- Haskell-Cafe - Haskell.org

Correct implementation of ListT besides the pipes package?
by Nicola Gigante 14 Nov '14

14 Nov '14

Hi! I understand the code on the wiki about what is wrong about the standard ListT and how to implement a correct version, but I’m wondering if there is already a package that provides such an implementation ready to use. I know the pipes package provides one, but since I don’t otherwise use pipes in my program I’d rather not depend on a big library like that only for a transformer. Is there any package that provides a correct ListT? Side question: is there a particular reason why Control.Monad.ListT doesn’t get fixed? Thank you, NIcola

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Presenting at Royal Holloway Colloquium
by Dominic Steinitz 14 Nov '14

14 Nov '14

All, I have been invited to give a TED style talk (20 mins) at the Royal Holloway Hewlett Packard Information Security Colloquium: https://www.royalholloway.ac.uk/isg/externalengagement/hpday.aspx. Now I could give an uncontroversial talk about Internet banking security using triple DES, role based access control, etc. but I am thinking about being controversial (I think that is in the spirit of TED). I’d like to say that the Information Security community is solving the wrong problems by e.g. performing security audits of code, developing tools for finding buffer overflows, etc. and what they should really be doing is encouraging development in languages that prevent this sort of behaviour. E.g. if openssl were written in Haskell, heartbleed (http://en.wikipedia.org/wiki/Heartbleed) would never have happened. What do people think about this? Are there other examples I can draw on? Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a)
by Evan Laforge 13 Nov '14

13 Nov '14

I've been using these functions lately: try :: Monad m => m (Maybe a) -> m (Maybe a) -> m (Maybe a) try action alternative = maybe alternative (return . Just) =<< action tries :: Monad m => [m (Maybe a)] -> m (Maybe a) tries = foldr try (return Nothing) It's sort of like (<|>) on Maybe, or MonadPlus, but within a monad. It seems like the sort of thing that should be already available, but hoogle shows nothing. I think 'm' has to be a monad, and I can't figure out how to generalize the Maybe to MonadPlus or Alternative. It's sort of a mirror image to another function I use a lot: justm :: Monad m => m (Maybe a) -> (a -> m (Maybe b)) -> m (Maybe b) justm op1 op2 = maybe (return Nothing) op2 =<< op1 ... which is just MaybeT for when I can't be bothered to put runMaybeT and lifts and hoists on everything. So you could say 'try' is like MaybeT with the exceptional case reversed. Is 'try' just the instantiation of some standard typeclass, or is it its own thing?

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Mechanize in Haskell or Revive Shpider?
by Cody Goodman 13 Nov '14

13 Nov '14

What would be a good way to model mechanize in Haskell? For instance the python mechanize example is quite simple: def google_search(keyword): br = mechanize.Browser() br.open("https://www.google.com") br.select_form(name="gbqf") br["q"] = keyword response = br.submit() print html_to_text(response.read()) I was thinking I could just use a StateT monad transformer with the current pages html as a cursor. Then have something like: get "http://www.google.com" >>= (\c -> c $// form (Name "gbqf") >=> input (Name "q") &| submit . modifyVal "query" "Haskell") That's kinda horrible but it's a place to start. Or would I be better off trying to revive Shpider?

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Fwd: Generating valid html
by Mike Meyer 13 Nov '14

13 Nov '14

On Thu, Nov 13, 2014 at 12:56 PM, Evan Laforge <qdunkan(a)gmail.com> wrote: > On Wed, Nov 12, 2014 at 2:44 PM, Wojtek Narczyński <wojtek(a)power.com.pl> > wrote: > > How to create a DSL for HTML generation, that would (statically) allow > only > > valid HTML to be generated? Parent - child relations, valid attributes > only. > > > > Or does such a DSL already exist, perhaps? > > WASH did that, a long time ago. They mentioned that they > intentionally relaxed the rules, since being totally correct was > annoying to actually use. There are docs here: > http://www2.informatik.uni-freiburg.de/~thiemann/WASH/ > Well, a lot of folks felt that any validation beyond "it works in my browser" was annoying to actually use. I, on the other hand, used NSGML in emacs and the actual DTD without much problem. But there were multiple DTDs, some of which also relaxed things because so many people felt the strict rules were annoying to actually use. It didn't seem to catch on, so maybe it was too heavy for its benefits. > Based on a decade of trying to get people to write valid HTML - so that it would work in browsers other than the most popular two or three on the most popular two or three platforms - most people don't care about "correct".. I've had people - including employees of the FSF, who I would expect otherwise from - tell me that their HTML was tested on some limited set of browser/platform pairs when I pointed out it was incorrect and hence broke on some obscure browser/platform combination. Or even on one of those browser/platform pairs with a configuration change. This also seems to be true in the Haskell community. The web platforms help catching the obvious problems you find testing against a browser. But - like most popular web platforms - break the tools that help with the creation of valid HTML Validating HTML against a DTD is much easier than doing so against browsers, but my clients never did it unless I set it up for them. Doing so helped find more than one really obscure bug, and on one occasion kept my clients web site working when a major browser tweaked how it handled broken code and broke a lot of web sites. I think this may be related to why ADTs are a hard sell. While they help a lot writing correct programs, but for just writing working programs not so much.

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[ANN] Haskell FFI Tutorial
by Alex Petrov 13 Nov '14

13 Nov '14

Hi Everyone, I've recently started working on Haskell FFI Tutorial [1]. The repository already contains code that explains how to: * (expressively) represent C `struct` in Haskell code * call C code from Haskell * call Haskell code from C * operate on nested `struct`s * operate on `struct` arrays * decode `unions` * read and write C fixed-length strings and Pointer-type Strings * how to import functions from, for example, stdlib I'm still working on more story-like writeups, but the first one (about how to call Haskell from C) is already available [2]. If you have any feedback / comments / questions, know how to improve it or would like to see more things covered, just ping me. Thanks [1] https://github.com/ifesdjeen/haskell-ffi-tutorial [2] https://github.com/ifesdjeen/haskell-ffi-tutorial#calling-haskell-from-c -- Alex https://twitter.com/ifesdjeen http://clojurewerkz.org/

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xs not in scope
by Roelof Wobben 13 Nov '14

13 Nov '14

Hello, Im trying to find several answers to problem1 of the 99 haskell problems. Find the last item in a list. Now im trying to find a guarded solution. So far I have this: last3::[a]-> Maybe a; last3 a | [] = Nothing | ([a]) = Just a | (_:xs) = last3 xs But I see this error messages : src/Main.hs@10:8-10:10Not in scope: xs How to solve this ? Roelof

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Using multiple versions of the Haskell Platform on Windows
by Yitzchak Gale 13 Nov '14

13 Nov '14

The win-hp-path project provides the use-hp command, which makes it easy to switch between different versions of Haskell Platform on Windows. https://github.com/ygale/win-hp-path We are using it for running cabal and GHC in command prompt windows. In particular, we can also use it in build scripts used by the build management team who are not Haskell programmers. Please let me know if you can use this in more complex dev environments, or if you have suggestions about how it could be enhanced to do that. Pull requests are welcome. Thanks, Yitz

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How is maintaining the iCalendar package?
by Alfredo Di Napoli 13 Nov '14

13 Nov '14

Hello fellow Haskellers, anybody knows who is maintaining https://github.com/tingtun/iCalendar ? I have tried to email the maintainer listed on Github, but his activity seems quite low. Posting it here hoping to get a bit more resonance. Alfredo

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open-source project looking for novice help?
by Richard Eisenberg 13 Nov '14

13 Nov '14

I'm teaching an introductory Haskell course this semester (http://cis.upenn.edu/~cis194/fall14/) and am about to assign students their final project. Is there anyone out there who would welcome getting some novice help on an open-source project? You certainly don't need to commit to accept their patch(es), but I know it would be a great experience for some of the students to contribute to the "real" world of Haskell, instead of just doing exercises. You can get an accurate summary of what we've covered by looking at the lecture titles at http://cis.upenn.edu/~cis194/fall14/lectures.html Students will be choosing projects starting next week, and will need to have them completed by Dec. 15 or so. A good project is about 20 hours of work. Thanks! Richard

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Representing record subtypes, sort of.
by Kannan Goundan 13 Nov '14

13 Nov '14

I have an API that, in a language with subtyping, would look like: class FsEntry id: String class FsFile extends FsEntry modified: Date size: Int class FsFolder extends FsEntry owner: String listFolder :: Path -> [FsEntry] createFile :: Path -> FsFile createFolder :: Path -> FsFolder (I'm assuming some way of specifying that FsEntry will only ever have those two subtypes.) How would you represent this in Haskell? My first thought was: newtype FsEntry = FsEntry FsCommon FsExtra data FsCommon = FsCommon { id: String } data FsExtra = FsFile { modified: Date, size: Int } | FsFolder { owner: String } But then I couldn't have precise return types for `writeFile` and `createFolder`. My next attempt was to use a type-parameterized top-level class: data FsCommon = FsCommon { id: String } data FsFileExtra = FsFileExtra { modified: Data, size: Int } data FsFolderExtra = FsFolderExtra { owner: String } data FsEither = FsFile FsFileExta | FsFolder FsFolderExtra newtype FsEntryBase extra = FsEntryBase FsCommon extra type FsEntry = FsEntryBase FsEither type FsFile = FsEntryBase FsFileExtra type FsFolder = FsEntryBase FsFolderExtra 1. This seems complicated. 2. I can't pass an `FsFolder` to a function expecting an `FsEntry`, but maybe that's just the nature of having subtyping and I have to give up on that (which I'm ok with). Any suggestions on how to do this? Thanks!

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IO monads, stream handles, and type inference
by Jeffrey Brown 12 Nov '14

12 Nov '14

In the thread "Precise timing <https://groups.google.com/forum/#!topic/haskell-cafe/sf8W4KBTYqQ>", in response to something ugly I was doing, Rohan Drape provided the following code: import Control.Concurrent import Control.Monad import System.IO import Sound.OSC main = withMax $ mapM_ note (cycle [1,1,2]) withMax = withTransport (openUDP "127.0.0.1" 9000) sin0 param val = sendMessage (Message "sin0" [string param,float val]) pause = liftIO . pauseThread . (* 0.1) note n = do sin0 "frq" 300 sin0 "amp" 1 pause n sin0 "amp" 0 pause n For days I have monkeyed with it, and studied the libraries it imports, and I remain sorely confused. *How can the "a" in "IO a" be a handle?* Here are two type signatures: openUDP :: String -> Int -> IO UDP withTransport :: Transport t => IO t -> Connection t a -> IO a Rohan's code makes clear that openUDP creates a handle representing the UDP connection. openUDP's type signature indicates that its output is an "IO UDP". How can I reconcile those two facts? When I read about the IO type, all sources seem to indicate that "IO a" represents a value of type "a" wrapped in an IO context. For instance, when putting Strings to the screen, one passes around "IO String" values. Until this OSC library, I had never seen the "a" in "IO a" represent a pipe; it had always represented data to be passed *through* a pipe. *Why the long signature?* When I ask for it, GHC provides the following additional type signatures: > :t pause pause :: Double -> transformers-0.3.0.0:Control.Monad.Trans.Reader.ReaderT UDP IO () What's up with that? *What type is note? (and related questions)* GHCI goes on: > :t sin0 sin0 :: (SendOSC m, Real n) => String -> n -> m () > :t note note :: Double -> transformers-0.3.0.0:Control.Monad.Trans.Reader.ReaderT UDP IO () note calls both sin0 and pause. It appears that note's type signature takes pause, but not sin0, into account, but I must be wrong about that. sin0 returns a SendOSC. pause applies liftIO to pauseThread. The result must be a SendOSC too, because sin0 and pause are both called in the same do loop. SendOSC implements these three classes: (Monad (ReaderT t io), Transport t, MonadIO io) => SendOSC (ReaderT t io) Is the liftIO that pause applies to pauseThread, then, the "default" liftIO defined in the MonadIO library? *How to read the "instances" portion of Hackage documentation?* In the Hackage documentation <https://hackage.haskell.org/package/hosc-0.13/docs/Sound-OSC-Transport-Mona…> for the SendOSC type, how should I be reading this line? (Monad (ReaderT t io), Transport t, MonadIO io) => SendOSC (ReaderT t io) I understand the middle two clauses: that io should be of type MonadIO, and t should be of type Transport. The outer two clauses, though, I don't know how to interpret. (I looked at the code <https://hackage.haskell.org/package/hosc-0.13/docs/src/Sound-OSC-Transport-…> and saw nothing that clearly corresponded to that line in the documentation.)

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A Very Simple Type Class Question
by Larry Lee 11 Nov '14

11 Nov '14

Hi I have a very simple problem. I have a class and want to define a function in that class that returns a different instance of the same class. I tried accomplishing this as follows: class A a where f :: A b => a -> b This fails however when I try to instantiate it. For example: instance A String where f x = x I get an error message that makes absolutely no sense to me: src/CSVTree.hs:12:9: Could not deduce (b ~ [Char]) from the context (A b) bound by the type signature for f :: A b => String -> b at src/CSVTree.hs:12:3-9 `b' is a rigid type variable bound by the type signature for f :: A b => String -> b at src/CSVTree.hs:12:3 In the expression: x In an equation for `f': f x = x In the instance declaration for `A String' make: *** [compile] Error 1 Can someone please explain: how I can achieve my goal; and why my code is failing; simply and in plain English. Thanks, Larry

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Hackage’s account creation down?
by Pablo Couto 10 Nov '14

10 Nov '14

Is Hackage’s account creation (at http://hackage.haskell.org/users/register-request) down? I tried it a few times since last night, without luck. This is the message I get: Server error: fd:57: hClose: resource vanished (Broken pipe) Best regards, Pablo

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New Functional Programming Job Opportunities
by Functional Jobs 10 Nov '14

10 Nov '14

Here are some functional programming job opportunities that were posted recently: Senior Software Engineer at McGraw-Hill Education http://functionaljobs.com/jobs/8760-senior-software-engineer-at-mcgraw-hill… Cheers, Sean Murphy FunctionalJobs.com

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list comprehension with multiple generator|targets
by Donn Cave 10 Nov '14

10 Nov '14

I'm guessing this isn't supported, but might be worth asking - can I extend a list comprehension like ['A' | A <- s] to multiple values? Like, data V = A | B | C pv :: [V] -> [Char] pv [] = [] pv (A:x) = 'A':(pv x) pv (B:x) = 'B':(pv x) pv (_:x) = pv x -- can that be a list comprehension, like pv s = [ 'A' | A <- s -- ?? ] thanks, Donn

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Test Framework, HUnit
by spopejoy 10 Nov '14

10 Nov '14

Hi, I started using Test Framework following the example of the lens library, specifically using Test.Framework.TH. It looks really nice, however I noticed that the TH project hasn't had any activity in 2 years ... plus the whole test framework project is very light on docs. I've been happy with straight HUnit, but I'm wanting something more structured to bring others into the project I'm working on. 1) Are there more resources on TF? 2) Is TF-TH actively maintained? Cheers, Stuart

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Can I check the length in a guarded expresion
by Roelof Wobben 10 Nov '14

10 Nov '14

Hello, I try to make a guarded solution to find the last item in a list. I thought I could do something like this | length a == 1 = a But then I see a lot of error messages. Is there another way I can check the length of a list with guards ? Roelof

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IO and exceptions
by Ömer Sinan Ağacan 10 Nov '14

10 Nov '14

Hi all, I'm having some trouble with IO functions and exceptions. When I call an IO function in Haskell, I feel very uncomfortable, because you never know what kind of exceptions your function may throw. There are a lot of cases where thrown exceptions are not specified in the documentation. What's worse, there are cases where it's impossible to catch thrown exceptions in a safe way. Example to not knowing what exceptions may be thrown: http://hackage.haskell.org/package/base-4.6.0.1/docs/Control-Concurrent-Cha… `readChan` blocks the thread until it reads something. But what happens when channel is closed while one thread is blocked to read? It's not mentioned in the documentation and it's not specified in terms of types. (e.g. by using Either, Maybe or similar) Example to not being able to catch the exception: http://hackage.haskell.org/package/network-2.6.0.2/docs/Network-Socket.html `recvFrom` blocks the thread until it reads something. However, what happens if socket is closed while `recvFrom` is blocking to read? Similar to `readChan`, we can't know this. What's worse is this: sockets have a `SO_RCVTIMEO` option, a timeout value for `recv_from` calls. What happens when we set that using `setSocketOption` in the same module and call `recvFrom` is unknown. This is really horrible, because a timeout exception is really something that you'd like to catch. Example: I have a thread that listens from a socket and handles some other events. I want to stop listening the socket after every second and handle events, then return listening the socket. But I can't know what it throws when timeout happens, so I have to catch all exceptions, which is horrible for a lot of reasons. I was wondering if I'm doing something wrong. I'm sure we can find a lot of cases like these and I feel like libraries are not designed very good for errors/exceptions. So I'd like to get some ideas/suggestions about this. Am I doing something wrong or did I misunderstand something? Should we improve the libraries for better types/error handling? Am I only one here that has problems with those functions? Thanks.

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Extensionality principles for kinds?
by Rob 09 Nov '14

09 Nov '14

Haskellers, I've waded into some pretty deep waters, but I'm hoping there's a GHC type system wizard in here who can help me. The following code fragment is extracted from something I've been working on involving some pretty fancy type-level programming. I'm using GHC 7.8.3. I've run into a problem where the typechecker refuses to believe that two types are equal; it seems to lack the necessary extensionality principles for lifted data kinds. Basically (see below for definitions) I get into a contex where I know that (TwiceLeft a ~ TwiceLeft b) and (TwiceRight a ~ TwiceRight b), but the typechecker will not believe that (a ~ b). Are there any tricks I can employ to make this code typecheck (using only safe operations)? Bonus if it doesn't add any runtime cost. Thanks, Rob Dockins ====== code follows ====== {-# LANGUAGE DataKinds #-} {-# LANGUAGE PolyKinds #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE TypeOperators #-} {-# LANGUAGE TypeFamilies #-} import Data.Type.Equality -- lifted kind (Twice k) consists of a pair of things of kind k data Twice k = TwiceVal k k -- type family accessor functions for the Twice kind type family TwiceLeft (x::Twice k) :: k where TwiceLeft (TwiceVal a b) = a type family TwiceRight (x::Twice k) :: k where TwiceRight (TwiceVal a b) = b -- Double is already taken, use Dbl instead... data Dbl (f :: k -> *) (tv :: Twice k) = Dbl (f (TwiceLeft tv)) (f (TwiceRight tv)) -- This function doesn't typecheck... doubleEq :: forall (f:: k -> *) . (forall (a::k) (b::k). f a -> f b -> Maybe (a :~: b)) -> (forall (a::Twice k) (b::Twice k). Dbl f a -> Dbl f b -> Maybe (a :~: b)) doubleEq teq (Dbl x y) (Dbl z w) = case teq x z of Nothing -> Nothing Just Refl -> case teq y w of Nothing -> Nothing Just Refl -> Just Refl

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`cabal repl` problems
by Ömer Sinan Ağacan 08 Nov '14

08 Nov '14

Hi all, I have a Cabal project that has library, executable and several test parts. I'd like to be able to load one of the test parts into GHCi using `cabal repl`. However, I'm having some problems: (To be more specific, I have these parts in my Cabal file: - library - executable myapp - test-suite test - test-suite doctests) - `cabal repl test` tries to load test suite into GHCi but it doesn't ask me which test suite I want to load. For example, in my Cabal file I have `test-suite test` and `test-suite doctests`. `cabal test doctests` runs only doctests but I can't use similar command for repl, like `cabal repl doctests`. - `cabal repl test` tries to load the library, but using wrong set of dependencies. It uses `build-depends` field of `test-suite test` but it loads `library` sources. Note that `hs-source-dirs` field of `test-suite test` and `library` are different, and I have my library in `test-suite test` as a dependency. So the problem is that it tries to load `library` part using the source, instead of compiled lib. Any ideas about those? (Note: `cabal test`, `cabal install`, `cabal run` etc. work fine, my Cabal file is working) Thanks.

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why is haddock so slow
by Johannes Waldmann 07 Nov '14

07 Nov '14

I have a larg-ish project (800 modules) and these ghc compilation times: cabal install : 19 min cabal install --disable-doc : 13 min the compilation includes the profiling build. That is, running haddock takes the same 6 minutes as running the compiler (with optimization) I also built with "optimization: False library-profiling: False" and this takes 3 min. I understand that haddockification goes via the compiler's front-end to resolve names and infer types. This takes 3 min (as the non-optimizing compiler run shows). What does haddock do in the other 3 min? What could I do to help analyze this? - J.W. (ghc-7.8.3, haddock 2.15.0, rotating disk (not SSD), Fedora 20, x86_64)

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Travis builds failing with spurious errors
by Francesco Mazzoli 07 Nov '14

07 Nov '14

Hi all, Apologies if this is not the right space to ask this question. We set up CI for Agda with Travis: <https://travis-ci.org/agda/agda>. The problem is that we're having trouble with older versions of GHC. For example, with GHC 7.4.2, the build complains that ScopedTypeVariables is not enabled, even if it is. You can check the build failure here: <https://travis-ci.org/agda/agda/jobs/39834738#L795>, and the relevant file here: <https://github.com/agda/agda/blob/9a82afaf7657a97e85e7e31ea23f6386f99acd35/…>. As you can see from the pragmas ScopedTypeVariables is enabled, and in fact when building locally I have no problems. We also got some spurious messages regarding syntax. Has anybody encountered similar problems? I don't know if it's something depending on Travis or some configuration mistakes, but I'm a bit at loss on how to research this. Moving the OPTIONS_GHC pragma after the LANGUAGE pragmas seems to fix the issue but I'd like to know what's going on. Thanks, Francesco

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Fwd: Increasing Haskell modularity
by Gesh hseG 07 Nov '14

07 Nov '14

Sorry, should have used reply-list. To clarify, under the proposal, the associated family instances would also be locally unique, with local instances overriding global instances. Gesh ---------- Forwarded message ---------- From: Gesh <gesh(a)gesh.uni.cx> Date: Wed, Oct 1, 2014 at 5:30 PM Subject: Re: [Haskell-cafe] Increasing Haskell modularity To: Richard Eisenberg <eir(a)cis.upenn.edu> On 10/1/2014 5:01 PM, Richard Eisenberg wrote: > > * I'm not sure exactly what you mean by "adopt [Oleg's] proposal". In Section 6.1, Oleg discusses how to keep principal typing while allowing local instances. He then suggests making this part of the semantics of local instances. > It seems the first part of your proposal just says > "Haskellers can no longer expect class instance coherence." > As such, this is a drastic change that would require all Haskell > programs to be inspected to see if they crucially rely on coherence No. Coherence is preserved, since you always use the unique in-scope dictionary. What *is* lost, however, is global uniqueness, which means that code that assumes that the same dictionary is used for the same type across calls to functions must be modified. Arguably, that code is semantically incorrect to begin with, but that's neither here nor there. > * As for associated vs. non-associated type families: > There is essentially no difference between associated > and non-associated type families, save concrete syntax. > If the Haskell house were on fire and we needed to eliminate > one of these forms, I would agree to it without much trouble, > because the two forms are equally expressive. > > > But, eliminating non-associated type families wouldn't solve > your problem at all: while class instance incoherence causes > wonky runtime behavior, associated type instance incoherence > breaks the type system. If I understand your proposal correctly, > users could implement unsafeCoerce through clever hackery around > associated type instance incoherence. This seems to be against > the Haskell tradition of strong typing. To clarify my proposal: Associated type families have the added guarantee over non-associated families that functions will be defined that allow one to work with them. As mentioned above, typeclass coherence is preserved under the proposal, so I see no danger of family incoherence. However, I'm inexperienced with these kinds of proposals, and the concern that unsafeCoerce will be possible with this proposal is worrying. Could you please construct a case implementing unsafeCoerce? Thanks, Gesh

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ICFP 2015: Call for Papers
by David Van Horn 07 Nov '14

07 Nov '14

===================================================================== 20th ACM SIGPLAN International Conference on Functional Programming ICFP 2015 Vancouver, Canada, August 31 - September 2, 2015 http://www.icfpconference.org/icfp2015 ===================================================================== Important Dates ~~~~~~~~~~~~~~~ Submissions due: Friday, February 27 2015, 23:59 UTC-11 Author response: Tuesday, April 21, 2015 through Thursday, 23 April, 2015 Notification: Friday, May 1, 2015 Final copy due: Friday, June 12, 2015 Scope ~~~~~ ICFP 2015 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. * Functional Pearls: elegant, instructive, and fun essays on functional programming. * Experience Reports: short papers that provide evidence that functional programming really works or describe obstacles that have kept it from working. If you are concerned about the appropriateness of some topic, do not hesitate to contact the program chair. Abbreviated instructions for authors ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * By Friday, 27 February 2015, 23:59 UTC-11 (anywhere in the world), submit a full paper of at most 12 pages (6 pages for an Experience Report) in standard ACM conference format, including bibliography, figures, and appendices. The deadlines will be strictly enforced and papers exceeding the page limits will be summarily rejected. * Authors have the option to attach supplementary material to a submission, on the understanding that reviewers may choose not to look at it. * Each submission must adhere to SIGPLAN's republication policy, as explained on the web at http://www.sigplan.org/Resources/Policies/Republication * Authors of resubmitted (but previously rejected) papers 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 program chair 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. Overall, a submission will be evaluated according to its relevance, correctness, significance, originality, and clarity. It 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. Functional Pearls and Experience Reports are separate categories of papers that need not report original research results and must be marked as such at the time of submission. Detailed guidelines on both categories are on the conference web site. Proceedings will be published by ACM Press. Authors of accepted submissions will have a choice of one of three ways to manage their publication rights. These choices are described at http://authors.acm.org/main.html Presentations will be videotaped and released online if the presenter consents. The proceedings will be freely available for download from the ACM Digital Library from one week before the start of the conference until two weeks after the conference. Formatting: Submissions must be in PDF format printable in black and white on US Letter sized paper and interpretable by Ghostscript. Papers must adhere to the standard ACM conference format: two columns, nine-point font on a ten-point baseline, with columns 20pc (3.33in) wide and 54pc (9in) tall, with a column gutter of 2pc (0.33in). A suitable document template for LaTeX is available at http://www.acm.org/sigs/sigplan/authorInformation.htm Submission: Submissions will be accepted on the web using a link that will be posted at http://icfpconference.org/icfp2015/ Improved versions of a paper may be submitted at any point before the submission deadline using the same web interface. Author response: Authors will have a 72-hour period, starting at 0:00 UTC on Tuesday, 21 April 2015, to read reviews and respond to them. 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 the definitive version of ACM article should reduce user confusion over article versioning. After your article has been published and assigned to your ACM Author Profile page, please visit http://www.acm.org/publications/acm-author-izer-service to learn how to create your links for free downloads from the ACM DL. Publication date: The official publication date of accepted papers is the date the proceedings 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. General Chair: Kathleen Fisher Tufts University (USA) Program Chair: John Reppy University of Chicago (USA) Program Committee: Amal Ahmed Northeastern University (USA) Jean-Philippe Bernardy Chalmers University of Technology (Sweden) Matthias Blume Google (USA) William Byrd University of Utah (USA) Andy Gill University of Kansas (USA) Neal Glew Google (USA) Fritz Henglein University of Copenhagen (Denmark) Gabriele Keller University of New South Wales and NICTA (Australia) Andrew Kennedy Microsoft Research Cambridge (UK) Neelakantan Krishnaswami Birmingham University (UK) Daan Leijen Microsoft Research Redmond (USA) Keiko Nakata Institute of Cybernetics at Tallinn University of Technology (Estonia) Mike Rainey INRIA Rocquencourt (France) Andreas Rossberg Google (Germany) Manuel Serrano INRIA Sophia Antipolis (France) Simon Thompson University of Kent (UK) David Van Horn University of Maryland (USA) Stephanie Weirich University of Pennsylvania (USA)

1 0

Haskell.org access problems
by Henk-Jan van Tuyl 07 Nov '14

07 Nov '14

L.S., When I try to browse haskell.org with Opera, I get the following message: --- Unable to complete secure transaction You tried to access the address http://www.haskell.org/, which is currently unavailable. Please make sure that the web address (URL) is correctly spelled and punctuated, then try reloading the page. Secure connection: fatal error (1066) https://www.haskell.org/ Unable to verify the website's identity (OCSP error). The response from the online certificate validation (OCSP) server was too old. --- The browser doesn't allow me in. Another thing that I get with Firefox: a captcha, probably because I am using the TOR network and the site is buffered by CloudFlare. Opera does not handle these captchas properly. Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

1 0

Cross ghc-7.8.3 for PowerPC build fails
by Jon Schneider 06 Nov '14

06 Nov '14

I got my ARM (v5) ghc cross compiler built the other day by making sure I had only the latest LLVM and Haskell Platform host packages, tweaking settings and commenting out haskeline and terminfo in the top level ghc.mk . There's something broken about terminfo but life's too short to worry about since it's not needed. The ARM cross ghc seems to product viable executables for simple code. Now a sister product is a PowerPC (MPC854E) and the ABI is gnuspe. In my PATH are (only) clangs and llcs from http://llvm.org/releases/3.4.2/clang+llvm-3.4.2-x86_64-unknown-ubuntu12.04.… and ghc from https://www.haskell.org/platform/download/2014.2.0.0/haskell-platform-2014.… Because of the none vendor I didn't seem able to put in the target I configured like this. ./configure --target=powerpc-linux-gnuspe --with-gcc=powerpc-none-linux-gnuspe-gcc --with-nm=powerpc-none-linux-gnuspe-nm --with-ld=powerpc-none-linux-gnuspe-ld --with-ar=powerpc-none-linux-gnuspe-ar --with-ranlib=powerpc-none-linux-gnuspe-ranlib This fails as follows. It seems to have generated x86 opcodes including movq unless it's something to do with GNU assembly trying to make everything have the same apparent instruction set then again %rcx and %esi are mentioned. ====== cp libffi/build/inst/lib/libffi.a rts/dist/build/libCffi_thr_l.a "inplace/bin/ghc-stage1" -hisuf hi -osuf o -hcsuf hc -static -H64m -O0 -package-name ghc-prim-0.3.1.0 -hide-all-packages -i -ilibraries/ghc-prim/. -ilibraries/ghc-prim/dist-install/build -ilibraries/ghc-prim/dist-install/build/autogen -Ilibraries/ghc-prim/dist-install/build -Ilibraries/ghc-prim/dist-install/build/autogen -Ilibraries/ghc-prim/. -optP-include -optPlibraries/ghc-prim/dist-install/build/autogen/cabal_macros.h -package rts-1.0 -package-name ghc-prim -XHaskell2010 -O -fllvm -no-user-package-db -rtsopts -odir libraries/ghc-prim/dist-install/build -hidir libraries/ghc-prim/dist-install/build -stubdir libraries/ghc-prim/dist-install/build -c libraries/ghc-prim/./GHC/Types.hs -o libraries/ghc-prim/dist-install/build/GHC/Types.o /tmp/ghc30000_0/ghc30000_5.s: Assembler messages: /tmp/ghc30000_0/ghc30000_5.s:8:0: Error: Unrecognized opcode: `movl' and so on with opcodes such as jbe, leal, jmpq and also "junk at end of line: `rcx),%esi'" ====== in libffi/build/powerpc-unknown-linux-gnuspe there's a sensible config.log with x86_64 in the build variables and powerpc in the host and target ones. How do I go about making this work ? Thanks, Jon

2 4

addCatch in conduit
by Yitzchak Gale 06 Nov '14

06 Nov '14

In getting the dtd library to compile with recent versions of conduit (yes, I know that it's deprecated and Michael longer supports it, but we still need it), we came across the following bit of code: -- (snip) -- CI.ConduitM $ addCatch $ CI.unConduitM src0 where -- (snip) -- addCatch :: (MonadThrow m, MonadBaseControl IO m) => CI.Pipe l i o u m r -> CI.Pipe l i o u m r addCatch (CI.HaveOutput src close x) = CI.HaveOutput (addCatch src) (addCatch' close) x addCatch (CI.NeedInput p c) = CI.NeedInput (addCatch . p) (addCatch . c) addCatch (CI.Done r) = CI.Done r addCatch (CI.PipeM msrc) = CI.PipeM (addCatch' $ liftM addCatch msrc) addCatch (CI.Leftover p i) = CI.Leftover (addCatch p) i addCatch' m = m `Lifted.catch` throw rr We adapted it to the new ConduitM type simply by changing the first line to: CI.ConduitM $ addCatch . CI.unConduitM src0 Not bad, a diff of exactly one character. It compiles and seems to work. Does this sound reasonable? Obviously, we would love to get rid of this use of conduit internals. addCatch seems like a general operation, not specific to this library. Is there a way to do this in modern conduit without dipping into internals? If not - can we propose to add it? Thanks, Yitz

2 1

Tutorial for creating haskell apps in the browser easy
by Alberto G. Corona 05 Nov '14

05 Nov '14

Hi Just in case you don´t noticed it, this is the first tutorial for creating Browser applications in Haskell for non haskellers. http://www.airpair.com/haskell/posts/haskell-tutorial-introduction-to-web-a… -- Alberto.

2 3

installing haskell on ubuntu using apt-get??
by Vasili I. Galchin 05 Nov '14

05 Nov '14

My config: runghc 7.4.1 3.2.0-61-generic #93-Ubuntu I installed Haskell using Ubuntu's "apt-get install" ======================================================== Please see results below ... (should I instead install Haskell from haskell.org??) vasili@vasili-VirtualBox:~/biolib$ cabal configure Resolving dependencies... Configuring bio-0.5.2... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package text-1.2.0.0 requires bytestring-0.10.4.0 package tagsoup-0.13.3 requires bytestring-0.10.4.0 package parsec-3.1.7 requires bytestring-0.10.4.0 package bio-0.5.2 requires bytestring-0.10.4.0 package binary-0.4.5 requires bytestring-0.10.4.0 package unix-2.5.1.0 requires bytestring-0.9.2.1 vasili@vasili-VirtualBox:~/biolib$

4 3

Debugging Wreq/http-client https requests
by Cody Goodman 05 Nov '14

05 Nov '14

I have a program that is POST'ing some data to a remote server. However, some part of the data I'm sending is wrong I believe. If this were plain http without encryption, wireshark would allow me to see the exact data being sent over the wire. However, with https it is encrypted. Is there an inbuilt way to debug requests sent by wreq or more likely, a way to output debug info for http-client? Alternatively, is there a way to use NSS support with either of these libraries: http://security.stackexchange.com/questions/35639/decrypting-tls-in-wiresha… Thanks in advance to all for your time.

4 5

[ANN] Simulation library aivika-3.0
by David Sorokin 04 Nov '14

04 Nov '14

Hi Cafe, I’m glad to announce a new release of my simulation library Aivika written in Haskell. It is focused on DES and System Dynamics. I strongly recommend to upgrade to this version for everyone who uses my library. It contains an important fix in that how the TimingStats statistics is gathered. A more simple SamplingStats is usually used. But in the recent versions I started using more widely TimingStats too. This is a kind of (immutable) statistics which is bound up to the simulation time. For example, the queue size statistics is collected as TimingStats, while other queue properties are usually collected as SamplingStats. The SamplingStats is used to be working. But I felt that there was something wrong with TimingStats. Now TimingStats is finally fixed. As a consequence, the queueRate function gives us a result, which suits very well to Little’s rule with good precision in the examples included in the distributive. So, please upgrade to a new version of Aivika (and related libraries) as possible. Besides, I added debugging facilities to the library. These are functions traceEvent, traceProcess, traceStream and so on. They allow tracing the behavior of the simulation model in time. Finally, the Aivika simulation library can be definitely translated to JavaScript with help of the Haste compiler and it works fine! Only I had to use random numbers different from that one, which is specified with help of the SimpleGenerator data constructor. Please read the notes [2] on the wiki page of my project on GitHub. Thanks, David [1] http://hackage.haskell.org/package/aivika <http://hackage.haskell.org/package/aivika> [2] https://github.com/dsorokin/aivika/wiki/Compiling-to-JavaScript

1 0

Re: [Haskell-cafe] How to use randomized algorithm within the implementation of pure data structures?
by Hiromi ISHII 04 Nov '14

04 Nov '14

Hi there, Thank you in advance for many people suggesting solution for this problem! On 2014/11/01 20:59, Travis Cardwell <travis.cardwell(a)extellisys.com> wrote: > If it works with the algorithm, you could use a pseudo-random number > generator with a fixed seed. For example, here is a program to estimate > the value of π (purely) using a Monte Carlo simulation: I got it. But, in my case, fixing initial seed might cause inefficiency, so I can't take this way in this case. By the way, this approach seems works well for other cases which is not so quality sensitive. On 2014/11/02 1:02, Carter Schonwald <carter.schonwald(a)gmail.com> wrote: > Hrm, you could make a num instance of a newtype wrapped state monad thats threading around your math! > > newtype MyNum = MN (State StdGen TheNumberType) This is almost the same as what I mean by "CPS-ing". We can omit generator argument by this approach, but we have to pass generator whenever we want to get the result or inspect the intermediate value. On 2014/11/02 1:15, Jun Inoue <jun.lambda(a)gmail.com> wrote: > Just an idea here, but would implicit-params work? It only gives you > Reader-monad capabilities, but you can always split random generators. > There might be repercussions for the quality of the generated numbers, > though, for which I have no idea. I think this is virtually the same as Travis's approach, because it returns same seed whenever we split the global value. And, yes, this causes repercussions for the random quality, and not suitable for my case. Perhaps we can parametrize `IORef StdGen` instead of `StdGen` so that we can change the state, but we have to call `unsafePerformIO` internally whenever accessing random generator, and then this appoarch became almost the same as my initial approach. By the way, we can control initial value with this implicit parameter approach, this might improve the random quality slightly than splitting global generator beforewards. -- Hiromi ISHII konn.jinro(a)gmail.com

4 5

Why does this work: 'instance Num (Show a) where'
by Tom Ellis 04 Nov '14

04 Nov '14

This is accepted by GHC 7.6.3. Is this a bug? What does it mean? instance Num (Show a) where

3 3

Obscure, important concepts
by Jeffrey Brown 04 Nov '14

04 Nov '14

What concepts relatively unique to Haskell are powerful, widely applicable, and not mentioned in most introductory texts? (By "relatively unique to Haskell" I mean maybe they're part of Lisp or Scheme, but not, say, Java.) Some of the concepts important to Haskell, such as lambda expressions and recursion, are also present in more popular languages. Many others, though, either have no equivalent in (most) other languages, or else are very unlike the equivalent: types, classes and kinds higher-order functions evaluation partial application tail recursion laziness currying pattern matching application and composition operations contexts functors, applicatives, monads monad transformers * lenses* * :def macros* * arrows* * continuation passing style* * software transactional memory* Most of those topics appear to be covered by introductory texts, but the last five are not. I found each of them by accident, and each kind of blew my mind. They all strike me as powerful, widely applicable, and obscure. Are there others?

3 2

Monads and scope
by Jeffrey Brown 04 Nov '14

04 Nov '14

*Question 1: Are monad stacks "transparent" -- that is, can one use a layer in the middle of a monad stack without thinking about other layers?* In learning about how to stack multiple monads (or more literally, how to stack monad transformers on top of a monad), I was expecting that in order to reach into the middle of the stack, one would have to stay aware of the order in which the stack was created, and unwrap outer transformers to get there. That appears to be false. For instance, the program UglyStack.hs in Chapter 18 (Monad Transformers) of Real World Haskell <http://book.realworldhaskell.org/read/monad-transformers.html> includes a function called "constrainedCount" that, in a ReaderT (StateT IO) stack, calls "ask", "get" and some IO functions as if no layer of the stack interferes with any of the others. Is that true in general? That is, if I build on top of a monad M a stack of monad transformers T1 T2 T3 ... TN M, and each of the Tn have different functions associated with them, call I call those functions without even remembering the order in which the transformers were stacked? Does the order of the stack constrain the programmer in how they chain execution functions like runReader, but not otherwise? *Question 2: Does a monad's scope extend maximally upward through the call stack?* All the programs I have seen that use monads keep them at the top scope: The "main" function will use them, and perhaps functions it calls, but if one descends low enough into the call stack, one escapes from the monad context into pure functional code. Can one ever escape a monad in the reverse direction? Stated differently: Monadic code can call pure code. Can pure code ever call monadic code?

4 3

[ANN]json-autotype: Tool generating JSON parsers and type declarations from example documents...
by Michal J Gajda 04 Nov '14

04 Nov '14

Dear Fellow Haskellers, I've just published a beta version of a tool facilitating generation of Haskell type declarations from example JSON documents. It is supposed to simplify implementing large JSON-based interfaces, without necessity of writing any boilerplate code. USAGE: ====== After installing with `cabal install json-autotype`, you might generate stub code for the parser: $ json-autotype input.json -o MyFormat.hs Then you might test the parser by running it on an input file: $ runghc MyFormat.hs input.json If everything is correct, then feel free to inspect the data structure generated automatically for you! The goal of this program is to make it easy for users of big JSON APIs to generate entries from example data. Occasionally you might find a valid JSON for which `json-autotype` doesn't generate a correct parser. You may either edit the resulting file _and_ send it to the author as a test case for future release. HOW IT WORKS: ============== The program uses union type unification to trim output declarations. The types of same attribute tag and similar attribute set, are automatically unified using recognition by attribute set matching. (This option can be optionally turned off, or a set of unified types may be given explicitly.) Either alternatives is used to assure that all JSON inputs seen in example input file are handled correctly. Details on official releases are on Hackage [1] 1. https://hackage.haskell.org/package/json-autotype Patches and suggestions are most welcome. -- Best regards Michał J. Gajda

3 3

Inlinable vs inline
by Charles Durham 04 Nov '14

04 Nov '14

Hi, I wanted to get some clarifications on how inlinable and inline behave. One particular case is where you have a module exporting some functions with helper functions that are not exported. You go ahead and add inlinable to the exported functions so that the call site can specialize on them. Is there any penalty to adding inline vs inlinable to the helper functions? For example, are there cases where a helper function will not inline and thus can not be specailized at the call site and it would be better to declare the helper function inlinable? I would be interested to hear any nuances related to the differences and practical consequences of these pragmas. Thanks! Charlie Durham

1 0

Code review request: Library and executable to read USB temperature device
by Bram Neijt 03 Nov '14

03 Nov '14

Dear reader, I have written a small executable and library to read values from a USB device and would like someone with more knowledge then hlint to review my code. If you find the time to read https://github.com/bneijt/temper/blob/master/src/lib/Temper.hs please consider placing comments/issues on Github or mailing the list or me directly with your comments. I also have the following doubts about the code: - It has some bit-shifting code on line 18 to 20. Is there a nicer "unpack two bytes to an integer" method? - I'm using test-framework with test-framework-hunit but it feels clunky, is hspec a more common approach to testing? - I'm ignoring a lot of return values between lines 61 to 70. In a language like Python or Java I would add assert statements or checks with exceptions there. How should I go about that in Haskell? Greetings, Bram

1 0

New Functional Programming Job Opportunities
by Functional Jobs 03 Nov '14

03 Nov '14

Here are some functional programming job opportunities that were posted recently: Senior Software Engineer at Soda Software Labs http://functionaljobs.com/jobs/8758-senior-software-engineer-at-soda-softwa… Cheers, Sean Murphy FunctionalJobs.com

1 0

Pattern variables in Template Haskell
by Alejandro Serrano Mena 03 Nov '14

03 Nov '14

Dear Haskell-cafe, I'm trying to use Template Haskell to generate patterns in Haskell code, but after a couple of days of trying, I have not come with a solution. I hope somebody could help me. My problem is basically that I want something like: example :: Maybe a -> a example $(j "x") = x to work correctly. The idea of this simple quoter is to generate code similar to: example :: Maybe a -> a example (Just x_123) = x_123 that is, to create the "Just" pattern, and to make "x" bind to the same element. Right now my code reads: j :: String -> Q Pat j s = return (ConP 'Just [VarP (mkName s)]) which generated a new variable "x" each time, thus unable to match with the "x" in the RHS. I have also tried with lookupValueName. Note that my problem is a bit more complicated, with variable names coming from a different source. However, I think that if I solve this problem where I can make variables in a pattern and in an expression match, I should be able to use them in my complete solution. Thanks in advance.

2 2

Nomyx (Beta 7): win 10000 Blings!
by Corentin Dupont 03 Nov '14

03 Nov '14

Join us on Nomyx: www.nomyx.net It's a unique game where you can change the rules of the game, while playing it. Log in, and you'll understand better what the title means (check rule 44 proposed by me in the game)! Cheers, Corentin

2 2

ANN: Released new version of Haskell plugin for IntelliJ
by Rik van der Kleij 03 Nov '14

03 Nov '14

Hi everyone, I'm glad to announce version 0.2. It's available in Jetbrains plugin repository. Besides improvements and minor changes, version 0.2 has support for HLint inspection and quick fixes based on HLint suggestions. For more information, see change notes. Complete feature list is now: - Syntax highlighting (which can be customized); - Error/warning highlighting; - Find Usages of identifiers; - Resolving references of identifiers (also to library code if library source code is added to project and resolves inside import declaration); - Code completion by resolving references; - Renaming variables (which first shows preview so refactoring scope can be adjusted); - View type info from (selected) expression; - View expression info; - View quick documentation; - View quick definition; - Structure view; - Navigate to declaration (called `Class` in IntelliJ menu); - Navigate to identifier (called `Symbol` in IntelliJ menu); - Code completion by looking to import declarations; - Simple form of code formatting; - Inspection by HLint; - Quick fixes for HLint suggestions; - View error, action to view formatted message from ghc-mod. Especially useful in case message consists of multiple lines (Ctrl-F10, Meta-F10 on Mac OSX); See for more information and getting started: https://github.com/rikvdkleij/intellij-haskell Any feedback is welcome! Regards, Rik

1 0

<|> that short-circuits in IO ?
by Semen Trygubenko / Семен Тригубен ко 02 Nov '14

02 Nov '14

Dear Haskell-Cafe, Is there a way to get Control.Applicative.<|> to short-circuit when combining two IO actions? E.g. > let a = putStrLn "A" >> return (Left "hehe") > let b = putStrLn "B" >> return (Right 42) > Prelude> liftA2 (<|>) a b A B Right 42 > liftA2 (<|>) b a B A Right 42 (In the latter case I don't want A in the output…) Many thanks for your help, Semen -- Семен Тригубенко http://trygub.com

4 6

How to use randomized algorithm within the implementation of pure data structures?
by Hiromi ISHII 01 Nov '14

01 Nov '14

Hi cafe, I'm currently implementing the data structure representing algebraic numbers. Implementing algebraic number arithmetic requires polynomial factorisation. Pure algorithm for factoring (Berlekamp's algorithm) is more expensive than the randomized one (Cantor-Zassenhaus algorithm) , so I want to use the latter algorithm. Since C-Z algorithm is a randomized algorithm, we have to have an access for random number generator when calculating algebraic number arithmetics e.g. writing Num instance for algebraic numbers. Here is the problem: how to pass-around random number generator throughout pure computaion? I think one immediate solution is to create global state with `newStdGen` and `unsafePerformIO` like below: ``` randSrc :: IORef StdGen randSrc = unsafePerformIO $ newIORef =<< newStdGen {-# NOINLINE randSrc #-} getRand :: IO Int getRand = atomicModifyIORef' randSrc (swap . next) -- We can probably use the following function to avoid calling -- `unsafePerformIO` whenever calling 'getRand`, -- assuming some optimization flag enabled: getRandUnsafe :: Int getRandUnsafe = unsafePerformIO getRand {-# NOINLINE getRandUnsafe #-} {-# RULES "getRandUnsafe" getRandUnsafe = unsafePerformIO getRand #-} ``` But this hack seems rather dirty and unsafe. Is there any workaround to achieve the same thing? -- Hiromi ISHII konn.jinro(a)gmail.com

4 4

Hackage inconsistent? (pandoc.cabal in 1.13.1)
by Magnus Therning 31 Oct '14

31 Oct '14

It seems pandoc 1.13.1 package on hackage has gotten into an insonsistent state. I just noticed this strange situation with the dependency on http-client: pandoc.cabal at [1] has if flag(https) Build-Depends: http-client >= 0.3.2 && < 0.5, http-client-tls >= 0.2 && < 0.3, http-types >= 0.8 && < 0.9 cpp-options: -DHTTP_CLIENT pandoc.cabal found in [2] has if flag(https) Build-Depends: http-client >= 0.3.2 && < 0.4, http-client-tls >= 0.2 && < 0.3, http-types >= 0.8 && < 0.9 cpp-options: -DHTTP_CLIENT How can something like this even happen? /M [1]: http://hackage.haskell.org/package/pandoc-1.13.1/pandoc.cabal [2]: http://hackage.haskell.org/package/pandoc-1.13.1/src/pandoc.cabal -- Magnus Therning OpenPGP: 0xAB4DFBA4 email: magnus(a)therning.org jabber: magnus(a)therning.org twitter: magthe http://therning.org/magnus Any fool can write code that a computer can understand. Good programmers write code that humans can understand. -- Martin Fowler

14 28

Getting the haddocks back (was: documentation build failing in hackage?)
by Rogan Creswick 30 Oct '14

30 Oct '14

On Sun, Oct 12, 2014 at 4:50 AM, Mateusz Kowalczyk <fuuzetsu(a)fuuzetsu.co.uk> wrote: > > The biggest problem is that there is no official statement as to why the > builds stopped and an estimate of when they are coming back. > I agree! My understanding (which is at /least/ 2nd or 3rd hand) is that the doc builds were turned off intentionally because it was a security issue, and that they are unlikely to come back. Now, assuming that is the case, how can we solve the "there is no documentation" issue? Some ideas to kick-start discussion: - Provide an option to include haddocks in the sdist bundle, and extract them on Hackage for display. - Add a 'cabal uploadHaddock' - Run all haddock builds in a VM/docker container / etc.. that mitigates the security concerns. - ??? --Rogan

10 18

Haskell Weekly News: Issue 311
by Daniel Santa Cruz 30 Oct '14

30 Oct '14

Welcome to issue 311 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers from October 19 to 25, 2014 Quotes of the Week * trap_exit: isn't darcs this haskell powered quantum physics simulator that happens to also keep revisions of files? Top Reddit Stories * "GHC uses GMP, and its performance is nothing short of extraordinary. GHC is clearly tuned to call into GMP with extremely low overheads." Domain: wilfred.me.uk, Score: 79, Comments: 26 Original: [1] http://goo.gl/n6tXA3 On Reddit: [2] http://goo.gl/zTH1v5 * Video games in Haskell: Chips Challenge demo Domain: vimeo.com, Score: 73, Comments: 22 Original: [3] http://goo.gl/EyUQ6t On Reddit: [4] http://goo.gl/8Lrj3I * HLint now spots bad unsafePerformIO Domain: neilmitchell.blogspot.it, Score: 56, Comments: 29 Original: [5] http://goo.gl/sxFywP On Reddit: [6] http://goo.gl/nA0rvs * Seemingly impossible functional programs Domain: math.andrej.com, Score: 56, Comments: 15 Original: [7] http://goo.gl/gtXO9 On Reddit: [8] http://goo.gl/XdTGd7 * New blog post: "Quasi-quoting DSLs for free" Domain: well-typed.com, Score: 55, Comments: 2 Original: [9] http://goo.gl/QbblTr On Reddit: [10] http://goo.gl/hv43hU * Flocking behaviour simulation written using helm. Source in comments. Domain: gfycat.com, Score: 46, Comments: 9 Original: [11] http://goo.gl/MegczW On Reddit: [12] http://goo.gl/aKma9A * Finished reading Learn You a Haskell, please recommend me book number 2: Domain: self.haskell, Score: 41, Comments: 24 Original: [13] http://goo.gl/uQiplX On Reddit: [14] http://goo.gl/uQiplX * Simple Haskell IRC client in "two lines of code" Domain: self.haskell, Score: 40, Comments: 62 Original: [15] http://goo.gl/7SQpZL On Reddit: [16] http://goo.gl/7SQpZL * GPU Programming in Haskell (video) Domain: looprecur.com, Score: 36, Comments: 0 Original: [17] http://goo.gl/7N0rgb On Reddit: [18] http://goo.gl/kjuFle * Are there ANY good game engines for haskell? Domain: self.haskell, Score: 33, Comments: 38 Original: [19] http://goo.gl/6ugfSM On Reddit: [20] http://goo.gl/6ugfSM * Darcs turns 12 Domain: hub.darcs.net, Score: 30, Comments: 8 Original: [21] http://goo.gl/tnNsyR On Reddit: [22] http://goo.gl/DXIYyW Top StackOverflow Questions * Dependent Types: How is the dependent pair type analogous to a disjoint union? votes: 20, answers: 6 Read on SO: [23] http://goo.gl/HOSXJO * How should I read this GHC Core “proof”? votes: 11, answers: 0 Read on SO: [24] http://goo.gl/3btTZT * more efficient type-level computations using type families? votes: 11, answers: 0 Read on SO: [25] http://goo.gl/LHYecG * How to make fmap rewrite rules fire? votes: 10, answers: 1 Read on SO: [26] http://goo.gl/JrjnJH * A workaround for the “Template Haskell + C” bug? votes: 8, answers: 1 Read on SO: [27] http://goo.gl/q9sIHZ * Transform a -> a -> Maybe a to take any combination of a's and Maybe a's votes: 8, answers: 2 Read on SO: [28] http://goo.gl/AS4kC0 * Making numeric functions an instance of Num? votes: 7, answers: 2 Read on SO: [29] http://goo.gl/6uNuWB * Inverting a Type Family votes: 7, answers: 2 Read on SO: [30] http://goo.gl/18XHro * FRP frameworks and IO votes: 7, answers: 1 Read on SO: [31] http://goo.gl/BbEWOM Until next time, [32]+Daniel Santa Cruz References 1. http://www.wilfred.me.uk/blog/2014/10/20/the-fastest-bigint-in-the-west/ 2. http://www.reddit.com/r/haskell/comments/2jt37u/ghc_uses_gmp_and_its_perfor… 3. https://vimeo.com/109613639 4. http://www.reddit.com/r/haskell/comments/2jwwlj/video_games_in_haskell_chip… 5. http://neilmitchell.blogspot.it/2014/10/hlint-now-spots-bad-unsafeperformio… 6. http://www.reddit.com/r/haskell/comments/2jsa4c/hlint_now_spots_bad_unsafep… 7. http://math.andrej.com/2007/09/28/seemingly-impossible-functional-programs/ 8. http://www.reddit.com/r/haskell/comments/2k0xg2/seemingly_impossible_functi… 9. http://www.well-typed.com/blog/2014/10/quasi-quoting-dsls/ 10. http://www.reddit.com/r/haskell/comments/2jowjc/new_blog_post_quasiquoting_… 11. http://gfycat.com/WearyAnguishedIggypops 12. http://www.reddit.com/r/haskell/comments/2k51kk/flocking_behaviour_simulati… 13. http://www.reddit.com/r/haskell/comments/2kbnzw/finished_reading_learn_you_… 14. http://www.reddit.com/r/haskell/comments/2kbnzw/finished_reading_learn_you_… 15. http://www.reddit.com/r/haskell/comments/2jvc78/simple_haskell_irc_client_i… 16. http://www.reddit.com/r/haskell/comments/2jvc78/simple_haskell_irc_client_i… 17. http://looprecur.com/blog/gpu-programming-in-haskell/ 18. http://www.reddit.com/r/haskell/comments/2jp9y9/gpu_programming_in_haskell_… 19. http://www.reddit.com/r/haskell/comments/2k93sm/are_there_any_good_game_eng… 20. http://www.reddit.com/r/haskell/comments/2k93sm/are_there_any_good_game_eng… 21. http://hub.darcs.net/darcs/darcs-screened/patch/20021020200105-e9342 22. http://www.reddit.com/r/haskell/comments/2jsvnd/darcs_turns_12/ 23. http://stackoverflow.com/questions/26542350/dependent-types-how-is-the-depe… 24. http://stackoverflow.com/questions/26524223/how-should-i-read-this-ghc-core… 25. http://stackoverflow.com/questions/26538595/more-efficient-type-level-compu… 26. http://stackoverflow.com/questions/26568262/how-to-make-fmap-rewrite-rules-… 27. http://stackoverflow.com/questions/26449154/a-workaround-for-the-template-h… 28. http://stackoverflow.com/questions/26520261/transform-a-a-maybe-a-to-take-a… 29. http://stackoverflow.com/questions/26515102/making-numeric-functions-an-ins… 30. http://stackoverflow.com/questions/26541893/inverting-a-type-family 31. http://stackoverflow.com/questions/26560100/frp-frameworks-and-io 32. https://plus.google.com/105107667630152149014/about

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Polymorphic functions over string libraries
by gonzaw 29 Oct '14

29 Oct '14

Hi. I was wondering what would be the best way to create a polymorphic function over any possible string library (Text, String, Bytestring, etc). For instance, imagine I have to read a file with text, transform this text in some way and output it to another file, or to the console. If I wanted to use String, I'd just do this: / transform :: String -> String main = readFile "input.txt" >>= writeFile "output.txt" . transform / But if I wanted to use Text instead, I'd have to use this: / import qualified Data.Text.IO as T transform :: Text -> Text main = T.readFile "input.txt" >>= T.writeFile "output.txt" . transform / Idem for ByteString. I was wondering if there was a way to create these computations in a generic way, for any kind of string library, something like this: / class StringLibrary s where: sReadFile :: FilePath -> IO s sWriteFile :: FilePath -> s -> IO () ... / So then I'd just have this: / transform :: StringLibrary s => s -> s main = sReadFile "input.txt" >>= sWriteFile "output.txt" . transform / Now I can perform the computation I want without being tied down to a specific library. At times when I create some quick scripts, I find myself using one library (for example using String to get it finished more quickly, since I have less experience with the other ones), but find that it's too slow or has some problem that is solved by using one of the other libraries. Yet swapping from one to the other is more cumbersome than expected at times. In the example above, I could easily swap between them, just by forcing the compiler to typecheck to a specific one (for instance by changing the type of "transform"). Or if I wanted to, I could leave it as it is and export it as a library of my own. Is there a way to do something like this in Haskell, with existing libraries? In terms of using the string datatype as some sort of container of characters, I think there are libraries like Lens and mono-traversable that allow you to do stuff like this. But I'm not too familiar with them (at least using them in this way). -- View this message in context: http://haskell.1045720.n5.nabble.com/Polymorphic-functions-over-string-libr… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Munich Haskell Meeting
by Heinrich Hördegen 29 Oct '14

29 Oct '14

Dear all, tomorrow, the 30th of October, our monthly Munich Haskell Meeting will take place again at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates If you plan to join, please click the button! Everybody is welcome! Heinrich

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