering positions in our Haskell development and operations team. At Soostone, you will be crafting and deploying products with various AI and optimization elements dealing with large quantities of data at their core to transform the way our clients tackle opportunities in consumer intelligence, marketing and pricing. We aim to develop high quality, reliable software while moving fast, being pragmatic and constantly improving our capabilities. Haskell usually makes this a little easier and we're big fans. We are located in New York City, but these are remote-friendly positions with little to no travel requirements. We will prefer candidates in North America, but will also consider qualified applications from elsewhere as long as the legal realities can be aligned. We are flexible in the work arrangement; depending on your jurisdiction and circumstances, these will either be direct hire or full-time contractor positions. Day-to-day, you will be interacting with technologies/components/libraries such as: - A relatively large Haskell codebase (500+ modules) spanning multiple applications and several reusable in-house libraries/packages - 200+ libraries from the Haskell ecosystem - Databases: PostgreSQL, Redis, DynamoDB - Analytics: Google BigQuery, tailor-made SQL EDSL/automation - A large suite of AWS and Google cloud solutions - APIs responding to millions of requests per day - Datasets spanning billions of rows …and typical activities will include: - Collaborative architecting and software design around new features - End-to-end implementation of entire components, from first line of code to the test suite - Interfacing with other languages, API services, foreign interfaces/functions as needed - Opportunities to work on all aspects of our platform, including both backend and frontend - Design/implementation of DSLs/EDSLs that supercharge tedious elements in the ecosystem - Abstract functionality into libraries and other reusable tools that ease the pain for other developers - Be on the lookout for opportunities to contribute back to the community through OSS work - Occasional trips to the land of Python, Typescript/JavaScript and Purescript for mission-specific needs Candidates of all skill/experience levels are invited to apply and will receive consideration, although these positions are likely not ideal for Haskell beginners or entry level developers without any experience. If you're interested, we encourage you to submit an application even if we've previously had conversations. If you have OSS contributions or a listing/linking of previous projects, please let us know as that's tremendously helpful to us in getting to know you. Please see the link below for more detail on the position and to submit an application. You can also reach out to us at jobs(a)soostone.com for any questions or ask here and we'll do our best to answer. Job Link: https://soostone-inc.workable.com/jobs/678589 The Soostone Team

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looking for yesod-newsfeed example
by Johannes Waldmann 16 Jan '19

16 Jan '19

Dear Cafe, I want to add news feeds to a yesod application. Is there some example code that shows real-life usage of https://hackage.haskell.org/package/yesod-newsfeed ? Thanks - J.W.

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How to set ghc options while building Setup.hs in cabal package?
by Magicloud Magiclouds 16 Jan '19

16 Jan '19

Hi, I have some code in Setup.hs in a cabalized project. I'd like them to be warning-free. But I could not find a way to set -Wall. I tried ghc-options in ~/.cabal/config. But seems like it did not kick in. ``` /bin/ghc --make -fbuilding-cabal-package -odir /home/shida/src/Personal/cairo-core/dist-newstyle/build/x86_64-linux/ghc-8.6.3/cairo-core-1.16.0/setup -hidir /home/shida/src/Personal/cairo-core/dist-newstyle/build/x86_64-linux/ghc-8.6.3/cairo-core-1.16.0/setup -i -i/home/shida/src/Personal/cairo-core/. -optP-include -optP/home/shida/src/Personal/cairo-core/dist-newstyle/build/x86_64-linux/ghc-8.6.3/cairo-core-1.16.0/setup/setup_macros.h -hide-all-packages -no-user-package-db -package-db /home/shida/.cabal/store/ghc-8.6.3/package.db -package-db /home/shida/src/Personal/cairo-core/dist-newstyle/packagedb/ghc-8.6.3 -package-id Cabal-2.4.0.1 -package-id base-4.12.0.0 -package-id directory-1.3.3.0 -package-id filepath-1.4.2.1 -package-id haskell-src-exts-1.21.0-f3e84a9ee8883b0dcaaaaba712d74996854b441aa843a4dedc055c685debcfdb /home/shida/src/Personal/cairo-core/dist-newstyle/build/x86_64-linux/ghc-8.6.3/cairo-core-1.16.0/setup/setup.hs -o /home/shida/src/Personal/cairo-core/dist-newstyle/build/x86_64-linux/ghc-8.6.3/cairo-core-1.16.0/setup/setup -threaded ``` -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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intero flymake not working
by Dennis Raddle 16 Jan '19

16 Jan '19

I recently updated a stack project to a later version of GHC, and Intero isn't quite behaving the same as before. I can't see any evidence that flycheck or flymake is working. Any idea what to look for? Mike

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PPDP 2019 CFP - Principles and Practice of Declarative Programming
by František Farka 15 Jan '19

15 Jan '19

[Apologies for cross-posting; please circulate] ====================================================================== CALL FOR PAPERS -- PPDP 2019 21st International Symposium on Principles and Practice of Declarative Programming 7–9 October 2019, Porto, Portugal Collocated with FM'19 http://ppdp2019.macs.hw.ac.uk ====================================================================== Important Dates --------------- Title and abstract registration 26 April 2019 (AoE) Paper submission 3 May 2019 (AoE) Rebuttal period (48 hours) 3 June 2019 (AoE) Author notification 14 June 2019 Final paper version 15 July 2019 Conference 7–9 October 2019 About PPDP ---------- The PPDP 2019 symposium brings together researchers from the declarative programming communities, including those working in the functional, logic, answer-set, and constraint handling programming paradigms. The goal is to stimulate research in the use of logical formalisms and methods for analyzing, performing, specifying, and reasoning about computations, including mechanisms for concurrency, security, static analysis, and verification. Scope ----- Submissions are invited on all topics related to declarative programming, from principles to practice, from foundations to applications. Topics of interest include, but are not limited to - Language Design: domain-specific languages; interoperability; concurrency, parallelism, and distribution; modules; probabilistic languages; reactive languages; database languages; knowledge representation languages; languages with objects; language extensions for tabulation; metaprogramming. - Implementations: abstract machines; interpreters; compilation; compile-time and run-time optimization; memory management. - Foundations: types; logical frameworks; monads and effects; semantics. - Analysis and Transformation: partial evaluation; abstract interpretation; control flow; data flow; information flow; termination analysis; resource analysis; type inference and type checking; verification; validation; debugging; testing. - Tools and Applications: programming and proof environments; verification tools; case studies in proof assistants or interactive theorem provers; certification; novel applications of declarative programming inside and outside of CS; declarative programming pearls; practical experience reports and industrial application; education. The PC chair (Ekaterina Komendanstkaya <e.komendantskaya(a)hw.ac.uk>) will be happy to advise on the appropriateness of a topic. Submission Categories --------------------- Submissions can be made in three categories: regular Research Papers, System Descriptions, and Experience Reports. Submissions of Research Papers must present original research which is unpublished and not submitted elsewhere. They must not exceed 12 pages ACM style 2-column (including figures, but excluding bibliography). Work that already appeared in unpublished or informally published workshop proceedings may be submitted (please contact the PC chair in case of questions). Research papers will be judged on originality, significance, correctness, clarity, and readability. Submission of System Descriptions must describe a working system whose description has not been published or submitted elsewhere. They must not exceed 10 pages and should contain a link to a working system. System Descriptions must be marked as such at the time of submission and will be judged on originality, significance, usefulness, clarity, and readability. Submissions of Experience Reports are meant to help create a body of published, refereed, citable evidence where declarative programming such as functional, logic, answer-set, constraint programming, etc., is used in practice. They must not exceed 5 pages **including references**. Experience Reports must be marked as such at the time of submission and need not report original research results. They will be judged on significance, usefulness, clarity, and readability. Supplementary material may be provided in a clearly marked appendix beyond the above-mentioned page limits. Reviewers are not required to study any material beyond the respective page limit. Format of a Submission ---------------------- For each paper category, you must use the most recent version of the "Current ACM Master Template" which is available at <https://www.acm.org/publications/proceedings-template>. The most recent version at the time of writing is 1.48. You must use the LaTeX sigconf proceedings template as the conference organizers are unable to process final submissions in other formats. In case of problems with the templates, contact ACM's TeX support team at Aptara <acmtexsupport(a)aptaracorp.com>. Authors should note ACM's statement on author's rights (http://authors.acm.org/) which apply to final papers. Submitted papers should meet the requirements of ACM's plagiarism policy (http://www.acm.org/publications/policies/plagiarism_policy) Requirements for Publication ---------------------------- At least one author of each accepted submission will be expected to attend and present the work at the conference. The pc chair may retract a paper that is not presented. The pc chair may also retract a paper if complaints about the paper's correctness are raised which cannot be resolved by the final paper deadline. Program Committee Chair ----------------------- Ekaterina Komendantskaya Heriot-Watt University, UK Program Committee ----------------- Henning Basold CNRS, ENS de Lyon, France Jasmin Christian Blanchette Vrije Universiteit Amsterdam, The Netherlands Maria Paola Bonacina University of Verona, Italy Dmitry Boulytchev Saint–Petersburg University, Russia William Byrd University of Alabama at Birmingham, USA Ornela Dardha University of Glasgow, UK Marco Gaboardi University at Buffalo, SUNY, USA Arie Gurfinkel University of Waterloo, Canada Zhenjiang Hu National Institute of Informatics, Japan Moa Johansson Chalmers University of Technology, Sweden Neelakantan Krishnaswami University of Cambridge, UK Ralf Lämmel University of Koblenz · Landau, Germany Anthony Widjaja Lin University of Oxford, UK Aart Middeldorp University of Innsbruck, Austria Gopalan Nadathur University of Minnesota, USA Keisuke Nakano Tohoku University, Japan Dominic Orchard University of Kent, UK Alberto Pardo University of the Republic, Uruguay Aleksy Schubert University of Warsaw, Poland Peter J. Stuckey The University of Melbourne, Australia Tarmo Uustalu Reykjavik University, Iceland Local Chair ----------- José Nuno Oliveira INESC TEC & University of Minho, Portugal For any queries about local issues please contact the local organiser, José Nuno Oliveira <jno(a)di.uminho.pt>. Publicity Chair --------------- František Farka University of St Andrews & Heriot-Watt University, UK

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ANN: New Haskell.org Committee Member
by George Wilson 15 Jan '19

15 Jan '19

Following the nomination period and discussion, the Haskell.org committee has selected the following member for a new three-year term, expiring 2021: * Emily Pillmore As per the rules of the committee, this discussion was held among the current members of the committee, and the outgoing member of the committee who was not seeking reappointment. Thank you to all the many candidates who submitted a self-nomination. We received a number of strong nominations. We would encourage all those who nominated themselves to consider self-nominating again in the future. The outgoing member is Gershom Bazerman. He served not one but two terms for the Haskell.org committee, and he was chair during his last term. During this long period, Gershom has undeniably made the community a better place with his leadership and efforts. Thank you for your service! Since Gershom was our chair, the committee, including the new member, will hold a discussion and elect a new chair from amongst ourselves. Regards, George Wilson

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About pointer taken by C lib in FFI.
by Magicloud Magiclouds 14 Jan '19

14 Jan '19

Hi, Say I have some data, and `poke` into a Ptr. Then I pass the Ptr to some function in C lib. The C lib stores the Ptr somewhere to use later. My question is, how does GHC handle the finalizing of the Ptr? Does it track the "used by foreign lib"? -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Binary Data Access via PIC…??
by Nick Rudnick 13 Jan '19

13 Jan '19

On NL FP day, it struck me again when I saw an almost 1 MB *.hs file with apparent sole purpose of getting a quantity of raw data incorporated to the binary – applying some funny text encoding constructs. I remembered that, to my best knowledge, with major downside that it's compile time, this appears to be the best solution to me… Another approach I did notice several times was, say, the use of super fast parsing, to read in binary data at run time. Did I miss something? Or, more specifically – I am speaking about that kind of binary data which is (1) huge! – the 1 MB mentioned above rather being at the lower limit, (2) completely independent from the version of the Haskell compiler, (3) guaranteed (externally!) to match the structural requirements of the application referred to, (4) well managed in some way, concerning ABI issues, too (e.g. versioning, metadata headers etc.), and the question is in how far – as I believe other languages do, too – we can exploit PIC (position independent code), to read in really large quantities of binary data at run time or immediately before run time, without the need for parsing at all. E.g., a textual data representation Haskell file will generate an an object file already, for which linking only should have a limited amount of assumptions regarding its inner structure. Imagining I have a huge but simple DB table, and a kind of converter which by some simplification of a Haskell compiler generates an object file that equally matches these (limited, as I believe) assumptions, and at the end can build a 'fake' the linker accepts instead of one dummy file skeleton – couldn't that be a way leading into the direction of directly getting in vast amounts of binary data in one part? In case there are stronger integrity needs, extra metadata like should be usable for verification of the origin from a valid code generator. Of course, while not completely necessary, true run time loading would be even greater… while direct interfacing to foreign (albeit simple) memory spaces deems much more intricate to me. I regularly stumbled about such cases – so I do believe this to useful. I would be happy to learn more about this – any thoughts…?? Cheers, and all the best, Nick

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Fwd: Void is not Monoid?
by Javran Cheng 13 Jan '19

13 Jan '19

(forgot to reply all, sorry) Hi Will, Thanks for the reply! > A monoid has an identity element, and void does not. now I feel dull never thought about that. > How would you write return with void as the writer? > You can accomplish what you want with the free monoid over Void - i.e. [Void], which is isomorphic to unit. So unit seems like the right choice. Unit does work fine, but I figure using Void is an interesting idea, as I can make sure that no one can use the "W" part of my RWST. Javran On Sun, Jan 13, 2019 at 12:23 AM William Yager <will.yager(a)gmail.com> wrote: > On Sun, Jan 13, 2019 at 4:00 PM Javran Cheng <javran.c(a)gmail.com> wrote: > >> Hi Cafe, >> >> I'm wondering why Data.Void does not have a Monoid instance, or, what >> would be the problem if we do "mempty = absurd mempty"? >> > > This diverges, does it not? > > A monoid has an identity element, and void does not. > > >> Long story: I was using a monad with some transformers, then I realize I >> can collapse State and Reader into RWST with Void being Writer output. >> (well, I could have just used Unit but I wanna give Void a try...) I know >> beforehand that Void is Semigroup but is a bit surprise it doesn't have >> Monoid instance. >> > > How would you write return with void as the writer? > > You can accomplish what you want with the free monoid over Void - i.e. > [Void], which is isomorphic to unit. So unit seems like the right choice. > > --Will > -- Javran (Fang) Cheng -- Javran (Fang) Cheng

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Void is not Monoid?
by Javran Cheng 13 Jan '19

13 Jan '19

Hi Cafe, I'm wondering why Data.Void does not have a Monoid instance, or, what would be the problem if we do "mempty = absurd mempty"? Long story: I was using a monad with some transformers, then I realize I can collapse State and Reader into RWST with Void being Writer output. (well, I could have just used Unit but I wanna give Void a try...) I know beforehand that Void is Semigroup but is a bit surprise it doesn't have Monoid instance. Javran Cheng

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Honest constant-space mergesort
by Ryan Reich 13 Jan '19

13 Jan '19

I am, for entertainment, trying to figure out how to implement a constant-space bottom-up mergesort in Haskell. This is quite easy to at least describe imperatively: break the list into ordered blocks, merge adjacent ones, and repeat with double the length until there's only one block and you're done. For a linked list like [a] in Haskell, this should be constant-space. Unfortunately, it is not. You can definitely express the above algorithm nicely if you organize those blocks into an [[a]], but then you are creating O(n) new lists. Even if you go to extremes to keep the blocks concatenated, though, you still have the issue that every time you merge two lists, you are creating a new list and that involves the allocator. (Note that the implementation of Data.List.sort does this.) The C equivalent would proceed by just overwriting the linking pointers in each node, without allocating new nodes to do so, but of course, that's not going to work in Haskell. I considered using something like STRef to represent the links (so, departing from the plain [a] type) but even then, if you overwrite the contents of one of those, the garbage collector still needs to know. This is better than allocating more space, but it is not necessary because the algorithm never actually makes any of the nodes unreachable. Also, I understand that using a lot of STRefs is bad for efficiency. Is there any way to do this in (GHC) Haskell and get the algorithm down to the bare minimum? If there is, is it any different than just "writing C in Haskell"? That is, can one coerce GHC into emitting the mutable-link operations and not doing collections, from high-level code? Although I've definitely seen examples of algorithms that compile down to such unboxed depths, this one has been really resistant to improvement. Thanks, Ryan Reich

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Re: [Haskell-cafe] About 'Overloaded functions are not your friend' line in GHC manual
by Johannes Waldmann 11 Jan '19

11 Jan '19

At run-time, only the branch (Left/Right) is "picked" at run-time. For each branch, the "show" instance is determined at compile-time. The problem is that such instances are represented by dictionaries, and this adds extra cost (extra arguments) to function calls - but only in those cases where these dictionaries are not inlined (that is, removed) statically. But I am thinking that GHC has become very good at inlining. - J.W.

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LambdaConf 2019 Call for Proposals
by John A. De Goes 11 Jan '19

11 Jan '19

Dear Haskell Enthusiasts: The LambdaConf 2019 Call for Proposals is open, and we warmly welcome Haskell proposals on topics of interest to aspiring and practicing functional programmers. Historically, Haskell content accounts for more than 50% of content across all 5-8 tracks of the event. Last year’s speakers included Michael Snoyberg, Dana Scott, Jeremy Siek, and many others from both industry and academia. To submit a proposal for LambdaConf 2019, please visit the following website: https://www.papercall.io/lambdaconf-2019 Travel assistance is available, including lodging. ## INTRODUCTION LambdaConf is the largest interdisciplinary functional programming conference in the Mountain West, and one of the largest and most respected functional programming conferences in the world. The conference takes place June 5rd - 7th, in Boulder, Colorado, at the University of Colorado Boulder, and is surrounded by commercial training opportunities. If you are an educator, a researcher, a speaker, a speaker coach, or someone aspiring to one of the preceding, then we warmly welcome you to submit a proposal for LambdaConf 2019. No prior experience is necessary for most proposals, and we welcome beginner-level content. The Call for Proposals closes at the end of January 2019. We recommend submitting as early as you can to ensure sufficient time for editing. LambdaConf attracts everyone from the FP-curious to researchers advancing state-of-the-art; hobbyists, professionals, academics and students. Material at all levels, including beginner content and very advanced content, will find an audience at LambdaConf. Historically, LambdaConf has enjoyed a large selection of sessions on statically-typed functional programming, and a smaller selection of sessions on dynamically-typed functional programming. Some sessions are not tied to specific programming languages, but rather cover topics in abstract algebra, category theory, type theory, programming language theory, functional architecture, and so on, either generically or in a way that applies across many programming languages. ## TOPICS LambdaConf looks for sessions in the following areas: - LANGUAGES. Proposals that overview or dive into specific features of functional, math, or logic programming languages (both new and existing), with the goal of exposing developers to new ideas or helping them master features of languages they already know. LIBRARIES. Proposals that discuss libraries that leverage functional or logic programming to help programmers solve real-world problems. - CONCEPTS. Proposals that discuss functional programming idioms, patterns, or abstractions; or concepts from mathematics, logic, and computer science, all directed at helping developers write software that’s easier to test, easier to reason about, and easier to change safely. - APPLICATIONS. Proposals that discuss how functional programming can help with specific aspects of modern software development, including scalability, distributed systems, concurrency, data processing, security, performance, correctness, user-interfaces, machine learning, and big data. - USE CASES. Proposals that discuss how functional programming enabled a project or team to thrive, or deliver more business value than possible with other approaches. - CHERRY PICKING. Proposals that show how techniques and approaches from functional programming can be adapted and incorporated into mainstream development languages and practices, to the benefit of developers using them. - CAUTIONARY TALES. Proposals that call attention to difficulties of functional programming (both as a cautionary tale but also to raise awareness), especially such proposals that suggest alternatives or a path forward. - EFFICACY. Proposals that present data, measurements, or analysis that suggests different techniques, paradigms, languages, libraries, concepts, or approaches have different efficacies for given specified metrics, which provide actionable takeaways to practicing functional and logic programmers. - OFF-TOPIC. Proposals that have appeal to a mainstream developer audience (the number of off-topic proposals we accept is small, but we do accept some, especially for keynotes). ## SESSION TYPES LambdaConf accepts proposals for the following types of sessions: - LEAP WORKSHOPS (6h). Leap Workshops are approximately 6 hours in length. They are in-depth, hands-on workshops designed to teach mainstream functional programming topics in enough detail, attendees can immediately apply what they learn in their jobs. We require that speakers follow our recommended format for Leap Workshops, although we allow exceptions for experienced teachers. - HOP WORKSHOPS (2h). Hop Workshops are 2 hours in length. Like Leap Workshops, these workshops are in-depth and hands-on, but they cover reduced content and may be specialized to topics that may not have mainstream appeal. We require that speakers follow our recommended format for Hop Workshops, although we allow exceptions for experienced teachers. - DE NOVO SESSIONS (50m). De Novo Sessions are 50 minutes in length. These sessions are designed to present original work from industry and academia. While the requirements for proposals are more rigorous, there is less competition for De Novo slots. - EDUCATIONAL SESSIONS (50m). Educational Sessions are 50 minutes in length. These sessions are designed to clearly and concisely teach one useful concept, skill, aspect, library, or language to attendees. - KEYNOTES (40m). Keynotes are 40 minutes in length, and are presented before all attendees (there are no other sessions concurrent with keynotes). Keynotes are designed to offer thought-provoking, opinionated, and insightful commentary on topics of interest to the community. If you are accepted for a specific type of proposal (e.g. Educational), we cannot guarantee that you will get a slot of this type. Based on scheduling requirements, feedback from the committee, or feedback from your speaker coach, we may require you to change the format of your session. Regards, -- John A. De Goes john(a)degoes.net Follow me on Twitter @jdegoes

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Second Call for Papers: PACMPL issue ICFP 2019
by William Fearon 11 Jan '19

11 Jan '19

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About 'Overloaded functions are not your friend' line in GHC manual
by Michail Pevnev 10 Jan '19

10 Jan '19

Hello everyone. I've been browsing through GHC manual some time ago, mostly to see what extensions there are, and stumbled upon the following paragraph in the 'Advice on: sooner, faster, smaller, thriftier' section, 'faster' subsection: Overloaded functions are not your friend: Haskell’s overloading (using type classes) is elegant, neat, etc., etc., but it is death to performance if left to linger in an inner loop. I don't quite understand the reason behind this. I was under impression that function overloading - existential shenanigans aside - is resolved mostly at compile time. Mostly, because obviously, say, 'show' for an Either still has to examine its argument at runtime and pick appropriate 'show' for one of the inner types. But the same happens for any examination of an Either, really. Does overloading make it worse, and if so, how and why? Or am I completely off track here, and the actual reason is something entirely different? -- Michail.

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overloading functions
by Damien Mattei 10 Jan '19

10 Jan '19

Hi, i have this definition: {-# LANGUAGE FlexibleInstances #-} class ConcatenateMaybeString a where cms :: Maybe String -> a -> Maybe String instance ConcatenateMaybeString (Maybe String) where cms mf ms = mf >>= (\f -> ms >>= (\s -> return (f ++ s))) instance ConcatenateMaybeString String where cms mf s = mf >>= (\f -> return (f ++ s)) when i use it on : f `cms` ("." ::String) `cms` s it works but not on this: f `cms` "." `cms` s "." is too ambigious to compile: *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:373:43: error: • Ambiguous type variable ‘a0’ arising from a use of ‘cms’ prevents the constraint ‘(ConcatenateMaybeString a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance ConcatenateMaybeString (Maybe String) -- Defined at UpdateSidonie.hs:169:11 instance ConcatenateMaybeString String -- Defined at UpdateSidonie.hs:177:11 • In the first argument of ‘cms’, namely ‘f `cms` "."’ In the expression: f `cms` "." `cms` s In the expression: let f = fmap head resBDwords s = fmap (head . tail) resBDwords mp = Just "." :: Maybe String .... in f `cms` "." `cms` s | 373 | in f `cms` "." `cms` s) :: Maybe String | ^^^^^^^^^^^ UpdateSidonie.hs:373:51: error: • Ambiguous type variable ‘a0’ arising from the literal ‘"."’ prevents the constraint ‘(Data.String.IsString a0)’ from being solved. Probable fix: use a type annotation to specify what ‘a0’ should be. These potential instances exist: instance Data.String.IsString Query -- Defined in ‘Database.MySQL.Simple.Types’ instance Data.String.IsString Tx.Text -- Defined in ‘Data.Text’ instance (a ~ Char) => Data.String.IsString [a] -- Defined in ‘Data.String’ ...plus six instances involving out-of-scope types (use -fprint-potential-instances to see them all) • In the second argument of ‘cms’, namely ‘"."’ In the first argument of ‘cms’, namely ‘f `cms` "."’ In the expression: f `cms` "." `cms` s | 373 | in f `cms` "." `cms` s) :: Maybe String | ^^^ Failed, no modules loaded. Prelude> any idea? Damien

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Second Call for Papers: PACMPL issue ICFP 2019
by Sam Tobin-Hochstadt 10 Jan '19

10 Jan '19

PACMPL Volume 3, Issue ICFP 2019 Call for Papers accepted papers to be invited for presentation at The 24th ACM SIGPLAN International Conference on Functional Programming Berlin, Germany http://icfp19.sigplan.org/ ### Important dates Submissions due: 1 March 2019 (Friday) Anywhere on Earth https://icfp19.hotcrp.com Author response: 16 April (Tuesday) - 18 Apri (Friday) 14:00 UTC Notification: 3 May (Friday) Final copy due: 22 June (Saturday) Conference: 18 August (Sunday) - 23 August (Friday) ### About PACMPL Proceedings of the ACM on Programming Languages (PACMPL <https://pacmpl.acm.org/>) is a Gold Open Access journal publishing research on all aspects of programming languages, from design to implementation and from mathematical formalisms to empirical studies. Each issue of the journal is devoted to a particular subject area within programming languages and will be announced through publicized Calls for Papers, like this one. ### Scope [PACMPL](https://pacmpl.acm.org/) issue ICFP 2019 seeks original papers on the art and science of functional programming. Submissions are invited on all topics from principles to practice, from foundations to features, and from abstraction to application. The scope includes all languages that encourage functional programming, including both purely applicative and imperative languages, as well as languages with objects, concurrency, or parallelism. Topics of interest include (but are not limited to): * *Language Design*: concurrency, parallelism, and distribution; modules; components and composition; metaprogramming; type systems; interoperability; domain-specific languages; and relations to imperative, object-oriented, or logic programming. * *Implementation*: abstract machines; virtual machines; interpretation; compilation; compile-time and run-time optimization; garbage collection and memory management; multi-threading; exploiting parallel hardware; interfaces to foreign functions, services, components, or low-level machine resources. * *Software-Development Techniques*: algorithms and data structures; design patterns; specification; verification; validation; proof assistants; debugging; testing; tracing; profiling. * *Foundations*: formal semantics; lambda calculus; rewriting; type theory; monads; continuations; control; state; effects; program verification; dependent types. * *Analysis and Transformation*: control-flow; data-flow; abstract interpretation; partial evaluation; program calculation. * *Applications*: symbolic computing; formal-methods tools; artificial intelligence; systems programming; distributed-systems and web programming; hardware design; databases; XML processing; scientific and numerical computing; graphical user interfaces; multimedia and 3D graphics programming; scripting; system administration; security. * *Education*: teaching introductory programming; parallel programming; mathematical proof; algebra. Submissions will be evaluated according to their relevance, correctness, significance, originality, and clarity. Each submission should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. The technical content should be accessible to a broad audience. PACMPL issue ICFP 2019 also welcomes submissions in two separate categories — Functional Pearls and Experience Reports — that must be marked as such at the time of submission and that need not report original research results. Detailed guidelines on both categories are given at the end of this call. Please contact the principal editor if you have questions or are concerned about the appropriateness of a topic. ### Preparation of submissions **Deadline**: The deadline for submissions is **Friday, March 1, 2019**, Anywhere on Earth (<https://en.wikipedia.org/wiki/Anywhere_on_Earth>). This deadline will be strictly enforced. **Formatting**: Submissions must be in PDF format, printable in black and white on US Letter sized paper, and interpretable by common PDF tools. All submissions must adhere to the "ACM Small" template that is available (in both LaTeX and Word formats) from <https://www.acm.org/publications/authors/submissions>. For authors using LaTeX, a lighter-weight package, including only the essential files, is available from <http://sigplan.org/Resources/Author/#acmart-format>. There is a limit of **25 pages for a full paper or Functional Pearl** and **12 pages for an Experience Report**; in either case, the bibliography will not be counted against these limits. Submissions that exceed the page limits or, for other reasons, do not meet the requirements for formatting, will be summarily rejected. Supplementary material can and should be **separately** submitted (see below). See also PACMPL's Information and Guidelines for Authors at <https://pacmpl.acm.org/authors.cfm>. **Submission**: Submissions will be accepted at <https://icfp19.hotcrp.com/> Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. **Author Response Period**: Authors will have a 72-hour period, starting at 14:00 UTC on **Tuesday, April 16, 2019**, to read reviews and respond to them. **Supplementary Material**: Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. This supplementary material should **not** be submitted as part of the main document; instead, it should be uploaded as a **separate** PDF document or tarball. Supplementary material should be uploaded **at submission time**, not by providing a URL in the paper that points to an external repository. Authors are free to upload both anonymized and non-anonymized supplementary material. Anonymized supplementary material will be visible to reviewers immediately; non-anonymized supplementary material will be revealed to reviewers only after they have submitted their review of the paper and learned the identity of the author(s). **Authorship Policies**: All submissions are expected to comply with the ACM Policies for Authorship that are detailed at <https://www.acm.org/publications/authors/information-for-authors>. **Republication Policies**: Each submission must adhere to SIGPLAN's republication policy, as explained on the web at <http://www.sigplan.org/Resources/Policies/Republication>. **Resubmitted Papers**: Authors who submit a revised version of a paper that has previously been rejected by another conference have the option to attach an annotated copy of the reviews of their previous submission(s), explaining how they have addressed these previous reviews in the present submission. If a reviewer identifies him/herself as a reviewer of this previous submission and wishes to see how his/her comments have been addressed, the principal editor will communicate to this reviewer the annotated copy of his/her previous review. Otherwise, no reviewer will read the annotated copies of the previous reviews. ### Review Process This section outlines the two-stage process with lightweight double-blind reviewing that will be used to select papers for PACMPL issue ICFP 2019. We anticipate that there will be a need to clarify and expand on this process, and we will maintain a list of frequently asked questions and answers on the conference website to address common concerns. **PACMPL issue ICFP 2019 will employ a two-stage review process.** The first stage in the review process will assess submitted papers using the criteria stated above and will allow for feedback and input on initial reviews through the author response period mentioned previously. At the review meeting, a set of papers will be conditionally accepted and all other papers will be rejected. Authors will be notified of these decisions on **May 3, 2019**. Authors of conditionally accepted papers will be provided with committee reviews (just as in previous conferences) along with a set of mandatory revisions. After four weeks (May 31, 2019), the authors will provide a second submission. The second and final reviewing phase assesses whether the mandatory revisions have been adequately addressed by the authors and thereby determines the final accept/reject status of the paper. The intent and expectation is that the mandatory revisions can be addressed within four weeks and hence that conditionally accepted papers will in general be accepted in the second phase. The second submission should clearly identify how the mandatory revisions were addressed. To that end, the second submission must be accompanied by a cover letter mapping each mandatory revision request to specific parts of the paper. The cover letter will facilitate a quick second review, allowing for confirmation of final acceptance within two weeks. Conversely, the absence of a cover letter will be grounds for the paper’s rejection. **PACMPL issue ICFP 2019 will employ a lightweight double-blind reviewing process.** To facilitate this, submitted papers must adhere to two rules: 1. **author names and institutions must be omitted**, and 2. **references to authors' own related work should be in the third person** (e.g., not "We build on our previous work ..." but rather "We build on the work of ..."). The purpose of this process is to help the reviewers come to an initial judgement about the paper without bias, not to make it impossible for them to discover the authors if they were to try. Nothing should be done in the name of anonymity that weakens the submission or makes the job of reviewing the paper more difficult (e.g., important background references should not be omitted or anonymized). In addition, authors should feel free to disseminate their ideas or draft versions of their paper as they normally would. For instance, authors may post drafts of their papers on the web or give talks on their research ideas. ### Information for Authors of Accepted Papers * As a condition of acceptance, final versions of all papers must adhere to the new ACM Small format. The page limit for the final versions of papers will be increased by two pages to help authors respond to reviewer comments and mandatory revisions: **27 pages plus bibliography for a regular paper or Functional Pearl, 14 pages plus bibliography for an Experience Report**. * Authors of accepted submissions will be required to agree to one of the three ACM licensing options: open access on payment of a fee (**recommended**, and SIGPLAN can cover the cost as described next); copyright transfer to ACM; or retaining copyright but granting ACM exclusive publication rights. Further information about ACM author rights is available from <http://authors.acm.org>. * PACMPL is a Gold Open Access journal. It will be archived in ACM’s Digital Library, but no membership or fee is required for access. Gold Open Access has been made possible by generous funding through ACM SIGPLAN, which will cover all open access costs in the event authors cannot. Authors who can cover the costs may do so by paying an Article Processing Charge (APC). PACMPL, SIGPLAN, and ACM Headquarters are committed to exploring routes to making Gold Open Access publication both affordable and sustainable. * ACM offers authors a range of copyright options, one of which is Creative Commons CC-BY publication; this is the option recommended by the PACMPL editorial board. A reasoned argument in favour of this option can be found in the article [Why CC-BY?](https://oaspa.org/why-cc-by/) published by OASPA, the Open Access Scholarly Publishers Association. * We intend that the papers will be freely available for download from the ACM Digital Library in perpetuity via the OpenTOC mechanism. * ACM Author-Izer is a unique service that enables ACM authors to generate and post links on either their home page or institutional repository for visitors to download the definitive version of their articles from the ACM Digital Library at no charge. Downloads through Author-Izer links are captured in official ACM statistics, improving the accuracy of usage and impact measurements. Consistently linking to the definitive version of an ACM article should reduce user confusion over article versioning. After an article has been published and assigned to the appropriate ACM Author Profile pages, authors should visit <http://www.acm.org/publications/acm-author-izer-service> to learn how to create links for free downloads from the ACM DL. * At least one author of each accepted submissions will be expected to attend and present their paper at the conference. The schedule for presentations will be determined and shared with authors after the full program has been selected. Presentations will be videotaped and released online if the presenter consents. * The official publication date is the date the papers are made available in the ACM Digital Library. This date may be up to *two weeks prior* to the first day of the conference. The official publication date affects the deadline for any patent filings related to published work. ### Artifact Evaluation Authors of papers that are conditionally accepted in the first phase of the review process will be encouraged (but not required) to submit supporting materials for Artifact Evaluation. These items will then be reviewed by an Artifact Evaluation Committee, separate from the paper Review Committee, whose task is to assess how the artifacts support the work described in the associated paper. Papers that go through the Artifact Evaluation process successfully will receive a seal of approval printed on the papers themselves. Authors of accepted papers will be encouraged to make the supporting materials publicly available upon publication of the papers, for example, by including them as "source materials" in the ACM Digital Library. An additional seal will mark papers whose artifacts are made available, as outlined in the ACM guidelines for artifact badging. Participation in Artifact Evaluation is voluntary and will not influence the final decision regarding paper acceptance. ### Special categories of papers In addition to research papers, PACMPL issue ICFP solicits two kinds of papers that do not require original research contributions: Functional Pearls, which are full papers, and Experience Reports, which are limited to half the length of a full paper. Authors submitting such papers should consider the following guidelines. #### Functional Pearls A Functional Pearl is an elegant essay about something related to functional programming. Examples include, but are not limited to: * a new and thought-provoking way of looking at an old idea * an instructive example of program calculation or proof * a nifty presentation of an old or new data structure * an interesting application of functional programming techniques * a novel use or exposition of functional programming in the classroom While pearls often demonstrate an idea through the development of a short program, there is no requirement or expectation that they do so. Thus, they encompass the notions of theoretical and educational pearls. Functional Pearls are valued as highly and judged as rigorously as ordinary papers, but using somewhat different criteria. In particular, a pearl is not required to report original research, but, it should be concise, instructive, and entertaining. A pearl is likely to be rejected if its readers get bored, if the material gets too complicated, if too much specialized knowledge is needed, or if the writing is inelegant. The key to writing a good pearl is polishing. A submission that is intended to be treated as a pearl must be marked as such on the submission web page, and should contain the words "Functional Pearl" somewhere in its title or subtitle. These steps will alert reviewers to use the appropriate evaluation criteria. Pearls will be combined with ordinary papers, however, for the purpose of computing the conference's acceptance rate. #### Experience Reports The purpose of an Experience Report is to help create a body of published, refereed, citable evidence that functional programming really works — or to describe what obstacles prevent it from working. Possible topics for an Experience Report include, but are not limited to: * insights gained from real-world projects using functional programming * comparison of functional programming with conventional programming in the context of an industrial project or a university curriculum * project-management, business, or legal issues encountered when using functional programming in a real-world project * curricular issues encountered when using functional programming in education * real-world constraints that created special challenges for an implementation of a functional language or for functional programming in general An Experience Report is distinguished from a normal PACMPL issue ICFP paper by its title, by its length, and by the criteria used to evaluate it. * Both in the papers and in any citations, the title of each accepted Experience Report must end with the words "(Experience Report)" in parentheses. The acceptance rate for Experience Reports will be computed and reported separately from the rate for ordinary papers. * Experience Report submissions can be at most 12 pages long, excluding bibliography. * Each accepted Experience Report will be presented at the conference, but depending on the number of Experience Reports and regular papers accepted, authors of Experience reports may be asked to give shorter talks. * Because the purpose of Experience Reports is to enable our community to accumulate a body of evidence about the efficacy of functional programming, an acceptable Experience Report need not add to the body of knowledge of the functional-programming community by presenting novel results or conclusions. It is sufficient if the Report states a clear thesis and provides supporting evidence. The thesis must be relevant to ICFP, but it need not be novel. The review committee will accept or reject Experience Reports based on whether they judge the evidence to be convincing. Anecdotal evidence will be acceptable provided it is well argued and the author explains what efforts were made to gather as much evidence as possible. Typically, more convincing evidence is obtained from papers which show how functional programming was used than from papers which only say that functional programming was used. The most convincing evidence often includes comparisons of situations before and after the introduction or discontinuation of functional programming. Evidence drawn from a single person's experience may be sufficient, but more weight will be given to evidence drawn from the experience of groups of people. An Experience Report should be short and to the point: it should make a claim about how well functional programming worked on a particular project and why, and produce evidence to substantiate this claim. If functional programming worked in this case in the same ways it has worked for others, the paper need only summarize the results — the main part of the paper should discuss how well it worked and in what context. Most readers will not want to know all the details of the project and its implementation, but the paper should characterize the project and its context well enough so that readers can judge to what degree this experience is relevant to their own projects. The paper should take care to highlight any unusual aspects of the project. Specifics about the project are more valuable than generalities about functional programming; for example, it is more valuable to say that the team delivered its software a month ahead of schedule than it is to say that functional programming made the team more productive. If the paper not only describes experience but also presents new technical results, or if the experience refutes cherished beliefs of the functional-programming community, it may be better to submit it as a full paper, which will be judged by the usual criteria of novelty, originality, and relevance. The principal editor will be happy to advise on any concerns about which category to submit to. ### ICFP Organizers General Chair: Derek Dreyer (MPI-SWS, Germany) Artifact Evaluation Co-Chairs: Simon Marlow (Facebook, UK) Industrial Relations Chair: Alan Jeffrey (Mozilla Research, USA) Programming Contest Organiser: Ilya Sergey (Yale-NUS College, Singapore) Publicity and Web Chair: Sam Tobin-Hochstadt (Indiana University, USA) Student Research Competition Chair: William J. Bowman (University of British Columbia, Canada) Workshops Co-Chair: Christophe Scholliers (Universiteit Gent, Belgium) Jennifer Hackett (University of Nottingham, UK) Conference Manager: Annabel Satin (P.C.K.) ### PACMPL Volume 3, Issue ICFP 2019 Principal Editor: François Pottier (Inria, France) Review Committee: Lennart Beringer (Princeton University, United States) Joachim Breitner (DFINITY Foundation, Germany) Laura M. Castro (University of A Coruña, Spain) Ezgi Çiçek (Facebook London, United Kingdom) Pierre-Evariste Dagand (LIP6/CNRS, France) Christos Dimoulas (Northwestern University, United States) Jacques-Henri Jourdan (CNRS, LRI, Université Paris-Sud, France) Andrew Kennedy (Facebook London, United Kingdom) Daan Leijen (Microsoft Research, United States) Kazutaka Matsuda (Tohoku University, Japan) Bruno C. d. S. Oliveira (University of Hong Kong, China) Klaus Ostermann (University of Tübingen, Germany) Jennifer Paykin (Galois, United States) Frank Pfenning (Carnegie Mellon University, USA) Mike Rainey (Indiana University, USA) Chung-chieh Shan (Indiana University, USA) Sam Staton (University of Oxford, UK) Pierre-Yves Strub (Ecole Polytechnique, France) German Vidal (Universitat Politecnica de Valencia, Spain) External Review Committee: Michael D. Adams (University of Utah, USA) Robert Atkey (University of Strathclyde, IK) Sheng Chen (University of Louisiana at Lafayette, USA) James Cheney (University of Edinburgh, UK) Adam Chlipala (Massachusetts Institute of Technology, USA) Evelyne Contejean (LRI, Université Paris-Sud, France) Germán Andrés Delbianco (IRIF, Université Paris Diderot, France) Dominique Devriese (Vrije Universiteit Brussel, Belgium) Richard A. Eisenberg (Bryn Mawr College, USA) Conal Elliott (Target, USA) Sebastian Erdweg (Delft University of Technology, Netherlands) Michael Greenberg (Pomona College, USA) Adrien Guatto (IRIF, Université Paris Diderot, France) Jennifer Hackett (University of Nottingham, UK) Troels Henriksen (University of Copenhagen, Denmark) Chung-Kil Hur (Seoul National University, Republic of Korea) Roberto Ierusalimschy (PUC-Rio, Brazil) Ranjit Jhala (University of California, San Diego, USA) Ralf Jung (MPI-SWS, Germany) Ohad Kammar (University of Oxford, UK) Oleg Kiselyov (Tohoku University, Japan) Hsiang-Shang ‘Josh’ Ko (National Institute of Informatics, Japan) Ondřej Lhoták (University of Waterloo, Canada) Dan Licata (Wesleyan University, USA) Geoffrey Mainland (Drexel University, USA) Simon Marlow (Facebook, UK) Akimasa Morihata (University of Tokyo, Japan) Shin-Cheng Mu (Academia Sinica, Taiwan) Guillaume Munch-Maccagnoni (Inria, France) Kim Nguyễn (University of Paris-Sud, France) Ulf Norell (Gothenburg University, Sweden) Atsushi Ohori (Tohoku University, Japan) Rex Page (University of Oklahoma, USA) Zoe Paraskevopoulou (Princeton University, USA) Nadia Polikarpova (University of California, San Diego, USA) Jonathan Protzenko (Microsoft Research, USA) Tiark Rompf (Purdue University, USA) Andreas Rossberg (Dfinity, Germany) KC Sivaramakrishnan (University of Cambridge, UI) Nicholas Smallbone (Chalmers University of Technology, Sweden) Matthieu Sozeau (Inria, France) Sandro Stucki (Chalmers | University of Gothenburg, Sweden) Don Syme (Microsoft, UK) Zachary Tatlock (University of Washington, USA) Sam Tobin-Hochstadt (Indiana University, USA) Takeshi Tsukada (University of Tokyo, Japan) Tarmo Uustalu (Reykjavik University, Iceland) Benoit Valiron (LRI, CentraleSupelec, Univ. Paris Saclay, France) Daniel Winograd-Cort (University of Pennsylvania, USA) Nicolas Wu (University of Bristol, UK)

2 2

Call for papers, MPC 2019, Portugal
by Graham Hutton 09 Jan '19

09 Jan '19

Dear all, The next Mathematics of Program Construction (MPC) conference will be held in Portugal in October 2019, co-located with the Symposium on Formal Methods (FM). Paper submission is 3rd May 2019. Please share, and submit your best papers! Best wishes, Graham Hutton Program Chair, MPC 2019 ====================================================================== *** CALL FOR PAPERS -- MPC 2019 *** 13th International Conference on Mathematics of Program Construction 7-9 October 2019, Porto, Portugal Co-located with Formal Methods 2019 https://tinyurl.com/MPC-Porto ====================================================================== TIMELINE: Abstract submission 26th April 2019 Paper submission 3rd May 2019 Author notification 14th June 2019 Camera ready copy 12th July 2019 Conference 7-9 October 2019 BACKGROUND: The International Conference on Mathematics of Program Construction (MPC) aims to promote the development of mathematical principles and techniques that are demonstrably practical and effective in the process of constructing computer programs. MPC 2019 will be held in Porto, Portugal from 7-9 October 2019, and is co-located with the International Symposium on Formal Methods, FM 2019. Previous conferences were held in Königswinter, Germany (2015); Madrid, Spain (2012); Québec City, Canada (2010); Marseille, France (2008); Kuressaare, Estonia (2006); Stirling, UK (2004); Dagstuhl, Germany (2002); Ponte de Lima, Portugal (2000); Marstrand, Sweden (1998); Kloster Irsee, Germany (1995); Oxford, UK (1992); Twente, The Netherlands (1989). SCOPE: MPC seeks original papers on mathematical methods and tools put to use in program construction. Topics of interest range from algorithmics to support for program construction in programming languages and systems. Typical areas include type systems, program analysis and transformation, programming language semantics, security, and program logics. The notion of a 'program' is interpreted broadly, ranging from algorithms to hardware. Theoretical contributions are welcome, provided that their relevance to program construction is clear. Reports on applications are welcome, provided that their mathematical basis is evident. We also encourage the submission of 'programming pearls' that present elegant and instructive examples of the mathematics of program construction. SUBMISSION: Submission is in two stages. Abstracts (plain text, maximum 250 words) must be submitted by 26th April 2019. Full papers (pdf, formatted using the llncs.sty style file for LaTex) must be submitted by 3rd May 2019. There is no prescribed page limit, but authors should strive for brevity. Both abstracts and papers will be submitted using EasyChair. Papers must present previously unpublished work, and not be submitted concurrently to any other publication venue. Submissions will be evaluated by the program committee according to their relevance, correctness, significance, originality, and clarity. Each submission should explain its contributions in both general and technical terms, clearly identifying what has been accomplished, explaining why it is significant, and comparing it with previous work. Accepted papers must be presented in person at the conference by one of the authors. The proceedings of MPC 2019 will be published in the Lecture Notes in Computer Science (LNCS) series, as with all previous instances of the conference. Authors of accepted papers will be expected to transfer copyright to Springer for this purpose. After the conference, authors of the best papers from MPC 2019 and MPC 2015 will be invited to submit revised versions to a special issue of Science of Computer Programming (SCP). For any queries about submission please contact the program chair, Graham Hutton <graham.hutton(a)nottingham.ac.uk>. KEYNOTE SPEAKERS: Assia Mahboubi INRIA, France Annabelle McIver Macquarie University, Australia PROGRAM COMMITTEE: Patrick Bahr IT University of Copenhagen, Denmark Richard Bird University of Oxford, UK Corina Cîrstea University of Southampton, UK Brijesh Dongol University of Surrey, UK João F. Ferreira University of Lisbon, Portugal Jennifer Hackett University of Nottingham, UK William Harrison University of Missouri, USA Ralf Hinze University of Kaiserslautern, Germany Zhenjiang Hu National Institute of Informatics, Japan Graham Hutton (chair) University of Nottingham, UK Cezar Ionescu University of Oxford, UK Mauro Jaskelioff National University of Rosario, Argentina Ranjit Jhala University of California, USA Gabriele Keller Utrecht University, The Netherlands Ekaterina Komendantskaya Heriot-Watt University, UK Chris Martens North Carolina State University, USA Bernhard Möller University of Augsburg, Germany Shin-Cheng Mu Academia Sinica, Taiwan Mary Sheeran Chalmers University of Technology, Sweden Alexandra Silva University College London, UK Georg Struth University of Sheffield, UK CONFERENE VENUE: The conference will be held at the Alfândega Porto Congress Centre, a 150 year old former custom's house located in the historic centre of Porto on the bank of the river Douro. The venue was renovated by a Pritzer prize winning architect and has received many awards. LOCAL ORGANISERS: José Nuno Oliveira University of Minho, Portugal For any queries about local issues please contact the local organiser, José Nuno Oliveira <jno(a)di.uminho.pt>. ====================================================================== This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

1 0

concatenate two Maybe String...
by Damien Mattei 08 Jan '19

08 Jan '19

hello, i have a variable resBDwords of type ( i expect) Maybe [String], for info it is integer and fractional part of a number example looks like this : resBDwords =Just ["-04","3982"] i want to concatanate "-04" and "3982" in the example, i begin to understand fmap to use the functor hidden in the Maybe ,it worked previously: let resBDstr = fmap Tx.unpack resBDtxt putStr "resBDstr =" putStrLn (show resBDtxt) let resBDwords = fmap words resBDstr putStr "resBDwords =" putStrLn (show resBDwords) which gives: resBDtxt ="-04 3982" resBDstr =Just "-04 3982" just after in my code i have this to concatanate the two strings f and s that are the first and second element of the array: putStr "resBDwords =" putStrLn (show resBDwords) let lgBDwords = length resBDwords let resBDstrFloat = if lgBDwords == 0 then trace "WARNING: BD contains no words" Nothing else if lgBDwords == 1 then trace "WARNING: BD contains only one word" fmap head resBDwords else let f = fmap head resBDwords s = fmap (head . tail) resBDwords in f ++ "." ++ S but i do not know how to concatanate the Maybe String in an elegant way, using somethin like fmap variable which have handled Nothing (from Maybe) automatically i need the counter part for multipe variable i do not want to do it using the hard way with case... of Just x -> nothing ......... i got this error : *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:339:43: error: • Couldn't match expected type ‘[Char]’ with actual type ‘Maybe String’ • In the first argument of ‘(++)’, namely ‘f’ In the expression: f ++ "." ++ s In the expression: let f = fmap head resBDwords s = fmap (head . tail) resBDwords in f ++ "." ++ s | 339 | in f ++ "." ++ s | ^ UpdateSidonie.hs:339:43: error: • Couldn't match expected type ‘Maybe String’ with actual type ‘[Char]’ • In the expression: f ++ "." ++ s In the expression: let f = fmap head resBDwords s = fmap (head . tail) resBDwords in f ++ "." ++ s In the expression: if lgBDwords == 1 then trace "WARNING: BD contains only one word" fmap head resBDwords else let f = fmap head resBDwords s = fmap (head . tail) resBDwords in f ++ "." ++ s | 339 | in f ++ "." ++ s | ^^^^^^^^^^^^^ UpdateSidonie.hs:339:55: error: • Couldn't match expected type ‘[Char]’ with actual type ‘Maybe String’ • In the second argument of ‘(++)’, namely ‘s’ In the second argument of ‘(++)’, namely ‘"." ++ s’ In the expression: f ++ "." ++ s | 339 | in f ++ "." ++ s | ^ Failed, no modules loaded. for now this page has been of valuable help: https://pbrisbin.com/posts/maybe_is_just_awesome/ i'm sure it's an obvious question but.... :-)

3 3

concatenate two Maybe String...
by Jake 08 Jan '19

08 Jan '19

On Mon, Jan 7, 2019 at 5:55 PM Jake <jake.waksbaum(a)gmail.com> wrote: > Like Josh mentioned, Applicative Functors > <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.h…> > are what you want. There are two idiomatic ways to do it: > > - You can just use liftA2, which has type (a -> b -> c) -> f a -> f b > -> f c. That means it lifts a binary function to some applicative functor > like maybe, so liftA2 (++) :: Maybe String -> Maybe String -> Maybe String > - In general, for any arity f that you want to lift to an applicative > functor. So you have a function g that takes a bunch of arguments of types > a, b, c, ... and gives you back an r and you want to get a function that > takes an f a, f b, f c, ... and gives you back an f r, you can write f <$> > a <*> b <*> c ... This works because <$> let's you apply g to type f a and > gives you an f (b -> c ..) -> f r, and <*> basically let's you take the > function back out of the f and apply it to the b to get a f (c ..) -> f r > and so on. *tl;dr* you can write (++) <$> s1 <*> s2. In fact, liftA2 > must satisfy the equation liftA2 > <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.h…> > f x y = f <$> x <*> > <http://hackage.haskell.org/package/base-4.12.0.0/docs/Control-Applicative.h…> > y so these are the same thing. > > בתאריך יום ב׳, 7 בינו׳ 2019, 17:41, מאת ☂Josh Chia (謝任中) < > joshchia(a)gmail.com>: > >> Firstly, because "resBDwords :: Maybe String", not "resBDwords :: >> String", "lgBDwords = length resBDwords" probably is not what you want -- >> it does not give you the number of words in the String that may be in there. >> >> Second, for the problem you asked about, you could just use a function >> that takes a String and do it "the hard way" like you said, using case >> outside before calling the function. Another way is to use an applicative >> functor to allow you to have a "Maybe String -> Maybe String -> Maybe >> String". This is used once for each "++" that you want to do. >> >> I don't know exactly what you need to accomplish but I would just write a >> function "f :: String -> Maybe String" implementing the logic you listed in >> the second code snippet but operating on String instead of "Maybe String" >> and do "join . fmap f $ resBDwords". >> >> On Tue, Jan 8, 2019 at 12:13 AM Damien Mattei <mattei(a)oca.eu> wrote: >> >>> hello, >>> >>> i have a variable resBDwords of type ( i expect) Maybe [String], for >>> info it is integer and fractional part of a number >>> >>> example looks like this : >>> resBDwords =Just ["-04","3982"] >>> >>> i want to concatanate "-04" and "3982" in the example, i begin to >>> understand fmap to use the functor hidden in the Maybe ,it worked >>> previously: >>> >>> let resBDstr = fmap Tx.unpack resBDtxt >>> putStr "resBDstr =" >>> putStrLn (show resBDtxt) >>> >>> let resBDwords = fmap words resBDstr >>> putStr "resBDwords =" >>> putStrLn (show resBDwords) >>> >>> which gives: >>> >>> resBDtxt ="-04 3982" >>> resBDstr =Just "-04 3982" >>> >>> >>> just after in my code i have this to concatanate the two strings f and s >>> that are the first and second element of the array: >>> >>> >>> putStr "resBDwords =" >>> putStrLn (show resBDwords) >>> >>> let lgBDwords = length resBDwords >>> >>> let resBDstrFloat = if lgBDwords == 0 >>> then trace "WARNING: BD contains no words" >>> Nothing >>> else >>> if lgBDwords == 1 >>> then trace "WARNING: BD contains only >>> one word" fmap head resBDwords >>> else let f = fmap head resBDwords >>> s = fmap (head . tail) >>> resBDwords >>> in f ++ "." ++ S >>> >>> but i do not know how to concatanate the Maybe String in an elegant way, >>> using somethin like fmap variable which have handled Nothing (from >>> Maybe) automatically i need the counter part for multipe variable >>> >>> i do not want to do it using the hard way with case... of Just x -> >>> nothing ......... >>> >>> i got this error : >>> *Main> :load UpdateSidonie >>> [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) >>> >>> UpdateSidonie.hs:339:43: error: >>> • Couldn't match expected type ‘[Char]’ >>> with actual type ‘Maybe String’ >>> • In the first argument of ‘(++)’, namely ‘f’ >>> In the expression: f ++ "." ++ s >>> In the expression: >>> let >>> f = fmap head resBDwords >>> s = fmap (head . tail) resBDwords >>> in f ++ "." ++ s >>> | >>> 339 | in f ++ "." ++ s >>> | ^ >>> >>> UpdateSidonie.hs:339:43: error: >>> • Couldn't match expected type ‘Maybe String’ >>> with actual type ‘[Char]’ >>> • In the expression: f ++ "." ++ s >>> In the expression: >>> let >>> f = fmap head resBDwords >>> s = fmap (head . tail) resBDwords >>> in f ++ "." ++ s >>> In the expression: >>> if lgBDwords == 1 then >>> trace "WARNING: BD contains only one word" fmap head >>> resBDwords >>> else >>> let >>> f = fmap head resBDwords >>> s = fmap (head . tail) resBDwords >>> in f ++ "." ++ s >>> | >>> 339 | in f ++ "." ++ s >>> | ^^^^^^^^^^^^^ >>> >>> UpdateSidonie.hs:339:55: error: >>> • Couldn't match expected type ‘[Char]’ >>> with actual type ‘Maybe String’ >>> • In the second argument of ‘(++)’, namely ‘s’ >>> In the second argument of ‘(++)’, namely ‘"." ++ s’ >>> In the expression: f ++ "." ++ s >>> | >>> 339 | in f ++ "." ++ s >>> | ^ >>> Failed, no modules loaded. >>> >>> for now this page has been of valuable help: >>> >>> https://pbrisbin.com/posts/maybe_is_just_awesome/ >>> >>> i'm sure it's an obvious question but.... :-) >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> To (un)subscribe, modify options or view archives go to: >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> Only members subscribed via the mailman list are allowed to post. >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. > >

4 3

conduit: consume part of input and push all other input downstream
by Qingbo Liu 08 Jan '19

08 Jan '19

Dear Cafe, I am using conduit to parse a file in a streaming fashion. In the middle of the conduit pipeline, I have a conduit that parses a value, returns the value, and then passes all other input downstream. Is there any way to express this? I looked up functions provided by the conduit and it seems that there is no such a function that provides such semantics. My conduit function looks as follows: -- | Run the given `Get` only once and push unconsumed bytes from upstream to -- downstream conduitParseOnce :: (MonadThrow m, MonadResource m) => Get a -> ConduitT BS.ByteString BS.ByteString m a conduitParseOnce g = go0 where go0 = do x <- await case x of Nothing -> undefined -- throwM SomeException Just bs -> go (runGetChunk g Nothing bs) go (Fail msg bs) = throwM (DecodeError bs msg "conduitParseOnce") go (Partial f) = await >>= maybe (go $ f mempty) (go . f) go (Done v bs) = leftover bs >> return v The downstream of this conduit will take all remaining inputs and parses them. The issue with this implementation is that the downstream will only be able to get the first chunk of data. Best Regards, Qingbo Liu

1 0

Hide internal modules but expose them for testing
by Georgi Lyubenov 07 Jan '19

07 Jan '19

Greetings, I want to have internal modules which I do not expose at all. I also want to be able to test them. This however also hides these modules from my tests. The best way I have found to work around this is to include my entire source directory in the test suite (source-dirs). This however means that I can't reuse builds that didn't include the tests. Is there some obvious and "best" solution that I am missing here? For reference I use Stack. Thanks in advance! ======= Georgi

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10 PhD studentships in Nottingham
by Graham Hutton 07 Jan '19

07 Jan '19

Dear all, *** FINAL CALL -- APPLICATION DEADLINE 18 JANUARY 2019 *** The School of Computer Science at the University of Nottingham is seeking applications for 10 fully-funded PhD studentships: https://tinyurl.com/10-phds-2019 Applicants in the area of the Functional Programming Laboratory (https://tinyurl.com/fp-notts) are strongly encouraged! If you are interested in applying, please contact a potential supervisor as soon as possible (the application deadline is 18th January): Thorsten Altenkirch - constructive logic, proof assistants, homotopy type theory, category theory, lambda calculus. Venanzio Capretta - type theory, mathematical logic, corecursive structures, proof assistants, category theory, epistemic logic. Graham Hutton - functional programming, program calculation and transformation, correctness and efficiency, category theory. Henrik Nilsson - functional reactive programming, modelling and simulation, domain-specific languages, probabilistic languages. Best wishes, Graham +-----------------------------------------------------------+ 10 Fully-Funded PhD Studentships School of Computer Science University of Nottingham, UK https://tinyurl.com/10-phds-2019 Applications are invited for up to ten fully-funded PhD studentships in the School of Computer Science at the University of Nottingham, starting on 1 October 2019. The topics for the studentships are open, but should relate to one of the School’s research groups: Agents Lab; Automated Scheduling and Planning; Computer Vision Lab; Data Driven Algorithms, Systems and Design; Functional Programming Lab; Intelligent Modelling and Analysis; Uncertainty in Data and Decision Making; Mixed Reality Lab. The studentships are for a minimum of three years and include a stipend of £14,777 per year and tuition fees. They are open to students of any nationality. Applicants are normally expected to have a first-class MSc or BSc in Computer Science or a related discipline, and must obtain the support of a supervisor in the School prior to submitting their application. Initial contact with supervisors should be made at least two weeks prior to the closing date for applications. Informal enquiries may be addressed to SS-PGR-JC(a)nottingham.ac.uk. To apply, please submit the following items by email to: Christine.Fletcher(a)nottingham.ac.uk: (1) a brief covering letter that describes your reasons for wishing to pursue a PhD, your proposed research area and topic, and the name of the potential supervisor whose support you have already secured; (2) a copy of your CV, including your actual or expected degree classes, and results of all University examinations; (3) an extended example of your technical writing, such as a project report or dissertation; (4) contact details for two academic referees. Closing date for applications: Friday 18 January 2019 +-----------------------------------------------------------+ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

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Call for Participation: BOB 2019 (March 22, Berlin)
by Matthias Fischmann 06 Jan '19

06 Jan '19

================================================================================ BOB 2019 Conference “What happens if we simply use what’s best?” March 22, 2019, Berlin http://bobkonf.de/2019/ Program: http://bobkonf.de/2019/en/program.html Registration: http://bobkonf.de/2019/en/registration.html ================================================================================ BOB is the conference for developers, architects and decision-makers to explore technologies beyond the mainstream in software development, and to find the best tools available to software developers today. Our goal is for all participants of BOB to return home with new insights that enable them to improve their own software development experiences. The program features 14 talks and 8 tutorials on current topics: http://bobkonf.de/2019/en/program.html The subject range of talks includes functional programming, formal methods, event sourcing, music, advanced SQL, logic, and feeling The tutorials feature introductions to Racket, Clojure, Functional Programming, TypeScript, type-level programming, SQL indexing, probabilistic programming, and hardware. Gabriele Keller will give the keynote talk. Registration is open online: http://bobkonf.de/2019/en/registration.html NOTE: The early-bird rates expire on February 19, 2019! BOB cooperates with the RacketFest conference on the following day: https://racketfest.com/

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Re: [Haskell-cafe] Consider operator fixity when deriving Show or Read
by Doaitse Swierstra 06 Jan '19

06 Jan '19

The current GHC implementation constructs Read instances out of other Read instances. In order to make this work some restictions are necessary. Given that a top-down paring strategy is used we cannot easily construct parsers for left-recursive data types. To cope with this problem, and to make sure that at least Read instances can read data written by Show instances, data is written out by Show instances with a sufficjently large number of parentheses. Unfortunately this may make reading data back extremely show. A solution for this problem is described in out Haskell 2008 Workshop paper: @inproceedings{1411296, Address = {New York, NY, USA}, Author = {Viera, Marcos and Swierstra, S. Doaitse and Lempsink, Eelco}, Booktitle = {Haskell Symposium}, Date-Added = {2009-06-06 22:24:07 +0200}, Date-Modified = {2009-06-06 22:24:07 +0200}, Doi = {http://doi.acm.org/10.1145/1411286.1411296}, Isbn = {978-1-60558-064-7}, Location = {Victoria, BC, Canada}, Pages = {63--74}, Publisher = {ACM}, Read = {Yes}, Title = {Haskell, do you read me?: constructing and composing efficient top-down parsers at runtime}, Year = {2008}, }} which I am attaching. Unfortunately the solution to solve all problems is rather involved. The code you can find in the package ChristmasTree: "ChristmasTree (Changing Haskell's Read Implementation Such That by Manipulating ASTs it Reads Expressions Efficiently) is an alternative approach of read that composes grammars instead of parsers. It reads data in linear time, while the function read has an exponential behavior in some cases of data types with infix operators.” Hope this helps answering your question, Doaitse > Op 6 jan. 2019, om 11:34 heeft Oleg Grenrus <oleg.grenrus(a)iki.fi> het volgende geschreven: > > Report says explicitly "ignoring associativity". Yet, I have written manual Show/Read to make Show of list-like data prettier: I don't know any problem with that. Would be good to know, why report is written as it is. > > - Oleg > > The result of show is a syntactically correct Haskell expression containing only constants, given the fixity declarations in force at the point where the type is declared. It contains only the constructor names defined in the data type, parentheses, and spaces. When labelled constructor fields are used, braces, commas, field names, and equal signs are also used. Parentheses are only added where needed, ignoring associativity. No line breaks are added. The result of showis readable by read if all component types are readable. (This is true for all instances defined in the Prelude but may not be true for user-defined instances.) > > Sent from my iPhone > > On 6 Jan 2019, at 3.53, Dannyu NDos <ndospark320(a)gmail.com> wrote: > >> Sorry for the mix-up. I mean associativity, not fixity! >> >> 2019년 1월 6일 (일) 오전 9:06, Dannyu NDos <ndospark320(a)gmail.com>님이 작성: >> For the following code as an example: >> >> {-# LANGUAGE TypeOperators #-} >> >> infixr 5 :. >> >> data List a = Null | a :. List a deriving (Eq, Ord, Show, Read) >> >> >> The Show instance and the Read instance are inaware of the fixity of (:.): >> >> *Main> 2 :. 3 :. Null >> 2 :. (3 :. Null) >> *Main> read "2 :. 3 :. Null" :: List Int >> *** Exception: Prelude.read: no parse >> *Main> read "2 :. (3 :. Null)" :: List Int >> 2 :. (3 :. Null) >> >> The derived instances should be: >> >> instance Show a => Show (List a) where >> showsPrec p Null = showParen (11 <= p) (showString "Null") >> showsPrec p (x :. xs) = showParen (5 <= p) (go p (x :. xs)) where >> go _ Null = showString "Null" >> go p (x :. xs) = showsPrec p x . showString " :. " . go p xs >> >> instance Read a => Read (List a) where >> readPrec = parens $ do >> Ident "Null" <- lexP >> return Null >> +++ (do >> x <- readPrec >> Symbol ":." <- lexP >> xs <- readPrec >> return (x :. xs) >> ) >> >> _______________________________________________ >> Libraries mailing list >> Libraries(a)haskell.org >> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries > _______________________________________________ > Libraries mailing list > Libraries(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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Maybe type filtered to remove Nothing
by Damien Mattei 05 Jan '19

05 Jan '19

i had filtered a [Maybe Text] type to remove Nothing from the list and now i want to put the result in a [Text] type but the compiler complains about the incompatible type : (bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String)) -- remove the records having N°BD NULL let fltWDS :: [Only Text] = Prelude.filter (\(Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:282:33: error: • Couldn't match type ‘Maybe Text’ with ‘Text’ Expected type: [Only Text] Actual type: [Only (Maybe Text)] • In the expression: Prelude.filter (\ (Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS In a pattern binding: fltWDS :: [Only Text] = Prelude.filter (\ (Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS In the expression: do conn <- connect defaultConnectInfo {connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie"} (rows :: [(Text, Double)]) <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" (names :: [Only Text]) <- query_ conn "SELECT Nom FROM AngularDistance WHERE distance > 0.000278" let resLstNames = Prelude.map fromOnly names .... | 282 | let fltWDS :: [Only Text] = Prelude.filter (\(Only a) -> | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^... Failed, no modules loaded. how can id do the type conversion,now i'm sure there is no more Nothing values?

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Ord methods too strict?
by V.Liepelt 04 Jan '19

04 Jan '19

I am surprised to find that `False <= undefined = undefined`. What justifies (<=) to be strict in both arguments? Vilem

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Re: [Haskell-cafe] Ord methods too strict?
by Olaf Klinke 04 Jan '19

04 Jan '19

Information-theoretically 'compare' must be stricter than <= because is gives more information: It decides equality, which <= doesn't. Therefore compare must be strict in both arguments. What follows is an argument that there can not be a lazy implementation of <= that behaves like logical implication. We try to model <= after the logical implication operator. Order-theoretically, the binary operation (<=) should be antitone in the first and monotone in the second argument. The preceding sentence only makes sense once we define a mathematical order on the semantics of Bool. Let's declare the "logical" order False < undefined < True. I give a formal justification in Appendix A below. Arrange all nine combinations of type (Bool,Bool) in a 3x3 grid where the first dimension is descending and the second dimension is ascending in the logical order. Now we consider: (undefined,True) must map to True because (True,True) maps to True and descending in the first argument means ascending in the result. (False,undefined) must map to True because (False,False) maps to True and ascending in the second argument means ascending in the result. Further we require (True,False) mapping to False. But now we're in trouble: A function giving the above three return values requires ambiguous choice, which we don't have available in pure, sequential Haskell, q.e.d. It's the same reason why we can't have (&&) or (||) operators which behave symmetrically w.r.t. the logical order. Olaf ========== Appendix A Let R be a binary relation on a set A. The Egli-Milner-lifting of R is a binary relation on the powerset of A. Among several possible liftings, it is the one with the most pleasing properties [1, Theorem 2.12]. Identify the "undefined" value of a type A with the maximal element of the powerset of A, which expresses that "undefined" may evaluate to anything. In Haskell terms: {-# LANGUAGE Rank2Types #-} module RelationLifting where import Prelude hiding (undefined) type Relation a = a -> a -> Bool type Lifting = forall a. Relation a -> Relation [a] egliMilner :: Lifting egliMilner r xs ys = hoare && smyth where hoare = all (\x -> any (\y -> r x y) ys) xs smyth = all (\y -> any (\x -> r x y) xs) ys true, false, undefined :: [Bool] true = [True] false = [False] undefined = [False,True] RelationLifting> egliMilner (<=) false undefined True RelationLifting> egliMilner (<=) undefined true True [1] https://www.cs.le.ac.uk/people/akurz/Papers/kv-relation-lifting.pdf

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RFC: in haddock, collapse instances by default
by Li-yao Xia 04 Jan '19

04 Jan '19

Hello Café, I would like to propose making haddock keep instance lists collapsed by default. Some discussion is in order since it would significantly affect the documentation of many packages on Hackage. Feel free to reply here or on the related Github thread: https://github.com/haskell/haddock/issues/698 1. Instance lists take a lot of screen estate For a motivating example, I can point to the Prelude we all love. https://hackage.haskell.org/package/base-4.12.0.0/docs/Prelude.html We are immediately welcomed by half a page of instances of Bool, which is not quite bad, but classes have the most impressive instance lists, as you may see when you reach Eq. Many packages, even commonly used ones, have the same issue. For an extreme example, see the scrollbar jump when you fold the instance list of Apply in singletons. https://hackage.haskell.org/package/singletons-2.5.1/docs/Data-Singletons.h… 2. Current workarounds They can be collapsed manually one by one, and we can jump to the middle of a module with the table of contents, but scrolling up from the bottom of an instance list is still a chore. Of course, instance lists also contain quite important information. Would it become too easy to miss if it were hidden by default? Would a more fine-grained alternative be better? Regards, Li-yao

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Left and right monadic folds
by Michail Pevnev 04 Jan '19

04 Jan '19

Hello everyone. Recently I've run into a situation where a monadic fold seems like an interesting option. However, I'm not particularly sure how to choose which fold - left or right - to use. For non-monadic folds the rule of thumb is 'avoid foldl, use foldr for infinite things and foldl' for performance'. What are the guidelines for the monadic ones? The fact that 'foldlM' is implemented (in base-4.12.0-0) using 'foldr' and 'foldrM' uses 'foldl' (also, non-strict foldl, which is worrying) adds somewhat to my confusion. I've played a bit with folding Eithers, and foldlM seems to work somewhat intuitively (breaks on the first Left, as is usual for Either), and also accepts infinite lists (which makes sense, given it's a foldr): import Data.Foldable main :: IO () main = do print $ sumNonNegative $ [1, 2, -4] ++ [5..] sumNonNegative :: [Int] -> Either String Int sumNonNegative = foldlM foo 0 foo :: Int -> Int -> Either String Int foo acc x | x >= 0 = Right $ x + acc | otherwise = Left $ "Negative number: " ++ show x This prints out "Negative number: -4". Odd things begin when I edit the above to use foldrM. It doesn't terminate on an infinite list, which is expected from a disguised foldl. But given several negative numbers in the input, it prints out the last one, seemingly turning the usual behaviour of Either on its head. It seems its foldl heritage results in applying side-effects (in case of Either, erroring out with a Left) in reverse order. Do I get this right and is this the general behaviour of foldrM for any monad? -- Michail.

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Call for papers: DBPL 2019
by Alvin Cheung 04 Jan '19

04 Jan '19

The 17th International Symposium on Database Programming Languages https://pldi19.sigplan.org/track/dbpl-2019-papers Phoenix, Arizona, USA June 23, 2019 hosted as part of PLDI 2019 Call for Papers For over 25 years, DBPL has established itself as the principal venue for publishing and discussing new ideas at the intersection of databases and programming languages. Many key contributions in query languages for object-oriented data, persistent databases, nested relational data, and semistructured data, as well as fundamental ideas in types for query languages, were first announced at DBPL. Today, this creative research area is broadening into a subfield of data-centric computation, currently scattered among a range of venues. DBPL is an established destination for such new ideas and solicits submissions from researchers in databases, programming languages or any other community interested in the design, implementation or foundations of data-centric computation. Scope ----- DBPL solicits practical and theoretical papers in all topics at the intersection of databases and programming languages. Papers emphasizing new topics or emerging areas are especially welcome. Suggested, but not exclusive, topics of interest for submissions include: - Compiling Query Languages to Modern Hardware - Data-Centric Programming Abstractions, Comprehensions, Monads - Data Integration, Exchange, and Interoperability - Data Synchronization and Bidirectional Transformations - Declarative Data Centers (e.g., distributed query processing, serverless computing platforms, social computing platforms, etc) - Emerging and Nontraditional Data Models - Language-Based Security in Data Management - Language-Integrated Query Mechanisms - Managing Uncertain and Imprecise Information - Metaprogramming and Heterogeneous Staged Computation - Programming Language Support for Data-Centric Programming (e.g., databases, web programming, machine learning, etc) - Query Compilation and In-memory Databases - Query Language Design and Implementation - Query Transformation and Optimization - Schema Mapping and Metadata Management - Semantics and Verification of Database Systems - Stream Data Processing and Query Languages - Type and Effect Systems for Data-Centric Programming Author Guidelines ----------------- Prospective authors are invited to submit full papers in English presenting original research. Submitted papers must be unpublished and not submitted for publication elsewhere. Submissions should be no more than 10 pages long, excluding references, in the two-column ACM proceedings format, following PLDI’s formatting requirements (https://pldi19.sigplan.org/track/pldi-2019-papers#Call-for-Papers) Each submission should begin with a succinct statement of the problem and a summary of the main results. Authors may provide more details to substantiate the main claims of the paper by including a clearly marked appendix at the end of the submission, which is not included in the page limit and is read at the discretion of the committee. At least one author of each accepted paper must attend the symposium to present their work. Short papers of at most 4 pages (same format as long papers) describing work in progress, demos, research challenges or visions are also welcome. Accepted short papers may be included or excluded from the formal proceedings, whichever the author(s) prefer. Full and short papers are both due on the deadline, February 15, 2019. Instructions on how to submit will be posted on the symposium website noted above. Review is single-blind, so authors do not need to anonymize their submissions. PC submissions are allowed, except for the co-chairs. Important Dates --------------- - Paper Submission: February 15, 2019 - Notification: March 29, 2019 - Final versions due: April 16, 2019 - Symposium: June 23, 2019 Proceedings ----------- Accepted papers will appear as part of the PLDI Proceedings for DBPL 2019. Program Committee ----------------- *Program Co-Chairs* Alvin Cheung, University of Washington Kim Nguyễn, Université Paris-Sud *Program Committee* William Cook, University of Texas at Austin Vasiliki Kalavri, ETH Harshad Kasture, Oracle Oleg Kiselyov, University of Tsukuba Sam Lindley, University of Edinburgh Tiark Rompf, Purdue University Stefanie Scherzinger, OTH Regensberg Amir Shaikhha, EPFL / University of Oxford Avi Shinnar, IBM Guido Wachsmuth, Oracle Melanie Wu, Pomona College History ------- The 17th Symposium on Data Base Programming Languages (DBPL 2019) continues the tradition of excellence initiated by its predecessors in Roscoff, Finistere (1987), Salishan, Oregon (1989), Nafplion, Argolida (1991), Manhattan, New York (1993), Gubbio, Umbria (1995), Estes Park, Colorado (1997), Kinloch Rannoch, Scotland (1999), Marino, Rome (2001), Potsdam, Germany (2003), Trondheim, Norway (2005), Vienna, Austria (2007), Lyon, France (2009), Seattle, Washington (2011), Riva del Garda, Italy (2013), Pittsburgh, Pennsylvania (2015), Munich, Germany (2017). DBPL was affiliated with VLDB from 1999-2013 and in 2017. In 2015, it is affiliated with SPLASH for the first time and in 2019, it is affiliated with PLDI for the first time.

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Concordium is hiring
by Bas Spitters 03 Jan '19

03 Jan '19

Concordium is the first ID/KYC ready blockchain network. Concordium focuses strongly on software quality and uses languages such as haskell, rust and Coq. There are a number of positions available at the offices in either Aarhus, Zurich, London and Marbella. In exceptional cases is off-site work possible. There is a preference for people with a relevant PhD or master's degree. https://www.concordium.com/careers/ In particular, the Concordium Aarhus office has a strong background in functional programming and type theory, and is looking to strengthen this team. There is a tight collaboration with the scientists at the Concordium Blockchain Research Center at Aarhus University. The center is also advertising several PhD and postdoc positions. https://www.iacr.org/jobs/#1717

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Maybe type filtered
by Damien Mattei 03 Jan '19

03 Jan '19

i had filtered a [Maybe Text] type to remove Nothing from the list and now i want to put the result in a [Text] type but the compiler complains about the incompatible type : (bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String)) -- remove the records having N°BD NULL let fltWDS :: [Only Text] = Prelude.filter (\(Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:282:33: error: • Couldn't match type ‘Maybe Text’ with ‘Text’ Expected type: [Only Text] Actual type: [Only (Maybe Text)] • In the expression: Prelude.filter (\ (Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS In a pattern binding: fltWDS :: [Only Text] = Prelude.filter (\ (Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS In the expression: do conn <- connect defaultConnectInfo {connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie"} (rows :: [(Text, Double)]) <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" (names :: [Only Text]) <- query_ conn "SELECT Nom FROM AngularDistance WHERE distance > 0.000278" let resLstNames = Prelude.map fromOnly names .... | 282 | let fltWDS :: [Only Text] = Prelude.filter (\(Only a) -> | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^... Failed, no modules loaded. how can id do the type conversion,now i'm sure there is no more Nothing values?

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