- 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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