- Haskell-Cafe - Haskell.org

Why is the Show instance of Text not used by showTerm?
by JP Moresmau 24 Jan '14

24 Jan '14

Hello cafe, I'm continuing my adventures with the GHC API. I'm following the code given at http://www.haskell.org/haskellwiki/GHC/As_a_library#Running_interactive_sta… run statements like in GHCi, but the behavior sometimes puzzles me. If I load a simple module: module Test where import qualified Data.Text as T And then run the statement T.pack \"test\", I expect to see "test" back. But I get Data.Text.Internal.Text _ 0 4. If I run "show $ T.Pack \"test\"" I get the expected result. Text _ 0 4 seems to be the output of the showText debugging function in Data.Text, that I don't see used in the Show instance. And looking at the GHC code (Debugger.hs) it does perform a show of the expression given. Why doesn't it work? What subtlety am I missing? Thanks! JP -- JP Moresmau http://jpmoresmau.blogspot.com/

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hpaste.org is expiring
by Bryan O'Sullivan 24 Jan '14

24 Jan '14

I've held the domain for a few years. I am not going to renew it. If you'd like to take it over, please let me know immediately and I'll send you an unlock code so you can transfer it.

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How does one read complicated type signatures?
by Jakub Arnold 24 Jan '14

24 Jan '14

Hello world, I’ve just started playing with the http-conduit package and started inspecting type signatures of some of the functions, and I stumbled upon many things that look like this *Main Network.HTTP.Conduit> :t withManager withManager :: (monad-control-0.3.2.2:Control.Monad.Trans.Control.MonadBaseControl IO m, transformers-0.3.0.0:Control.Monad.IO.Class.MonadIO m) => (Manager -> resourcet-0.4.10:Control.Monad.Trans.Resource.Internal.ResourceT m a) -> m a The question is … how am I supposed to read these? It seems that they wrap around (maybe) 80 characters, which makes them unreadable … and the package prefixes also don’t really help. I’d like to point out here that I’m really a noob without any practical experience in Haskell, I’ve just read a few books. So I don’t have a problem understanding the concepts, but more and more often I find myself struggling just to read the type signature correctly Thanks for any tips, Jakub

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Sorting a Repa Array
by Hans Georg Schaathun 23 Jan '14

23 Jan '14

Hi, I am trying to parallellise genetic algorithms using haskell, but I get stuck at the stage where I need to sort the array of chromosomes based on their fitness. Using Repa, I am neither able to find a nice library implementation to sort an array, nor can I figure out how to generate a new array modifying only a few entries. I am not at a stage where I need to optimise execution time. It is still at the stage of learning exercise, wrt both to genetic algorithms, haskell, and splittable pseudo-random number generators. Thus, I would be grateful for both naïve and clever suggestions :-) Except for the sorting stage, the problem seems to well suited for Repa, with a chain of operations which can be mapped on the array. I am also planning to explore accelerate later for GPU work, and Repa seems to be a better step on the way than most other alternatives. My arrays are 1-D boxed arrays BTW. The entries are tuples including an Unboxed Vector (and a Double on which the sort order is defined). Doing it as a 2-D Unboxed Array is further down on the agenda; I need more practice before I get my head around using slices correctly. So, any advice? Whether it involves conversion to a data type where a library sort routine is available, a library sort routine I have missed, or how to create new deferred arrays by updating selected entries in the input array so that I can implement sorting myself. TIA -- :-- Hans Georg

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Re: [Haskell-cafe] off-topic question: how well do you think linguistic relativity applies to PLs and programming?
by Simon Yarde 23 Jan '14

23 Jan '14

> programming ... languages, and in particular the way that programmers use them, afford a > capacity for essentially limitless amounts of abstraction, unlike human > language and human communication. Au contraire! Humans use just such powerful abstractions in language, arts and sciences — what we call humanity is built on no less. They're called *memes*. The only difference is that humans communicate on a foundation of shared experience that is a little more fuzzy and infinitely richer than the result of a data-base lookup. Simon Yarde On 22 Jan 2014, at 23:35, Keshav Kini <keshav.kini(a)gmail.com> wrote: > Lucas Paul <reilithion(a)gmail.com> writes: >> This is my take, as a CS undergraduate. >> >> I'm not sure if we can say that a programmer's language of choice >> determines the way they think about programming (the strong version of >> linguistic relativity for programming, as I see it). But I think it's >> fairly obvious that the language we choose to use to solve a problem >> affects how we think about the solution. That's basically the entire >> raison d'être for domain-specific languages (DSLs)! >> >> DSLs are popular (and becoming more so) precisely because the right >> choice of DSL can make expressing the solution to a particular kind of >> problem almost trivial. A poor choice can almost doom an endeavor. >> Imagine trying to query a database in assembly language. No SQL. It >> would at the very least require some mental gymnastics that a SELECT >> statement simply obviates. > > I think this can be partially explained by noting that programming > languages, and in particular the way that programmers use them, afford a > capacity for essentially limitless amounts of abstraction, unlike human > language and human communication. For example, I might easily have a > magic library that does exactly what I want, with bindings for my > programming language of choice, in which case I don't need to think > about what to do, I just call the appropriate function. In human > communication, while someone might have perfectly formulated exactly the > idea I want to communicate, it is rarely sufficient for the purpose of > communication to say "insert pages 204 through 356 of _Foobar_ by John > Doe here" in the middle of a conversation :) > > -Keshav > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe

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off-topic question: how well do you think linguistic relativity applies to PLs and programming?
by Ömer Sinan Ağacan 23 Jan '14

23 Jan '14

Hi all, I recently got myself thinking about programming languages and their effects on programmers. I already knew that concept of "linguistic relativity" ( http://en.wikipedia.org/wiki/Linguistic_relativity ) and I was thinking that this may be relevant with programming too, although I don't have any concrete evidence. I was wondering if anyone else also find that idea of programming language's effect of the programmer interesting. Do we have any research on that kinds of things? Thanks,

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Why Maybe exists if there is Either?
by Vlatko Basic 23 Jan '14

23 Jan '14

Hello Cafe, With my current knowledge of Haskell, I do not see why is there Maybe if we have Either. For example, Functor and Monad instances (and many others) of Maybe and Either are the same (except for fail). In other words, this should hold: Maybe a = Either String a -- String or something else Nothing = Left "" Just a = Right a I'm curious to find out what was the reasoning to make Maybe? What is the added value with introducing it? In which situations the above substitution does not hold? Best regards, vlatko

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ANN: ZuriHac 2014
by Bas van Dijk 22 Jan '14

22 Jan '14

Dear Haskellers, After a very successful ZuriHac 2013 we are delighted to announce ZuriHac 2014! When: Friday 6 June 2014 - Sunday 8 June 2014 Where: Erudify offices, Zurich, Switzerland ZuriHac is an international Haskell hackathon: a grassroots, collaborative coding festival with a simple focus, to build and improve Haskell libraries, tools, and infrastructure. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools, or even start new ones! Registration ------------ If you wish to attend, please register by filling in this form: http://bit.ly/ZuriHac2014 Please note that we have a limited number of places -- first come, first served. Full details can be found on the wiki page: http://www.haskell.org/haskellwiki/ZuriHac2014 We look forward to seeing you there! -- The organisers of ZuriHac 2014 ZuriHac 2014 is sponsored by Erudify & Google

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Haddock Constructor parameter documentation without record syntax
by satvik chauhan 22 Jan '14

22 Jan '14

Hi, Does anybody know how to document parameters of a constructor in haddock. The following code gives parse error on the second parameter while generating haddock documentation. ``` data Foo = Foo Int -- ^ First Int -- ^ Second ``` -Satvik

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Help on simpleHttp in Network.HTTP.Conduit problem
by Stuart Mills 22 Jan '14

22 Jan '14

I copied and pasted some html parsing demo source from FP School. While the code works on the FP demo site (in the IDE), I get the following error on my Windows 7 64 bit: InternalIOException getAddrInfo: does not exist (error 10093). Thanks

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ANNOUNCE: Hac NYC: Haskell Hackathon in NYC, April 4-6
by MightyByte 22 Jan '14

22 Jan '14

Greetings, On behalf of the organizers, I am pleased to officially announce the first Hac NYC, a Haskell hackathon/get-together to be held April 4-6 in New York City. Space will be provided by Etsy on Saturday and Sunday. Details for Friday will be announced as they become finalized. We want to stress that everyone is welcome---you do not have to be a Haskell guru to attend! Helping hack on someone else's project could be a great way to increase your Haskell-fu. If you plan on coming, you must officially register by filling out our registration form [1]. Other details for travel, lodging, etc can be found on the Hac NYC wiki [2]. We're also looking for a few people interested in giving short (15-20 min.) talks, probably on Saturday afternoon. Anything of interest to the Haskell community is fair game---a project you've been working on, a paper, a quick tutorial. If you'd like to give a talk, add it on the wiki [3]. Hope to see you in April! -The Hac NYC Team [1] https://docs.google.com/forms/d/1taZtjgYozFNebLt1TR2VnKv-ovD2Yv5sOdSZzmi_xF… [2] http://www.haskell.org/haskellwiki/Hac_NYC [3] http://www.haskell.org/haskellwiki/Hac_NYC/Talks

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Constraint on data type
by Vlatko Basic 22 Jan '14

22 Jan '14

Hi Cafe, I have these typeclasses defined: class UA a where ... class RA a where ... and this data definition data (UA u, RA r) => AD u r = AD { ... } I have another data definition data ACT aur c = ACT { x :: aur, ... } but the type 'aur' should be restricted to (AD u r) only. (I would like to keep 'aur' as 1 parameter, not as data ACT u r c = ACT { x :: AD u r, ... } ) How can such constraint be defined? Best regards, vlatko

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Any projected data for new Haskell Platform (2013.4? 2014.2?)
by Stuart Mills 22 Jan '14

22 Jan '14

Thanks

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Associated patterns
by Alejandro Serrano Mena 21 Jan '14

21 Jan '14

Dear haskell-cafe, I've read in Reddit that pattern synonyms have been merged in GHC HEAD [ http://www.reddit.com/r/haskell/comments/1vpaey/pattern_synonyms_merged_int… ]. I would like to know whether associated patterns, that is, patterns which come under the umbrella of a type class, have also been implemented. I think that associated patterns would fill the gap in difference of features between type classes and common data types. For plain data types, you can declare both functions and patterns (either via constructors or now via pattern synonyms). However, you can only declare functions (either term-level or type-level) in type classes. This means that the pattern match mechanism, very useful to get clear code, is not useful if you want to use type classes. Alejandro.

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MissingH fails to compile
by Peter Colberg 21 Jan '14

21 Jan '14

Dear Haskell developers, I am trying to build a current version of git-annex, and stumble over a compilation failure of one of its dependencies, MissingH. I am using GHC 7.4.2 and Cabal 1.18.0.2, both bootstrapped on RHEL 5.x/6.x using a helper makefile [1] that has worked fine so far. The command to install git-annex is cabal install git-annex-5.20140108 --flags=-assistant --flags=-dbus --flags=-webapp --flags=-webdav --flags=-xmpp The build log for MissingH-1.2.0.2 is attached. I would be glad for any pointers on how to resolve this issue. Thanks, Peter [1] http://git.colberg.org/packages/plain/packages.mk

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ghc-mod list: cannot satisfy -package cairo
by Brian Victor 21 Jan '14

21 Jan '14

I'm receiving an error from ghc-mod in my cabal sandbox: %./.cabal-sandbox/bin/ghc-mod list Dummy:0:0:Error:<command line>: cannot satisfy -package cairo (use -v for more information) The obvious potential problem is that cairo isn't installed. However, %cabal repl Preprocessing executable 'Notation' for Notation-0.1.0.0... GHCi, version 7.6.3: http://www.haskell.org/ghc/ :? for help ... Prelude Main> import Graphics.Rendering.Cairo Prelude Graphics.Rendering.Cairo Main> My cabal file includes: build-depends: base >=4.6 && <4.7, mtl >=2.1 && <2.2, containers >=0.4 && <0.5, cairo >= 0.12 && <= 0.13 I'm on OSX 10.9.1. I had to point cairo to gcc-4.9 instead of clang to make it compile. Where else can I look for the problem? -- Brian

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[Help] Trying to rewrite the NetBSD kernel using Haskell language.
by Kiwamu Okabe 21 Jan '14

21 Jan '14

Hi Haskell hackers, We start trying to rewrite NetBSD kernel using Haskell language. https://github.com/metasepi/netbsd-arafura-s1 But we find some problems. Could you have any ideas for them? ## Status * Trying rewrite AC''97 sound driver using Haskell * Before rewriting. (C language) https://github.com/metasepi/netbsd-arafura-s1/blob/d1b19c686de5573ef8a8342a… * After rewriting. (Haskell language) https://github.com/metasepi/netbsd-arafura-s1/blob/d1b19c686de5573ef8a8342a… * Have rewrited 2 functions, auich_open() and auich_close() * Not yet touch Haskell heap ## Problems Some time, C programmer writes following code. ~~~ static int auich_open(void *addr, int flags) { struct auich_softc *sc; sc = (struct auich_softc *)addr; mutex_spin_exit(&sc->sc_intr_lock); sc->codec_if->vtbl->lock(sc->codec_if); ~~~ Note the line "sc->codec_if->vtbl->lock(sc->codec_if);". It traces pointer tree regionally. Haskell's Strorable class is not good for the use case, because it copies the entire pointer tree! We avoid the problem using following messy code. https://github.com/metasepi/netbsd-arafura-s1/blob/d1b19c686de5573ef8a8342a… ~~~ auichOpen :: Ptr AuichSoftc -> Int -> IO Int auichOpen sc flags = do mutexp <- p_AuichSoftc_sc_intr_lock sc codeif <- peek =<< p_AuichSoftc_codec_if sc lock <- peek =<< p_Ac97CodecIfVtbl_lock =<< peek =<< p_Ac97CodecIf_vtbl codeif mutexSpinExit mutexp call_Ac97CodecIfVtbl_lock lock codeif -- snip -- p_AuichSoftc_sc_intr_lock :: Ptr AuichSoftc -> IO (Ptr KmutexT) p_AuichSoftc_sc_intr_lock p = return $ plusPtr p offsetOf_AuichSoftc_sc_intr_lock p_AuichSoftc_codec_if :: Ptr AuichSoftc -> IO (Ptr (Ptr Ac97CodecIf)) p_AuichSoftc_codec_if p = return $ plusPtr p offsetOf_AuichSoftc_codec_if p_Ac97CodecIf_vtbl :: Ptr Ac97CodecIf -> IO (Ptr (Ptr Ac97CodecIfVtbl)) p_Ac97CodecIf_vtbl p = return $ plusPtr p offsetOf_Ac97CodecIf_vtbl p_Ac97CodecIfVtbl_lock :: Ptr Ac97CodecIfVtbl -> IO (Ptr (FunPtr Ac97CodecIfVtbl_lock)) p_Ac97CodecIfVtbl_lock p = return $ plusPtr p offsetOf_Ac97CodecIfVtbl_lock ~~~ Haskell world has the answer for it already? Best regards, -- Kiwamu Okabe

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Re: [Haskell-cafe] Help with Binary Serialization
by Lennart Kolmodin 21 Jan '14

21 Jan '14

Hi, the format is quite simple, and it's the same in both binary and cereal. For data types with only one constructor, only the values of the constructor are encoded - not the constructor itself. data Foo = Foo Int Int deriving (Generic) So Foo is encoded as if it was only the Ints without the Foo constructor. encode (Foo 1 2) = 0x 00000001 00000002 If there are multiple constructors a tag is encoded representing the constructor, and then the values of that constructor. The tag will use as many bytes as it needs to be to fit the tag. 2-255 constructors will use 1 byte, and so on. data Fruit = Apple Int | Orange Foo encode (Apple 3) = 0x 00 00000003 First byte is zero to represent the Apple constructor, then the Int. encode (Orange (Foo 4 5)) = 0x 01 00000004 00000005 First byte is 0x01 to represent the Orange constructor, then the Foo value which is just to subsequent Ints. The format will not change without warning, and has not changed since it was implemented. That said, the generic format is mostly meant for being written and read by the haskell app that defines the data types, as it's easy to break compatibility even within the same app. JSON is probably not a bad choice, try it out http://hackage.haskell.org/package/aeson-0.6.1.0/docs/Data-Aeson.html Hope it helps! Lennart On Monday, 20 January 2014 21:28:33 UTC+4, Joey Eremondi wrote: > > Makes sense. I've had trouble finding documentation on the format used by > aeson, any links to that? > On 2014-01-19 10:46 AM, "John Lato" <jwl...(a)gmail.com <javascript:>> > wrote: > >> I think this approach will likely lead to problems later on. First, >> undocumented formats are liable to change without warning. Secondly, it's >> conceivable that the format could change due to changes in ghc's generics >> support, or internal (and hence unstable) data structures of some component >> type. >> >> Would it be possible to just define your own format instead, or use >> something like JSON that's already well-defined? >> >> On Jan 18, 2014 12:17 PM, "Joey Eremondi" <jmit...(a)gmail.com<javascript:>> >> wrote: >> > >> > I was wondering if somebody could talk me through the default derived >> format for binary serialization used, either by binary or by cereal. >> > >> > I'm trying to share data between Haskell and another function language >> (Elm) which also supports algebraic data types, so the conversion of data >> should be pretty trivial. I'd like to be able to just derive encode and >> decode in Haskell using either binary/cereal, and then write a parser for >> the same format in Elm. The trick is, I don't know what that format is. >> > >> > Is there any documentation on it, or if not, is anybody familiar enough >> with it that they could explain the format to me? >> > >> > Thanks! >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskel...(a)haskell.org <javascript:> >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> >

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Hacking GHC's Stack for Fun and Profit (featuring The Glorious Haskell Debugger v0.0.1 Pre-alpha)
by Tim C. Schroeder 20 Jan '14

20 Jan '14

Hello, I have developed various profiling and debugging tools in the past, but haven't done any work in that direction for GHC compiled Haskell programs. I thought a good start might be to write a document detailing how the call stack works in GHC, and also write a basic debugger stack trace tool. Here: https://github.com/blitzcode/ghc-stack I hope this is helpful to people trying to pin down crashes, and people interested in working on debugging / profiling tools. I'd also really appreciate if somebody with more GHC / RTS knowledge than me would have a look and see if I got things right. Cheers, Tim

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Re: [Haskell-cafe] different behaviours with or without putStrLn
by Corentin Dupont 20 Jan '14

20 Jan '14

One question: Since it works with putStrLn, can I simulate the behaviour of putStrLn without actually... printing anything? As a workaround... On Mon, Jan 13, 2014 at 12:40 AM, Corentin Dupont <corentin.dupont(a)gmail.com > wrote: > I doesn't seems to be easy, either with deepseq-th or deepseq-generic... > With deepseq-generic, the derivation of Generic doesn't work because I'm > using a GATD. > It says *"NewVar must be a vanilla data constructor"*. > > For deepseq-th, I have this error: > > *deriveNFData: 'forall' not supported in constructor declaration* > I have some complex data constructor... > > > So it looks like both deepseq-th and deepseq-generic are out of the game > :(( > > > > > On Mon, Jan 13, 2014 at 12:08 AM, Erik de Castro Lopo < > mle+hs(a)mega-nerd.com> wrote: > >> James ‘Twey’ Kay wrote: >> >> > You can (probably) automatically derive it via Template Haskell using >> > the deepseq-th package: >> > >> > {-# LANGUAGE TemplateHaskell #-} >> > import Control.DeepSeq.TH >> > >> > data MyState = ... >> > >> > $(deriveNFData ''MyState) >> >> In most cases where you think >> >> You should probably try the deepseq-generics package before you try >> deepseq-th. >> >> The generics version was suggested to me by the author of the TH version, >> Herbert Valerio Riedel. It seems to be just as fast as the TH version and >> doesn't require TH. >> >> Cheers, >> Erik >> -- >> ---------------------------------------------------------------------- >> Erik de Castro Lopo >> http://www.mega-nerd.com/ >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >

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Combining databases from packages installed with cabal install
by Marco Túlio Pimenta Gontijo 20 Jan '14

20 Jan '14

Hi. I'm configuring haddock via cabal install (see [0]) to build the hoogle database. The database is being installed in ~/.cabal/share/doc/$package-$version/html/$package.txt, but is not being combined with the default database. That is, if right after the installation I try to search with the hoogle command for some a function, it will not work. I wrote the following script, which I called cabal-install: #!/bin/sh set -e set -x cabal \ install \ --enable-documentation \ --enable-library-profiling \ --haddock-hyperlink-source \ --haddock-hoogle \ --haddock-html \ "$@" cd ~/.cabal/share/hoogle-4.2.13/databases/ for file in ~/.cabal/share/doc/*/html/*.txt do hoo=`echo $file | sed 's/.txt$/.hoo/;s#.*/##'` if [ ! -f $hoo ] then hoogle convert $file $hoo || true hoogle combine default.hoo $hoo -o /tmp/cabal-install-$$.hoo mv /tmp/cabal-install-$$.hoo default.hoo fi done Basically, it searches for .txt hoogle databases installed that were not combined yet with the default database, and combines them. I think it would be good if this was the default behaviour of cabal install when called with --haddock-hoogle. Is this a bug? Greetings. 0: http://hackage.haskell.org/trac/hackage/ticket/517 -- marcot http://marcot.eti.br/

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ANN: unsafely, Flexible access control for unsafe operations and instances
by Hiromi ISHII 19 Jan '14

19 Jan '14

Yesterday, I uploaded the library `unsafely` to Hackage: http://hackage.haskell.org/package/unsafely This package provides you the functionality for access control for unsafe operations and instances. This purpose is somewhat similar to GHC's `NullaryTypeClasses`[^1] extension, but permits more flexible access control. With this package, you can tag functions and type-class instances as *unsafe* in type constraint. This library is useful when: * You want to restrict the access to *unsafe* operations by type constraint * You have to provide some *unsafe* type-instances for practical reasons. For example, when writing computer algebra system with type-classes, `Double` type doesn't even form a semi ring, but we need the instance `Semiring Double` if we want to combine the symbolic computations and the numerical methods. A simple example: ```haskell {-# LANGUAGE FlexibleContexts, FlexibleInstances, RankNTypes #-} {-# LANGUAGE UndecidableInstances #-} module Main where import Data.Constraint.Unsafely import Data.IORef import Data.Proxy import System.IO.Unsafe saferUnsafePerformIO :: Unsafely IO => IO a -> a saferUnsafePerformIO = unsafePerformIO global :: Unsafely IO => IORef Int global = saferUnsafePerformIO $ newIORef 0 unsafelyIO :: (Unsafely IO => a) -> a unsafelyIO = unsafely (Proxy :: Proxy IO) main :: IO () main = do unsafelyIO $ readIORef global -- | uncommenting following line causes type-error! -- readIORef global return () ``` For more detail, please read Haddock[^2]. [^1]: https://ghc.haskell.org/trac/ghc/ticket/7642 [^2]: http://hackage.haskell.org/package/unsafely-0.1.0.0.1/docs/Data-Constraint-… -- Hiromi ISHII konn.jinro(a)gmail.com

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Help with Binary Serialization
by Joey Eremondi 19 Jan '14

19 Jan '14

I was wondering if somebody could talk me through the default derived format for binary serialization used, either by binary or by cereal. I'm trying to share data between Haskell and another function language (Elm) which also supports algebraic data types, so the conversion of data should be pretty trivial. I'd like to be able to just derive encode and decode in Haskell using either binary/cereal, and then write a parser for the same format in Elm. The trick is, I don't know what that format is. Is there any documentation on it, or if not, is anybody familiar enough with it that they could explain the format to me? Thanks!

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Standard implementation of asynchronous helpers?
by Tristan Seligmann 19 Jan '14

19 Jan '14

I was wondering if a standard implementation of these helpers (or something similar) existed somewhere: startAsync :: IO a -> IO (MVar a) startAsync action = do var <- newEmptyMVar forkIO $ action >>= putMVar var return var wait :: MVar a -> IO a wait = readMVar I've found a few throwaway examples that implement something similar, and I can imagine some improvements already (I guess the MVar should be wrapped, for example, and some higher) but I was unable to find any released library that does something like this. Is it just too trivial for anyone to have bothered? -- mithrandi, i Ainil en-Balandor, a faer Ambar

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Building ghc-7.6.3 under cygwin
by Dmitry Malikov 18 Jan '14

18 Jan '14

Trying to build 7.6.3-x86_64-unknown-linux under cygwin. $ ./configure checking for path to top of build tree... ./configure: line 2121: utils/ghc-pwd/dist-install/build/tmp/ghc-pwd: cannot execute binary file configure: error: cannot determine current directory Not sure it is a libgmp issue, because linking doesn't help: $ ln -s /usr/lib/libgmp.dll.a /usr/lib/libgmp.so.3 Any ideas what is going on and how to fix this?

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Re: [Haskell-cafe] [Haskell] Just started working with Haskell. Need some help
by Henning Thielemann 18 Jan '14

18 Jan '14

moving to Haskell-Cafe ... Am 18.01.2014 18:25, schrieb Pyro Crane: > But, when I try to run a simple script, I keep getting the error : > "*Could not find module XXXXXX*".............. What precisely did you try? > And, now, I've come up against a module/package, which I am unable to > locate in Google. It's missing from my installation, and Google does > not know what it is, or where I can find it > > Is there any way to simply install ALL required packages??? I thought > I already did this, but obviously I missed something $ cabal install wantedpkg should install all dependencies of 'wantedpkg'. If it does not I am interested in how you tried to install 'wantedpkg'.

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ANNOUNCE: Diagrams 1.0
by Brent Yorgey 18 Jan '14

18 Jan '14

The diagrams team is very pleased to announce the 1.0 release of diagrams [1], a framework and embedded domain-specific language for declarative drawing in Haskell. Check out the gallery [2] for some examples of what it can do. Diagrams can be used for a wide range of purposes, from data visualization [3] to illustration [4] to art [5], and diagrams code can be seamlessly embedded in blog posts [6], LaTeX documents [7], and Haddock documentation [8], making it easy to incorporate diagrams into your documents with minimal extra work. [1] http://projects.haskell.org/diagrams [2] http://projects.haskell.org/diagrams/gallery.html [3] http://idontgetoutmuch.wordpress.com/2013/10/23/parking-in-westminster-an-a… [4] https://www.fpcomplete.com/user/edwardk/cellular-automata/part-1 [5] http://mathlesstraveled.com/2013/04/06/stars-of-the-minds-sky-with-diagrams/ [6] http://byorgey.wordpress.com/2012/08/28/creating-documents-with-embedded-di… [7] http://projects.haskell.org/diagrams/doc/latex.html [8] http://byorgey.wordpress.com/2013/03/23/introducing-diagrams-haddock/ What's new ---------- Brent recently gave a talk at the New York Haskell Users' Group [12] presenting the new release. You can find videos of the talk on vimeo: part 1 presents a basic introduction to the library [13], and part 2 talks about mathematical abstraction and DSL design [14]. The slides are also available [15]. [12] http://www.meetup.com/NY-Haskell/ [13] http://vimeo.com/84104226 [14] http://vimeo.com/84249042 [15] http://www.cis.upenn.edu/~byorgey/pub/13-11-25-nyhaskell-diagrams.pdf This release includes a number of significant new features and improvements. Highlights include: - Support for drawing arrows between given points or between diagrams, with many options for customization (tutorial [16], documentation [17], API [18]). [16] http://projects.haskell.org/diagrams/doc/arrow.html [17] http://projects.haskell.org/diagrams/doc/manual.html#arrows [18] http://projects.haskell.org/diagrams/haddock/Diagrams-TwoD-Arrow.html - A new framework for creating custom command-line-driven executables for diagram generation (tutorial [19], API [20]). [19] http://projects.haskell.org/diagrams/doc/cmdline.html [20] http://projects.haskell.org/diagrams/haddock/Diagrams-Backend-CmdLine.html - Offsets of trails and paths, i.e. compute the trail or path lying a constant distance from the given one (documentation [21], API [22]). [21] http://projects.haskell.org/diagrams/doc/manual.html#offsets-of-segments-tr… [22] http://projects.haskell.org/diagrams/haddock/Diagrams-TwoD-Offset.html - A new API, based on Metafont, for constructing cubic splines with control over things like tangents and "tension" (tutorial [23], API [24]). [23] http://projects.haskell.org/diagrams/doc/metafont.html [24] http://projects.haskell.org/diagrams/haddock/Diagrams-TwoD-Path-Metafont.ht… - Tangent and normal vectors of segments and trails (API [25]). [25] http://projects.haskell.org/diagrams/haddock/Diagrams-Tangent.html - Alignment can now be done by trace in addition to envelope (API [26]). [26] http://projects.haskell.org/diagrams/haddock/Diagrams-TwoD-Align.html - The lens [27] package is now used consistently for record fields throughout the library (documentation [28]). [27] http://hackage.haskell.org/package/lens [28] http://projects.haskell.org/diagrams/doc/manual.html#faking-optional-named-… - Across-the-board improvements in performance and size of generated files. See the release notes [29] for full details, and the migration guide [30] for help porting your diagrams 0.7 code to work with diagrams 1.0. [29] http://projects.haskell.org/diagrams/releases.html [30] http://www.haskell.org/haskellwiki/Diagrams/Dev/Migrate1.0 Try it out ---------- For the truly impatient: cabal install diagrams Diagrams is supported under GHC 7.4 and 7.6. To get started, read the quick start tutorial [31], which will introduce you to the fundamentals of the framework and provide links for further reading. For those who are less impatient and want to really dig in and use the power features, read the extensive user manual [32]. There is also a growing collection of tutorials [33] on specific topics. [31] http://projects.haskell.org/diagrams/doc/quickstart.html [32] http://projects.haskell.org/diagrams/doc/manual.html [33] http://projects.haskell.org/diagrams/documentation.html Get involved ------------ Diagrams has a friendly and growing community of users and developers. To connect with the community, subscribe to the project mailing list [34], and/or come hang out in the #diagrams IRC channel on freenode.org for help and discussion. Development continues stronger than ever, and there are a wide range of projects available for new contributors of all levels of Haskell skill. Make some diagrams. Fix some bugs [35]. Submit your cool examples for inclusion in the gallery [36] or your cool code for inclusion in the diagrams-contrib [37] package. [34] http://groups.google.com/group/diagrams-discuss [35] http://github.com/diagrams/ [36] http://projects.haskell.org/diagrams/gallery.html [37] http://hackage.haskell.org/package/diagrams%2Dcontrib Happy diagramming! Brought to you by the diagrams team: - Daniel Bergey - Jeff Rosenbluth - Ryan Yates - Brent Yorgey with contributions from: - Jan Bracker - Conal Elliott - Daniil Frumin - Sam Griffin - Niklas Haas - Peter Hall - Claude Heiland-Allen - Deepak Jois - John Lato - Felipe Lessa - Chris Mears - Ian Ross - Carlos Scheidegger - Vilhelm Sjöberg - Michael Sloan - Jim Snavely - Luite Stegeman - Kanchalai Suveepattananont - Michael Thompson - Scott Walck

2 2

Automatic differentiation and dimension types
by Douglas McClean 18 Jan '14

18 Jan '14

Has anyone explored the intersection between automatic differentiation and dimension types (like those in the dimensional package or along the lines of any of the approaches discussed at http://www.haskell.org/haskellwiki/Physical_units)? It's tricky because for ordinary automatic differentiation the types are all the same, but when dimensions get involved that isn't the case, you have to keep dividing by the dimension of the infinitesimal. -Doug McClean

2 1

Open researcher position at Create-Net Italy
by Corentin Dupont 18 Jan '14

18 Jan '14

Dear Haskell-cafe, CREATE-NET, a research center located in Trento, Italy, is hiring one researcher on distributed system management. The responsibility of the candidate will be to contribute to the research topic of Energy Efficiency in the Cloud with particular focus on making cloud-running applications energy-aware. The techniques used will include Constraint Programming and Functional Programming to some extent. We offer competitive compensation and a stimulating work environment in the research and innovation sector. The scenic landscape of Italian Alps doesn't hurt either. If you are interested, please submit your application to careers(a)create-net.org (with me in copy). Details on the open position and ideal profile may be found on http://www.create-net.org/it/node/3197 Don't hesitate to contact me for more details! Cheers, Corentin Dupont CREATE-NET www.create-net.org www.corentindupont.info

1 0

Silencing 'cabal configure' warning
by Jan Stolarek 17 Jan '14

17 Jan '14

Hi all, When running 'cabal configure' I get a warning that my package database is old: [killy@xerxes : /dane/projekty/singletons] cabal configure Warning: The package list for 'hackage.haskell.org' is 25 days old. Run 'cabal update' to get the latest list of available packages. Is there a way to silence this warning other than running 'cabal update'? I don't want to update my package database and getting a warning every time I configure a package is irritating. Janek

2 1

ANN: pooled-io
by Henning Thielemann 17 Jan '14

17 Jan '14

I uploaded the package pooled-io to Hackage: https://hackage.haskell.org/package/pooled-io It is intended to perform parallelism in the IO monad. I needed it for computations that must write intermediate data to disk. The main task of the package is to make sure that no more than a maximum number of actions is run in parallel. There are three modules: For actions without monadic results, for actions with monadic results and for actions that depend on each others results.

3 2

Re: [Haskell-cafe] Restrict values in type
by Simon Yarde 17 Jan '14

17 Jan '14

If it's not been mentioned, this tutorial regarding Type Families and the kinds of restrictions you mention might provide a few pointers: https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/t… In general, I've found key concepts to hold in mind are 'exclusion of bad programs' mentioned in the paper Fun With Types: http://research.microsoft.com/en-us/um/people/simonpj/papers/assoc-types/fu… I'm sorry if any of this has been mentioned previous. I asked a related question yesterday about the relative merits of creating type restrictions using using newtypes (unions), which I agree seem unwieldy, and using multi-parameter-type-classes (and/or type families) to enable exclusion-of-bad-programms via instances. I'd be interested to follow along and see what you discover. Simon Yarde

1 0

Re: [Haskell-cafe] Restrict values in type
by oleg＠okmij.org 17 Jan '14

17 Jan '14

The problem you have posed calls for so-called open unions. Open unions come up all the time, and lots of solutions exists. Alas, they are all a bit ad hoc. At Haskell Symposium I was advocating designing a good solution once and for all. The paper that introduced monad transformers showed one implementation of open unions (of effects). The paper `data types a la carte' showed another (essentially the same, trying to deemphasize its use of overlapping instances). The Extensible effects paper has two more solutions, one with Typeable and one without. You can use OpenUnions from that paper if you install extensible-effects package. Using singletons is yet another, quite heavy-weight solution. I'd like to stress a much simpler solution, which requires no type equality or GADTs or bleeding edge. It is a tagless-final solution. In fact, it has been demonstrated already by Jake McArthur. I elaborate and show the whole code. Your original code defined PenShape as a data structure > data PenShape = Circle Float > | Rectangle Float Float > | ArbitraryPen -- Stuff (not relevant) I will define it as an expression in a very simple domain-specific language of pen shapes. > class CirclePen repr where > circle :: Float -> repr > -- other ways of constructing circles go here > > class RectPen repr where > rectangle :: Float -> Float -> repr > > class ArbitraryPen repr where > arbitrary :: () -> repr -- () stands for irrelevant stuff Here repr is the meaning of a pan shape in a particular interpretation. The same term can be interpreted in many ways (compare: a Haskell code can be loaded into GHCi, compiled with GHC or processed with Haddoc). One interpretation of pen shapes is to print them out nicely: data S = S{unS :: String} instance CirclePen S where circle x = S $ "circle pen of radius " ++ show x instance RectPen S where rectangle x y = S $ "rect pen " ++ show (x,y) instance ArbitraryPen S where arbitrary () = S $ "arbitrary pen" There probably will be other representations: defined only for specific sets of pens (rather than all of them), see below for an example. You ask how can you pattern-match on pen shapes. The answer is that in taggless-final style, you don't pattern-match. You interpret. Quite often the code becomes clearer. Enclosed is the complete code. For (far) more explanation of tagless-final, please see the first part of http://okmij.org/ftp/tagless-final/course/lecture.pdf {-# LANGUAGE RankNTypes #-} module Im where data Image = Image [Stroke] -- As a data structure {- data PenShape = Circle Float | Rectangle Float Float | ArbitraryPen -- Stuff (not relevant) -} -- As a term in a simple language of shapes class CirclePen repr where circle :: Float -> repr -- other ways of constructing circles go here class RectPen repr where rectangle :: Float -> Float -> repr class ArbitraryPen repr where arbitrary :: () -> repr -- () stands for irrelevant stuff -- Let's define a few interpretations of pens -- the Show interpretation, to print them -- All pens support this interpretation data S = S{unS :: String} instance CirclePen S where circle x = S $ "circle pen of radius " ++ show x instance RectPen S where rectangle x y = S $ "rect pen " ++ show (x,y) instance ArbitraryPen S where arbitrary () = S $ "arbitrary pen" -- Another interpretation: finite-dim pens. Only CirclePen and RectPen -- support it data FiniteDim = FiniteDim{unFD:: Float} instance CirclePen FiniteDim where circle x = FiniteDim x instance RectPen FiniteDim where rectangle x y = FiniteDim $ max x y {- data Stroke = Line Point Point PenShape | Arc Point Point Point PenShape | Spot Point PenShape -} type Point = (Float,Float) p0 = (0,0) p1 = (1,1) data Stroke = Line Point Point (forall repr. (CirclePen repr, RectPen repr) => repr) | Arc Point Point (forall repr. (CirclePen repr) => repr) | Spot Point (forall repr. (CirclePen repr, RectPen repr, ArbitraryPen repr) => repr) -- Let's make a an image im1 = Image [ Line p0 p1 (circle 10), Line p0 p1 (rectangle 1 2), -- The following will be a type error, as expected -- Arc p0 p1 (rectangle 1 2), Arc p0 p1 (circle 3), Spot p0 (rectangle 1 2), Spot p0 (arbitrary ()) ] -- If we add -- Line p0 p1 (arbitrary ()) -- we get a type error with an informative message {- Could not deduce (ArbitraryPen repr) arising from a use of `arbitrary' from the context (CirclePen repr, RectPen repr) bound by a type expected by the context: (CirclePen repr, RectPen repr) => repr -} -- Let's print the list of strokes show_strokes :: Image -> [String] show_strokes (Image l) = map f l where f (Line p1 p2 pensh) = unwords ["Line", show (p1,p2), unS pensh] f (Arc p1 p2 pensh) = unwords ["Arc", show (p1,p2), unS pensh] f (Spot p1 pensh) = unwords ["Spot", show p1, unS pensh] tshow = show_strokes im1 {- ["Line ((0.0,0.0),(1.0,1.0)) circle pen of radius 10.0", "Line ((0.0,0.0),(1.0,1.0)) rect pen (1.0,2.0)", "Arc ((0.0,0.0),(1.0,1.0)) circle pen of radius 3.0", "Spot (0.0,0.0) rect pen (1.0,2.0)","Spot (0.0,0.0) arbitrary pen"] -}

3 5

Can't install yesod-platform due to vector package
by Sukit Tretriluxana 16 Jan '14

16 Jan '14

Hi all, I'm trying to install Yesod as suggested by this page. http://www.yesodweb.com/page/quickstart with the following command cabal update cabal install yesod-platform yesod-bin --max-backjumps=-1 --reorder-goals But I constantly get the following error. ... 136 warnings and 6 errors generated. Failed to install vector-0.10.9.1 cabal: Error: some packages failed to install: aeson-0.6.2.1 depends on vector-0.10.9.1 which failed to install. ... The error appears to be from the fact that cabal cannot install vector package. Any suggestion to fix the problem. I'm on Maverick, Xcode5, Haskell Platform 2013.2.0.0 64bit Thanks, Ed

3 3

flip1 through flip9, useful?
by Christian Marie 16 Jan '14

16 Jan '14

I have defined a bunch of functions like this: -- | Move the fourth argument to the first place rotate4 :: (a -> b -> c -> d -> e) -> (d -> a -> b -> c -> e) -- | Reverse four arguments flip4 :: (a -> b -> c -> d -> e) -> (d -> c -> b -> a -> e) I decided to upload this as a library to hackage, as I personally find it useful, especially for writing FFI bindings. It seems like I can't be the first to write a library like this though, am I missing something obvious? Is this useful or stupid? Does it exist already? Full definition here: https://github.com/christian-marie/flippers/blob/master/src/Control/Flipper…

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ANN: castle 0.1.0.0
by Eric Rochester 16 Jan '14

16 Jan '14

I'd like to announce the first release of castle ( http://hackage.haskell.org/package/castle and https://github.com/erochest/castle) From the README: > I really like having sandboxes baked into cabal-install<http://hackage.haskell.org/package/cabal-install>(see Cabal > Sandboxes <http://coldwa.st/e/blog/2013-08-20-Cabal-sandbox.html> for > more information). > > I got tired of waiting for big packages like Yesod<http://www.yesodweb.com/>and > Lens <http://hackage.haskell.org/package/lens> to compile in project > after project that used them. However, I still didn't want to install them > in the user database. I wanted to maintain *some* sandboxing among a > group of projects that all share a common set of packages, but I wanted to > be able to switch from them or upgrade them easily. > > That's the itch I was trying to scratch with castle. > > It allows you to share one Cabal sandbox between multiple projects. This > keeps the package versions for all of these projects in line. It also means > that you don't have to constantly be re-installing everything, but you > still get the ability to blow away a set of packages without borking your > whole system. > This tool is still pretty rough around the edges, but I've been using it some, and it's to the point that more feedback would be helpful. Let me know what bugs and rough patches you find. Thanks, Eric

4 9

mdo with multiple values
by Joachim Breitner 16 Jan '14

16 Jan '14

Dear List, a little puzzle. Given a monad M with a MonadFix instance, and these two functions: act1 :: T -> M () act2 :: a -> M T I morally want to write this function: foo :: [a] -> M () foo = mdo mapM_ act1 xs xs <- mapM act2 return () Unfortunately, that will not work: mapM_ will force xs before any of it can be generated. But morally it should be possible, as the lists passed to mapM_ and mapM have the same, already known list. So here is my solution (which is a bit more general, because I happen to need some that in one place): mapFstMapSnd :: MonadFix m => [(a -> m (), m a)] -> m () mapFstMapSnd xs = const () `liftM` go xs (return []) where go [] cont = cont go ((f,s):xs) cont = mdo f v (v:vs) <- go xs $ do vs <- cont v <- s return (v:vs) return vs Using that, I can write foo = mapFstSnd . map (x -> (act1, act2 x)) Are there better solutions? Simpler ones? Or possibly ones that do not require a partial pattern? Hmm, and thinking while writing lets me come up with data FunSplit m where FunSplit :: forall m a . (a -> m ()) -> m a -> FunSplit m mapFstMapSnd :: forall m. MonadFix m => [FunSplit m] -> m () mapFstMapSnd xs = const () `liftM` go xs (return ()) where go :: [FunSplit m] -> m b -> m b go [] cont = cont go (FunSplit f s:xs) cont = mdo f v (v,vs) <- go xs $ do vs <- cont v <- s return (v,vs) return vs foo :: [SPut] -> SPut foo = mapFstMapSnd . map go where go x = FunSplit act1 (act2 x) Any suggestions for improvement? Thanks, Joachim -- Joachim Breitner e-Mail: mail(a)joachim-breitner.de Homepage: http://www.joachim-breitner.de Jabber-ID: nomeata(a)joachim-breitner.de

3 4

Error management in Happstack
by Corentin Dupont 16 Jan '14

16 Jan '14

Hi Jeremy, all, In Happstack when I throw an error, I obtain a blank page with "server error: my message". How can I decorate this page? I'd like to have it look like the other pages of my website, and a "back to login" link, for example. Thanks! Corentin

2 2

Restrict values in type
by Luke Clifton 16 Jan '14

16 Jan '14

Hi, I'm quite new to Haskell, and have been loving exploring it. I've always been a huge fan of languages that let me catch errors at compile time, finding dynamic languages like Python a nightmare to work in. I'm finding with Haskell I can take this compile time checking even further than most static languages and it has gotten me rather excited. So I was wondering if there is a Haskell way of solving my problem. I'm trying to represent an image made up of a list of strokes. Strokes are either lines, arcs or spots, and can be made using different pen shapes. data Image = Image [Stroke] data Stroke = Line Point Point PenShape | Arc Point Point Point PenShape | Spot Point PenShape data PenShape = Circle Float | Rectangle Float Float | ArbitraryPen -- Stuff (not relevant) And this is all great and works. But now I have a problem. I want to extend this such that Arc strokes are only allowed to have the Circle pen shape, and Lines are only allowed to have the Rectangle or Circle pen shapes. What is the best way of enforcing this in the type system. I could make more Strokes like LineCircle, LineRectangle, Arc, PointCircle, PointRectangle, PointArbitrary and get rid of the PenShape type altogether. But this doesn't really feel good to me (and seems like the amount of work I have to do is bigger than it needs to be, especially if I added more basic pen shapes). I thought about making the different PenShapes different types, using typeclasses and making Stroke an algebraic data type, but then my strokes would be of different types, and I wouldn't be able to have a list of strokes. I have been looking at DataKinds and GADTs, but I can't quite figure out if they actually help me here at all. I'm sure there is a way to do this, I'm just not googling properly. What I want to write is... data Image = Image [Stroke] data Stroke = Line Point Point (Circle or Rectangle) | Arc Point Point Point Circle | Spot Point PenShape data PenShape = Circle Float | Rectangle Float Float | ArbitraryPen -- Stuff (not relevant) Regards, Luke

7 14

Postdocs / Research Programmer for Efficient First-Class Automatic Differentiation
by Barak A. Pearlmutter 15 Jan '14

15 Jan '14

Postdocs / Research Programmer for Compositional Learning via Generalized Automatic Differentiation The goal of this project is to make a qualitative improvement in our ability to write sophisticated numeric code, by giving numeric programmers access to _fast_, _robust_, _general_, _accurate_ differentiation operators. To be technical: we are adding exact first-class derivative calculation operators (Automatic Differentiation or AD) to the lambda calculus, and embodying the combination in a production-quality fast system suitable for numeric computing in general, and compositional machine learning methods in particular. Our research prototype compilers generate object code competitive with the fastest current systems, which are based on FORTRAN. And the combined expressive power of first-class AD operators and function programming allows very succinct code for many machine learning algorithms, as well as for some algorithms in computer vision and signal processing. Specific sub-projects include: compiler and numeric programming environment construction; writing, simplifying, and generalising, machine learning and other numeric algorithms; and associated Type Theory/Lambda Calculus/PLT/Real Computation issues. TO THE PROGRAMMING LANGUAGE COMMUNITY, we seek to contribute a way to make numeric software faster, more robust, and easier to write. TO THE MACHINE LEARNING COMMUNITY, in addition to making it easier to write efficient numeric codes, we seek to contribute a system that embodies "compositionality", in that gradient optimisation can be automatically and efficiently performed on systems themselves consisting of many components, even when such components may internally take derivatives or perform optimisation. (Examples of this include, say, optimisation of the rules of a multi-player game to cause the players' actions to satisfy some desiderata, where the players themselves optimise their own strategies using simple models of their opponents which they optimise according to their opponents' observed behaviour.) To this end, we are seeking to fill three positions (postdoctoral or research programmer, or in exceptional cases graduate students) with interest and experience in at least one of: programming language theory, automatic differentiation, machine learning, numerics, mathematical logic. Informal announcement: http://www.bcl.hamilton.ie/~barak/ad-fp-positions.html Formal job postings on http://humanresources.nuim.ie/vacancies.shtml, in particular http://humanresources.nuim.ie/documents/JobSpecPostdoc2_Final.pdf and http://humanresources.nuim.ie/documents/JobSpecProgrammer_Final.pdf Inquiries to: -- Barak A. Pearlmutter <barak(a)cs.nuim.ie> Hamilton Institute & Dept Computer Science NUI Maynooth, Co. Kildare, Ireland

1 0

GHC API: runStmt not taking into account reloaded module
by JP Moresmau 15 Jan '14

15 Jan '14

It's late here and I'm probably overlooking something stupid, so I'd like if somebody could put my nose on it... I'm using the GHC API to evaluate statements. I use runStmt to get a RunResult, lookupName to get the ID for the bound names, obtainTermFromId to get the term and showTerm to display it. So I can call a function from the loaded module with some parameters and get the result. Good! However, if I reload a module and I change the implementation of the function, runStmt still returns the old value! I know the reload worked because if I added new names, getNamesInScope returns the new names. What do I need to do to make sure the new function definitions are used? I've perused the source code of InteractiveEval and ghci but nothing stood out. I am calling setContext after load. Thanks a million! -- JP Moresmau http://jpmoresmau.blogspot.com/

1 0

Future imports in GHC/Base
by João Cristóvão 14 Jan '14

14 Jan '14

Hello all, I was wondering if there is any particular reason for Haskell/GHC not having a kind of "import from Future", similar to Python import __future__, or, in a different take, stuff like modernizr for javascript. Was this done already? My quite simple use case: I could really use the Foldable instance for Either, and also ended up defining my own 'isLeft' and 'isRight'. Felt guilty of re-inventing the wheel after seeing it defined elsewhere, it was so obvious, of course. Then I see it defined in GHC Head - but unfortunatelly not all packages support it yet. And I can only guess that installing base 4.7 on top of GHC 7.6.3 would certainly result in Cabal Hell. I ended up copying the parts that I needed. Any one else doing this? My proposal is simple, and quite restricted: 1) Include only stuff with no dependencies on new compiler features 2) Focus mainly on additional typeclasses instances 3) Or new functions, that otherwise should not interfere with existing code. 4) Use CPP to translate the code into NOP when the new compiler finally enters the Haskell Platform. Thus, a very simple "FutureGHC" package (I'm open to alternative names) would, for isLeft/isRight, be like the code proposed below - taken from GHC Head libraries, of course. What do you think? Joao {-# LANGUAGE CPP #-} -- (...) module Data.Either.GHC707 ( module Data.Either #if __GLASGOW_HASKELL__ < 707 , isLeft , isRight #endif ) where #if __GLASGOW_HASKELL__ < 707 import Data.Either -- | Return `True` if the given value is a `Left`-value, `False` otherwise. -- -- /Since: 4.7.0.0/ isLeft :: Either a b -> Bool isLeft (Left _) = True isLeft (Right _) = False -- | Return `True` if the given value is a `Right`-value, `False` otherwise. -- -- /Since: 4.7.0.0/ isRight :: Either a b -> Bool isRight (Left _) = False isRight (Right _) = True #endif

3 4

Re: [Haskell-cafe] Records vs Tuples
by oleg＠okmij.org 14 Jan '14

14 Jan '14

Cary Cherng wrote: > What exactly is the purpose in having both records and tuples? They > seem to behave rather similarly although records do have the extra > naming of the fields that tuples don't. In SML there is really no difference: In fact, in Standard ML, tuples are simply a special case of records; for example, the type int * string * bool is the same as the type { 1 : int, 2 : string, 3 : bool }. http://en.wikibooks.org/wiki/Standard_ML_Programming/Types In some programming languages (C, OCaml, Haskell), records, or structures, have to be declared and are not extensible. OCaml also has extensible records, which don't have to be declared and whose field names may be reused. Tuples may be thought of as `extensible' records that don't have to be declared (and HList takes this thought quite seriously). > data Rec1 = Rec1 { a :: Int, b :: Bool} > data Rec2 = Rec2 { a :: Int, b :: Bool} > foo :: Rec1 -> Bool > > Rec1 and Rec2 could be in totally different code libraries. I've read > that preventing Rec2 being used in foo is good for the type safety in > that Rec1 and Rec2 are likely intended to have semantically different > meanings and allowing interchangeability breaks this. The reason is not type safety but type inference, or the ease of it. Given > data Rec1 = Rec1 { rec1a :: Int, rec1b :: Bool} > data Rec2 = Rec2 { rec2a :: Int, rec2b :: Bool} and the term \x -> rec2a x + 1 the type checker quickly determines that x must be of a type Rec2, and the inferred type is Rec2 -> Int. With extensible records, using OCaml: # fun x -> x#rec2a + 1;; - : < rec2a : int; .. > -> int = <fun> we infer a polymorphic type. That's all nice, until we look at the OCaml type checker and see the complexity of inference. Open records bring up lots of interesting problems (e.g., variance), which interact in interesting and not yet worked out ways with other features (e.g., GADTs). Also, open records can often mask problems. For example, in Haskell \x -> rec1a x + rec2a x won't type check but the corresponding code in OCaml will -- although the programmer may have never intended a record with both fields rec1a and rec2a. OCaml community has had a lot of experience with extensible and non-extensible records (and their dual -- extensible and non-extensible variants). The conclusion seems to be that both are needed. Often a programmer does mean closed records/variants; and non-extensible forms match the programmer's intent closely and give better, and earlier, error messages.

1 0

Records vs Tuples
by Cary Cherng 14 Jan '14

14 Jan '14

What exactly is the purpose in having both records and tuples? They seem to behave rather similarly although records do have the extra naming of the fields that tuples don't.

5 5

OT - targeting many systems
by Marc Weber 14 Jan '14

14 Jan '14

I love Haskell, and it gets many things right. However eg its hard to target JS in an efficient way. Sometimes I just need a quick and dirty (somewhat typesafe) program which gets the job done - sometimes its js, sometimes a tool in console, sometimse web/client code (also smartphone applets). haxe.org comes close, but while the language is fun to use it still has some shortcomings - and sometimes I hit unexpected bugs. The real question I have is: - Haskell is cool - being lazy is cool - having OO is cool (because you can target Java/Flash/.. easily) - how to get best of all worlds? So I think about creating my own language is should look like more like a source code generator than language which contains different sub universes. The idea is to code some parts - such as a parser - in a Haskell like dialect, then reuse it in quick and dirty OO world (Java like). Of course additional targets could be just plain old C, because C++ is known to be very complex - and there are tons of legacy code which should be maintained (such as Vim) - but there is no true upstream, because coding C takes so much more time. I also just don't get why it should not be possible to write code once (such as parsing snippets or highlighting), then reuse it for Vim, Emacs, Yi, .. There is no way, you cannot merge such editors - but eventually there is a way to "code once" and compile down to elisp, C, Haskell ... Live is too short to duplicate much work .. I'm writing to this mailinglist because the knowledge in the Haskell community is incredible - and I want to know * who would join such an effort (maybe write drafts, get funded by kickstarter) * does such attempt already exist ? Languages like impredicative.com/ur, Haskell, lisp, Java, Haxe cleary also illustrate the power of "meta programming" or "source code generation at compile time", yet they all come with their own limitations, eg ur cannot create generate code which is useful outside of a web server and so on. So I feel I'm reinventing the wheel again and again - I'd like to stop this. So the goal would be trying to write "functors/code" which you can instantiate with different memeory management models, tell to target different backends etc. I love Haskell, its community and what it achieved, but somehow I feel that its only 80% of the true story getting everything done. Eg haxe already allows me to write code like such - and then compile it twice: once to create a .js file, once to create server side js/php/neko/whatsoever. class FormComponentGeoLocation { new (opts){ this.opts = opts; } function check(value){ // check with regular expression that value has lat lon numbers } #if SERVER function assignValuesFromInputs(post_data, values){ var value = post_data[opts.name]; check(value); values[opts.name] = post_data[opts.name]; } #end #if js additional client code eg use geo location apis to fill in lat/lon function assignValuesFromInputs(values); var value = post_data[opts.name]; check(value); values[opts.name] = value; } #end } So this is what I'd call modular code I can reuse which has all logic it needs. While Haskell is great I'm unsure wether it allows me to create modules by code (I know about template haskell, but that's only about adding code) - and while being lazy is a feature sometimes I'd like to be strict. (I know that people work on simplifying this) ideas/thougts/links/you'd like to join understanding whether such a goal would be reachable at all? Marc Weber

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any idea why binary isn't bulding on OSX
by Anatoly Yakovenko 13 Jan '14

13 Jan '14

src/Data/Binary/Get.hs:423:3: error: invalid preprocessing directive #-} ^ src/Data/Binary/Get.hs:511:53: warning: missing terminating ' character [-Winvalid-pp-token] -- host order, host endian form, for the machine you're on. On a 64 bit ^ 12 warnings and 1 error generated. Failed to install binary-0.7.1.0 cabal: Error: some packages failed to install: JuicyPixels-3.1.2 depends on binary-0.7.1.0 which failed to install. binary-0.7.1.0 failed during the building phase. The exception was: ExitFailure 1

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Structural typing of records in Haskell?
by Cary Cherng 13 Jan '14

13 Jan '14

Are there statically typed languages that treat records with structural typing, either imperative or functional? Why should records not be structurally typed in Haskell? From what I understand, in the below foo cannot take a Rec2 even though Rec1 and Rec2 are essentially the same. data Rec1 = Rec1 { a :: Int, b :: Bool} data Rec2 = Rec2 { a :: Int, b :: Bool} foo :: Rec1 -> Bool Rec1 and Rec2 could be in totally different code libraries. I've read that preventing Rec2 being used in foo is good for the type safety in that Rec1 and Rec2 are likely intended to have semantically different meanings and allowing interchangeability breaks this. But then why is map structurally typed. map takes an argument of type a -> b and suppose some other higher order function bar also takes an argument of type a -> b. Should map instead have the below type which prevents a function of type a -> b semantically intended for bar from being accidentally used in map. newtype Mapper a b = Mapper { fn :: a -> b } map :: Mapper a b -> [a] -> [b] map _ [] = [] map f (x:xs) = (fn f) x : map f xs If there is a mechanism that prevents something of type Rec2 from accidentally being used in foo, then why shouldn't there be something analogous that prevents something of type a -> b (meant for bar) from accidentally being used in map?

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upgrading from Xcode4 to Xcode5 on Mountain Lion
by Angel Alvarez (GMAIL) 13 Jan '14

13 Jan '14

Hi, I have pending the Xcode 4 -> 5 upgrade for a while in my mountain lion, have any of you any pointers to fixes needed in order to get Xcode 5 runnignwith the current haskell platform? from time to time I've seen people having trouble and don't know really if its recommended to upgrade on Mountain Lion... regards, Angel Alvarez (GMAIL) angeljalvarezmiguel(a)gmail.com

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High level overview of GHCi?
by Maarten Faddegon 13 Jan '14

13 Jan '14

Dear Café, I was reading http://www.aosabook.org/en/ghc.html. Figure 5.2 gives a high level overview of the compiler passes when compiling Haskell with GHC. Is anyone aware of a similar figure that gives an overview of the passes when Haskell code is interpreted with GHCi? Thank you! Maarten Faddegon

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ANN: tree-view-0.1
by Emil Axelsson 13 Jan '14

13 Jan '14

tree-view is a package for rendering trees as foldable HTML and Unicode art. http://hackage.haskell.org/package/tree-view Example: *Data.Tree.View> drawTree $ Node "Add" [Node "Sub" [Node "3" [], Node "Mul" [Node "1" [], Node "2" []]], Node "4" []] Add ├╴Sub │ ├╴3 │ └╴Mul │ ├╴1 │ └╴2 └╴4 / Emil

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