- Haskell-Cafe - Haskell.org

Creating Repa arrays from unboxed vectors
by Janek S. 04 Oct '12

04 Oct '12

Dear list, I'm trying to create unboxed REPA array from unboxed Vector, but I keep getting this type error: ghci> :m + Data.Array.Repa ghci> :m + Data.Array.Repa.Repr.Unboxed ghci> :m + Data.Vector.Unboxed ghci> fromUnboxed Z (Data.Vector.Unboxed.singleton 1) <interactive>:5:16: Couldn't match expected type `vector-0.9.1:Data.Vector.Unboxed.Base.Vector e0' with actual type `Data.Vector.Unboxed.Vector a0' In the return type of a call of `Data.Vector.Unboxed.singleton' In the second argument of `fromUnboxed', namely `(Data.Vector.Unboxed.singleton 1)' In the expression: fromUnboxed Z (Data.Vector.Unboxed.singleton 1) I am confused, because REPA documentation lists Vector from Data.Vector.Unboxed package as its second parameter. Why am I getting this error? I have Repa 3.2 an vector 0.9.1. Janek

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Problems installing DPH
by Janek S. 04 Oct '12

04 Oct '12

Dear list, I ran into problems when installing DPH examples: [root@GLaDOS : ~] cabal --global install dph-examples Resolving dependencies... Configuring dph-examples-0.6.1.3... Building dph-examples-0.6.1.3... Preprocessing executable 'dph-spectral-smvm' for dph-examples-0.6.1.3... [1 of 3] Compiling Vectorised ( examples/spectral/SMVM/dph/Vectorised.hs, dist/build/dph-spectral-smvm/dph-spectral-smvm-tmp/Vectorised.o ) exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE ghc-prim:GHC.Types.Double{(w) tc 3u} exprType TYPE (ghc-prim:GHC.Types.Int{(w) tc 3J}, ghc-prim:GHC.Types.Double{(w) tc 3u}) opt: Bitcode stream should be a multiple of 4 bytes in length cabal: Error: some packages failed to install: dph-examples-0.6.1.3 failed during the building phase. The exception was: ExitFailure 1 Gogling the error message leads to various LLVM pages, which suggests that LLVM is the problem. I'm using Debian Squeeze with LLVM 2.6 and I'm thinking that maybe DPH needs newer version. What is the minimum required version of LLVM for DPH to work? Or is this problem caused by semthing else? Janek

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Haskell Weekly News: Issue 246
by Daniel Santa Cruz 04 Oct '12

04 Oct '12

Welcome to issue 246 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of September 23 to September 23, 2012. Want to contribute quotes for HWN? Until today the only way to help with those was to use @remember in the IRC channel. Now you can also use @HWNQuotes in twitter to submit quotes for the weekly newsletter. Quotes of the Week * carpi: all said ... the brain of someone of who writes compilers for haskell scares me nuts. I wouldn't want them concentrating on me, lest I catch fire. * Igloo: The major problem, as far as I can see, is how to construct the mumble'. * edwardk: i want to be able to know if the compiler is being stupid or if its just me, and without any sort of model to reason about it with, i always have to assume it could be the compiler; i already have to deal with that problem, its called 'scala' * ddarius: Now I can just point people at a readable and relevant paper instead of having to Cale it. * rwbarton: also, a generally safe answer to "was there an update to lambdabot recently" is "no" * camccann: you want a debugger? GHC has one. Except that stepping through lazy evaluation is like listening to a stream of consciousness narrative told by a 5-year-old with ADHD. Top Reddit Stories * My Haskell book is not dead! I'd love some feedback now that it's six chapters long and growing. Domain: sthaskell.com, Score: 61, Comments: 18 On Reddit: [1] http://goo.gl/AfLwS Original: [2] http://goo.gl/oB0e8 * What commercial users say they want from Haskell Domain: fpcomplete.com, Score: 46, Comments: 54 On Reddit: [3] http://goo.gl/JTtMG Original: [4] http://goo.gl/ZoHrh * Haskell rewarding moments: Orc Battle translation Domain: self.haskell, Score: 41, Comments: 22 On Reddit: [5] http://goo.gl/5xjJJ Original: [6] http://goo.gl/5xjJJ * Does Stroustroup have some FP envy? This is a pretty neat library implementation of pattern matching in C++. Domain: open-std.org, Score: 40, Comments: 44 On Reddit: [7] http://goo.gl/PH7ZU Original: [8] http://goo.gl/KuGrw * Functional TDD: a clash of cultures? Domain: facebook.com, Score: 34, Comments: 34 On Reddit: [9] http://goo.gl/9xBLo Original: [10] http://goo.gl/kOi2G * Dovetailing Domain: self.haskell, Score: 33, Comments: 21 On Reddit: [11] http://goo.gl/GdlMi Original: [12] http://goo.gl/GdlMi * Oh the fun of trying to install wxhaskell Domain: self.haskell, Score: 33, Comments: 33 On Reddit: [13] http://goo.gl/4F3Fu Original: [14] http://goo.gl/4F3Fu * Lazy v. Yield: Incremental, Linear Pretty-printing (Kiselyov, Peyton-Jones, Sabry) [pdf] Domain: cs.indiana.edu, Score: 31, Comments: 54 On Reddit: [15] http://goo.gl/PfVHl Original: [16] http://goo.gl/ub46l * Call for discussion: OverloadedLists extension Domain: gist.github.com, Score: 24, Comments: 52 On Reddit: [17] http://goo.gl/0O1G8 Original: [18] http://goo.gl/xYyTq * Real World Haskell: is there a second edition coming? Domain: self.haskell, Score: 24, Comments: 9 On Reddit: [19] http://goo.gl/rjjQb Original: [20] http://goo.gl/rjjQb * Introduction to Functional Reactive Programming [video] Domain: blog.edwardamsden.com, Score: 23, Comments: 3 On Reddit: [21] http://goo.gl/vSVKP Original: [22] http://goo.gl/IH3MF Top StackOverflow Questions * What optimizations can GHC be expected to perform reliably? votes: 27, answers: 2 Read on SO: [23] http://goo.gl/E6bH4 * A simple example showing that IO doesn't satisfy the monad laws? votes: 22, answers: 4 Read on SO: [24] http://goo.gl/YptHT * Are there any good use cases for OverlappingInstances? votes: 12, answers: 3 Read on SO: [25] http://goo.gl/FLChO * Why is ListT monad transformer considered buggy - what monad laws it breaks? votes: 11, answers: 1 Read on SO: [26] http://goo.gl/HX7Oa * How to build and use a bootstrap executable in a Cabal package votes: 10, answers: 1 Read on SO: [27] http://goo.gl/tvA03 * Continuation passing style representation of types votes: 10, answers: 1 Read on SO: [28] http://goo.gl/6R60n * GHC code generation for type class function calls votes: 10, answers: 3 Read on SO: [29] http://goo.gl/3ggvE * Why inductive datatypes forbid types like `data Bad a = C (Bad a -> a)` where the type recursion occurs in front of ->? votes: 10, answers: 2 Read on SO: [30] http://goo.gl/jtG6d * Are functions of arity-n really just an n-category due to currying? Can they be made into a 1-category? votes: 9, answers: 2 Read on SO: [31] http://goo.gl/JfJLF * Setting socket options on OSX votes: 9, answers: 0 Read on SO: [32] http://goo.gl/aybHY Until next time, Daniel Santa Cruz References 1. http://www.sthaskell.com/ 2. http://www.reddit.com/r/haskell/comments/10fx9w/my_haskell_book_is_not_dead… 3. http://fpcomplete.com/commercialuserneeds/ 4. http://www.reddit.com/r/haskell/comments/10me87/what_commercial_users_say_t… 5. http://www.reddit.com/r/haskell/comments/10c7n5/haskell_rewarding_moments_o… 6. http://www.reddit.com/r/haskell/comments/10c7n5/haskell_rewarding_moments_o… 7. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3449.pdf 8. http://www.reddit.com/r/haskell/comments/10i859/does_stroustroup_have_some_… 9. https://www.facebook.com/notes/kent-beck/functional-tdd-a-clash-of-cultures… 10. http://www.reddit.com/r/haskell/comments/10jk06/functional_tdd_a_clash_of_c… 11. http://www.reddit.com/r/haskell/comments/10d7gx/dovetailing/ 12. http://www.reddit.com/r/haskell/comments/10d7gx/dovetailing/ 13. http://www.reddit.com/r/haskell/comments/10lyaw/oh_the_fun_of_trying_to_ins… 14. http://www.reddit.com/r/haskell/comments/10lyaw/oh_the_fun_of_trying_to_ins… 15. http://www.cs.indiana.edu/~sabry/papers/yield-pp.pdf 16. http://www.reddit.com/r/haskell/comments/10nsmw/lazy_v_yield_incremental_li… 17. https://gist.github.com/3761252 18. http://www.reddit.com/r/haskell/comments/10c8ld/call_for_discussion_overloa… 19. http://www.reddit.com/r/haskell/comments/10gcpp/real_world_haskell_is_there… 20. http://www.reddit.com/r/haskell/comments/10gcpp/real_world_haskell_is_there… 21. http://blog.edwardamsden.com/2012/09/video-introduction-to-functional.html 22. http://www.reddit.com/r/haskell/comments/10nho4/introduction_to_functional_… 23. http://stackoverflow.com/questions/12653787/what-optimizations-can-ghc-be-e… 24. http://stackoverflow.com/questions/12617664/a-simple-example-showing-that-i… 25. http://stackoverflow.com/questions/12628700/are-there-any-good-use-cases-fo… 26. http://stackoverflow.com/questions/12617916/why-is-listt-monad-transformer-… 27. http://stackoverflow.com/questions/12571205/how-to-build-and-use-a-bootstra… 28. http://stackoverflow.com/questions/12589765/continuation-passing-style-repr… 29. http://stackoverflow.com/questions/12645254/ghc-code-generation-for-type-cl… 30. http://stackoverflow.com/questions/12651146/why-inductive-datatypes-forbid-… 31. http://stackoverflow.com/questions/12571161/are-functions-of-arity-n-really… 32. http://stackoverflow.com/questions/12590202/setting-socket-options-on-osx

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Re: [Haskell-cafe] Explicitly passing an argument to an arrow
by Alfredo Di Napoli 03 Oct '12

03 Oct '12

I see. There exists an equivalent version but more generic? Just out of curiosity, I'm still pretty new to arrows, as you may have read :) Thanks, A. On 3 October 2012 15:59, Ertugrul Söylemez <es(a)ertes.de> wrote: > Alfredo Di Napoli <alfredo.dinapoli(a)gmail.com> wrote: > > > Solution was simple, after reading that functions are arrows: > > > > swapA' t = (swapFirst >>> swapSecond) (t,t) > > where > > swapFirst = first snd > > swapSecond = second fst > > No, that's not a solution, unless a = (->) is acceptable. This code is > specific to the function arrow. > > > Greets, > Ertugrul > > -- > Key-ID: E5DD8D11 "Ertugrul Soeylemez <es(a)ertes.de>" > FPrint: BD28 3E3F BE63 BADD 4157 9134 D56A 37FA E5DD 8D11 > Keysrv: hkp://subkeys.pgp.net/ >

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Explicitly passing an argument to an arrow
by Alfredo Di Napoli 03 Oct '12

03 Oct '12

Hello everyone, sorry for the dumb question but I'm wrapping my head around arrow just from this morning. Consider this toy function to swap argument of a tuple: swapA' :: (Arrow a) => a ((b,c), (b,c)) (c,b) swapA' = swapFirst >>> swapSecond where swapFirst = first $ arr snd swapSecond = second $ arr fst It works but requires to pass a tuple of tuple, namely ((b,c), (b,c)). How can I explicitly pass my tuple of tuple to swapFirst so I can simply invoke swapA' (1,2) and get the correct result? ps. I know that swap can be easily and elegantly be written as: swap = snd &&& fst but the point of the exercise was to experiment with first, second and arrow concatenation Thanks in advance, Alfredo

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Data.Text and quasi quoting
by Jake Wheat 03 Oct '12

03 Oct '12

Hello, I have a parser which parses to an ast which contains Text values. I am trying to use this parser with quasiquoting, but the implementation of Data for Text is incomplete. I've attached a smallish test case, when I try to compile Text.hs I get: Text.hs:17:9: Exception when trying to run compile-time code: Data.Text.Text.toConstr Code: Language.Haskell.TH.Quote.quoteExp expr " test " Is there a way to get this working? Thanks, Jake.

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Strange things with type literals of kind Nat
by Troels Henriksen 03 Oct '12

03 Oct '12

Consider the following definition of lists with a statically determined minimum length. data Nat' = Z | S Nat' data NList (n::Nat') a where Rest :: [a] -> NList Z a (:>) :: a -> NList n a -> NList (S n) a uncons :: NList (S n) a -> (a, NList n a) uncons (x :> xs) = (x, xs) This works fine. Now consider an implementation using the new type literals in GHC: data NList (n::Nat) a where Rest :: NList 0 [a] (:>) :: a -> NList n a -> NList (n+1) a uncons :: NList (n+1) a -> NList n a uncons (x :> l) = l This gives a type error: /home/athas/code/nonempty_lists.hs:41:19: Could not deduce (n1 ~ n) from the context ((n + 1) ~ (n1 + 1)) bound by a pattern with constructor :> :: forall a (n :: Nat). a -> NList n a -> NList (n + 1) a, in an equation for `uncons' at /home/athas/code/nonempty_lists.hs:41:9-14 `n1' is a rigid type variable bound by a pattern with constructor :> :: forall a (n :: Nat). a -> NList n a -> NList (n + 1) a, in an equation for `uncons' at /home/athas/code/nonempty_lists.hs:41:9 `n' is a rigid type variable bound by the type signature for uncons :: NList (n + 1) a -> NList n a at /home/athas/code/nonempty_lists.hs:40:11 Expected type: NList n a Actual type: NList n1 a In the expression: l In an equation for `uncons': uncons (x :> l) = l I don't understand why GHC cannot infer that the two types are the same. My guess is that 'n+1' is not "structural" to GHC in the same way that 'S n' is. The page http://hackage.haskell.org/trac/ghc/wiki/TypeNats/MatchingOnNats mentions that "type level numbers of kind Nat have no structure", which seems to support my suspicion. What's the complete story, though? -- \ Troels /\ Henriksen

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ANNOUNCE: cmaes, a generic optimizer
by Takayuki Muranushi 02 Oct '12

02 Oct '12

Hello, I'm pleased to announce "cmaes" http://hackage.haskell.org/package/cmaes , a library for optimization (in mathematical sense). That is, given a function f :: a->Double, to find x of type a so that (f x) is as small as possible. CMA-ES is an optimization algorithm developed by Nikolaus Hansen and his folks http://www.lri.fr/~hansen/cmaesintro.html . It is a stochastic approach, it does not require gradient information. These properties makes it robust against noise and makes it widely applicable. The cmaes packages is a Haskell wrapper to their elaborate Python code. The robustness of CMA-ES and generic programming put together, you can now describe optimization problems for various types of functions just in one line. CMA-ES is not a silver bullet, of course, but it's been working fine for many application including mine, and I hope this library will drastically reduce the initial cost of trying one for you. Type will tell what cmaes can do. Please refer to the running document http://hackage.haskell.org/packages/archive/cmaes/0.2.1/doc/html/Numeric-Op… . You can optimize functions of type ([Double]-> Double) and ([Double]-> IO Double) , that's usual. Next up, you can optimize (Traversable t => t Double-> Double) and (Traversable t => t Double-> IO Double) where you are pretty sure about which parameter you're tuning. Finally, you can do (Data a => a -> Double) and (Data a => a -> IO Double) , where SYB will find the hidden Doubles to be optimized over for you. It was my joy to see optimization of function of type such as (Triangle -> Double) works out of the box. Any comments, suggestions, bug reports are welcome! Thanks, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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Copying a syntax tree
by Sergey Mironov 02 Oct '12

02 Oct '12

Hi! I have a syntax tree defined like this: > > data A a = A Int Int (B a) > > data B a = B String String (C a) > > data C a = C Int Int (D a) > and so on, all the data are parametrized with a type variable. This variable is actually used as a field in the very end of a hierarchy: > > data D a = D Int a > Now I have to write a function which would copy (A Int) to (A String). Is it possible to do so using TH/syb without writing copyA (A i1 i2 b) = A i1 i2 (copyB b) copyB = ... copyC = ... .. copyD (D i a) = D i (show a) ? Could you provide me with a hint? Thanks, Sergey

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Possible bug in Criterion or Statistics package
by Till Berger 02 Oct '12

02 Oct '12

Dear all, I may have stumbled upon a bug in the Criterion package. When running the attached Haskell program (Benchmark.hs, a simple test case) on multiple cores (with +RTS -N, +RTS -N2, +RTS -N3 etc.) it sooner or later crashes with the following exception: Benchmark: thread blocked indefinitely in an MVar operation With profiling support enabled and run with the xc flag I get the following output before the crash: *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main, called from Main.CAF --> evaluated by: Main.main, called from Main.CAF *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main *** Exception (reporting due to +RTS -xc): (THUNK_STATIC), stack trace: Statistics.Resampling.Bootstrap.bootstrapBCA, called from Main.main, called from Main.CAF I have tested this with GHC versions 7.0.4 and 7.4.2 and Criterion 0.6.0.1. So I am not sure if this is a bug in Criterion itself, the Statistics package or any dependency or if I am doing something obviously wrong. I would be grateful if someone could look into this as it is holding me back from using Criterion for benchmarking my code. Regards, Till Berger

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ANNOUNCE: Sylvia, a lambda calculus visualizer
by Chris Wong 02 Oct '12

02 Oct '12

Hello all Some of you in the audience may have read Dave Keenan's paper, [To Dissect a Mockingbird][]. A subset of that may have wondered if it was possible to generate those pretty pictures programmatically. For that subset, I can answer to you -- yes, yes you can. [To Dissect a Mockingbird]: http://dkeenan.com/Lambda/ Sylvia is a lambda calculus visualizer. It takes in an expression in the untyped lambda calculus and spits out a pretty picture. This is still in very early alpha, but it renders a fair number of combinators correctly. I plan to add animated reduction (once I figure out how to do it), and eventually develop this into a sandbox game of some sort. I'm hoping to get some comments and ideas on how I can take it from here. Obligatory links ---------------- Hackage: http://hackage.haskell.org/package/sylvia Source: https://github.com/lfairy/sylvia Documentation: https://github.com/lfairy/sylvia/wiki ----------------

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Monads
by Vasili I. Galchin 02 Oct '12

02 Oct '12

Hello, I would an examples of monads that are pure, i.e. no side-effects. Thank you, Vasili

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List indexer
by Alex Stangl 02 Oct '12

02 Oct '12

Hi, I have written a small "wrapper" to speed random-access to a list. The usage scenario I have in mind is a "stream" computation yielding an infinite list, and I want to randomly access elements w/o having to traverse the entire list from the beginning for each access. I suspected something similar must already exist, but nothing I looked at seemed to do the trick. IntMap seems to want a finite input list. Ditto for the various array types, except possibly dynamic array. Attached is the list indexer I came up with, and a small test program (I swap the commented-out lines to switch btw. list & list index tests). I am interested to hear any feedback on this -- whether it duplicates something that already exists, or whether there's a better approach, and comments on the code, etc. Also if somebody can suggest a better name (so as not to overlay the word index too much.) I'll publish it on hackage (or at least github) if people think it's useful. It sped up the program I initally wrote it for enormously. Thanks, Alex

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Compiler jobs
by Tom Hawkins 01 Oct '12

01 Oct '12

Hi, BAE Systems has some open positions for principal level Haskell programmers with compiler experience. Our DARPA project[1] is building a compiler for a new programming language with dynamic information flow control, targeting a custom ISA with hardware enforced information security. On the project, we are partnered with academics from the University of Pennsylvania, Harvard, and Northeastern University. Send a resume If interested. Thanks! -Tom [1] http://www.crash-safe.org/

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ANN: monadLib-3.7.1 on Hackage
by Iavor Diatchki 01 Oct '12

01 Oct '12

Hello, I am pleased to announce the availability of monadLib-3.7.1 on Hackage. MonadLib is a library intended to help programmers to quickly and easily construct various monads. The library has support for a wide range of effects: threading state, read-only variables, collecting output, exception handling, back-tracking computation, and working with explicit continuations. It provides similar functionality to libraries such as MTL, but it has a simpler interface and improved functionality for a number of the transformers. The main change in this version is improved support for working with continuations: now we support invoking the same continuation in contexts that have different types. Happy hacking! -Iavor

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Is inspectable recursion in Arrows possible?
by Alessandro Vermeulen 01 Oct '12

01 Oct '12

Dear all, I am trying to find a way to translate normal recursive notation such as the |fib| function below to an arrow, retaining as much of the structure of the recursive notation as possible. In addition I would like to inspect the arrow. For this I created a datatype containing a constructor for each Arrow{..} class: Fib: > fib 0 = 0 > fib 1 = 1 > fib n = fib (n-2) + fib (n-1) My R datatype, the instances for this datatype consist of the mapping to the appropriate constructor: > data R x y where > -- Category > Id :: R a a > Comp :: R b c -> R a b -> R a c > -- Arrow > Arr :: (a -> b) -> R a b > Split :: R b c -> R b' c' -> R (b,b') (c,c') > Cache :: (a -> a -> Bool) -> R a a > -- ArrowChoice > Choice :: R b c -> R b' c' -> R (Either b b') (Either c c') > -- ArrowLoop > Loop :: R (b, d) (c, d) -> R b c > -- ArrowApply > Apply :: R (R b c, b) c Translating the |fib| function from above first resulted in the following definition. It is not allowed however due to the proc n on the RHS of the declaration for |fibz|. I know that the grammar of the Arrow Notation prevents this, but what is the underlying reason for this? > fib' :: (ArrowChoice r, ArrowLoop r) => r Int Int > fib' = proc x -> do > rec fibz <- proc n -> case n of > 0 -> returnA -< 0 > 1 -> returnA -< 1 > n' -> do l <- fibz -< (n'-2) > r <- fibz -< (n'-1) > returnA -< (l+r) > fibz -<< x Rewriting the function above to use a let statement compiles. However, here my second problem arises. I want to be able to inspect the recursion where it happens. However, in this case |fibz| is an infinite tree. I would like to capture the recursion into fibz, I hoped the rec would help me with that in combination with |loop| but maybe I am wrong? > fib'' :: (ArrowChoice r, ArrowLoop r, ArrowApply r) => r Int Int > fib'' = proc x -> do > let fibz = proc n -> case n of > 0 -> returnA -< 0 > 1 -> returnA -< 1 > n' -> do l <- fibz -< (n'-2) > r <- fibz -< (n'-1) > returnA -< (l+r) > fibz -<< x Basically, is it possible to observe this kind of recursion? (Perhaps even within the boundaries of Arrow Notation) I could perhaps add another constructor like fix. Any thoughts on this? Cheers, Alessandro Vermeulen

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One of the new buzz phrases is "Event-Sourcing"; is Haskell suitable for this?
by KC 01 Oct '12

01 Oct '12

http://martinfowler.com/eaaDev/EventSourcing.html http://martinfowler.com/articles/lmax.html -- -- Regards, KC

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Re: [Haskell-cafe] Call for discussion: OverloadedLists extension
by George Giorgidze 30 Sep '12

30 Sep '12

Hi Michael, Here at the University of Tübingen, I am co-supervising (together with Jeroen Weijers) a student project implementing the OverloadedLists extension for GHC. Achim Krause is the student who is working on the project. We took into consideration earlier discussions on this topic [1,2] before embarking on the project. Achim has worked on two approaches. The first approach is very simple, both from the user's and the extension implementor's perspective (it follows the implementation of OverloadedStrings closely) and typechecks and desugars lists like [] ; [x,y,z] ; ['a' .. 'z'] ; as fromList [] ; fromList [x,y,z] ; fromList ['a' .. 'z'] ; where fromList is whatever is in scope with that name. That said, we do provide the FromList type class that can be used to overload fromList. In the following I give the definition of the class, as well as, example instances: class FromList l where type Item l fromList :: [Item l] -> l instance FromList [a] where type Item [a] = a fromList = id instance (Ord a) => FromList (Set a) where type Item (Set a) = a fromList = Set.fromList instance (Ord k) => FromList (Map k v) where type Item (Map k v) = (k,v) fromList = Map.fromList instance FromList (IntMap v) where type Item (IntMap v) = (Int,v) fromList = IntMap.fromList instance FromList Text where type Item Text = Char fromList = Text.pack This approach has already been implemented by Achim as patch against GHC head. This approach is very simple, but can be inefficient as it may result into unnecessary construction of lists at runtime. This can be a serious issue when constructing large structures from arithmetic sequences (e.g., from the [ .. ] notation) or when using non-literal expressions (e.g., variables) inside the square brackets. Our second approach to OverloadedLists is to avoid the construction of lists altogether. By typechecking and desugaring lists like [] ; [x,y,z] ; ['a' .. 'z'] ; as mempty ; singleton x `mappend` singleton y `mappend` singleton z ; genericEnumFromTo 'a' 'z' ; We provide the Singleton and GenericEnum type classes for overloading singleton and genericEnum(..) functions. In the following, I give the definitions of the classes, as well as, example instances: -- Singleton class class Singleton l where type SingletonItem l singleton :: SingletonItem l -> l -- Singleton instances instance Singleton [a] where type SingletonItem [a] = a singleton a = [a] instance (Ord a) => Singleton (Set a) where type SingletonItem (Set a) = a singleton = Set.singleton instance (Ord k) => Singleton (Map k v) where type SingletonItem (Map k v) = (k,v) singleton (k,v) = Map.singleton k v instance Singleton (IntMap v) where type SingletonItem (IntMap v) = (Int,v) singleton (k,v) = IntMap.singleton k v instance Singleton Text where type SingletonItem Text = Char singleton = Text.singleton -- GenericEnum class class GenericEnum l where type EnumItem l genericEnumFrom :: EnumItem l -> l genericEnumFromThen :: EnumItem l -> EnumItem l -> l genericEnumFromTo :: EnumItem l -> EnumItem l -> l genericEnumFromThenTo :: EnumItem l -> EnumItem l -> EnumItem l -> l -- GenericEnum instances instance (Enum a) => GenericEnum [a] where type EnumItem [a] = a genericEnumFrom = enumFrom genericEnumFromThen = enumFromThen genericEnumFromTo = enumFromTo genericEnumFromThenTo = enumFromThenTo instance (Ord a,Enum a) => GenericEnum (Set a) where type EnumItem (Set a) = a genericEnumFrom a = Set.fromList (enumFrom a) genericEnumFromThen a b = Set.fromList (enumFromThen a b) genericEnumFromTo a b = Set.fromList (enumFromTo a b) genericEnumFromThenTo a b c = Set.fromList (enumFromThenTo a b c) instance (Ord k,Enum (k,v)) => GenericEnum (Map k v) where type EnumItem (Map k v) = (k,v) genericEnumFrom a = Map.fromList (enumFrom a) genericEnumFromThen a b = Map.fromList (enumFromThen a b) genericEnumFromTo a b = Map.fromList (enumFromTo a b) genericEnumFromThenTo a b c = Map.fromList (enumFromThenTo a b c) instance (Enum (Int,v)) => GenericEnum (IntMap v) where type EnumItem (IntMap v) = (Int,v) genericEnumFrom a = IntMap.fromList (enumFrom a) genericEnumFromThen a b = IntMap.fromList (enumFromThen a b) genericEnumFromTo a b = IntMap.fromList (enumFromTo a b) genericEnumFromThenTo a b c = IntMap.fromList (enumFromThenTo a b c) instance GenericEnum Text where type EnumItem Text = Char genericEnumFrom a = Text.pack (enumFrom a) genericEnumFromThen a b = Text.pack (enumFromThen a b) genericEnumFromTo a b = Text.pack (enumFromTo a b) genericEnumFromThenTo a b c = Text.pack (enumFromThenTo a b c) Note that the GenericEnum instances can be implemented more efficiently, but for now I give simple definitions that go through lists. Our second approach avoids the construction of intermediate lists at runtime and directly constructs the target data structure for which the list notation is used. We will release GHC patches for both approaches, meanwhile the feedback from the community on the approaches that we took would be very much appreciated. Which one those would you prefer? or would you suggest a different one. Note that we intend to make fromList in the first approach and singleton, genericEnum(..), mempty and mapped rebindable. This means that the definitions of the type classes that overload this functions can be easily changed. Having said that, altering the changes that Achim already made to the GHC source code (including typechecking and desugaring rules) will be more work and we hope that one of the approaches that we took will be acceptable for the community. Cheers, George [1] http://www.mail-archive.com/glasgow-haskell-users@haskell.org/msg20447.html [2] http://www.mail-archive.com/glasgow-haskell-users@haskell.org/msg20518.html

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If I want to make a pure Haskell eigenvalue, etc. package without calling to LAPACK, etc ...
by KC 30 Sep '12

30 Sep '12

What are some good basis (pun intended) to start from? e.g. REPA? or ....? -- -- Regards, KC

1 0

ANNOUNCE: bindings-gobject-0.4
by Yuras Shumovich 29 Sep '12

29 Sep '12

Hello, I uploaded new release of bindings-gobject, low level binding to gobject library: http://hackage.haskell.org/package/bindings-gobject-0.4 (I maintain it now) Now it exposes internals of GObject and GObjectClass, so it is possible to create custom GObject subclasses from haskell land. Source repository contains basic example: https://github.com/Yuras/bindings-gobject/blob/master/examples/custom-objec… The use case I'm personally is interested in is implementing custom SourceCompletionProvider for gtksourceview2. I hope it will be useful for other applications. Right now we don't (afaik) have low level bindings to gtk and gtksourceview. But once we will have them, it will be possible to create custom controls in haskell and even expose them to C (C++, python, etc) land. Thanks, Yuras

1 0

Class constraints with "free" type variables and fundeps
by Francesco Mazzoli 29 Sep '12

29 Sep '12

I would expect this to work, maybe with some additional notation (a la ScopedTypeVariables) {-# LANGUAGE FunctionalDependencies #-} {-# LANGUAGE MultiParamTypeClasses #-} class Foo a b | a -> b class Foo a b => Bar a where foo :: a -> b -> c The type family equivalent works as expected: {-# LANGUAGE TypeFamilies #-} class Foo a where type T a :: * class Bar a where foo :: a -> T a -> c I can't use type families because the `Foo' I'm using is in an external library. Is there any way to achieve what I want without adding `b' to `Bar'? -- Francesco * Often in error, never in doubt

3 5

Discovery of unnecessary build-depends
by Jason Whittle 29 Sep '12

29 Sep '12

Is there a tool available that will tell me if the cabal file for my library or application has any unnecessary build-depends? I have a habit of developing new modules within an application and then moving them out to separate libraries. For instance, I might move module Foo out of my application and into a new or existing library package. If Foo was the only module in my application to explicitly import any modules from the Bar package, I would like to know that I can now remove the dependency on Bar from my application's build-depends in the cabal file. My strategy right now would be to search my whole application for other imports of any of the modules from the Bar package, or just remove Bar from build-depends and check if the application still compiles. Neither solution is particularly scalable or satisfying. Cheers, Jason Whittle

2 1

Re: [Haskell-cafe] Class constraints with "free" type variables and fundeps
by Francesco Mazzoli 29 Sep '12

29 Sep '12

CCing the list back. At Fri, 28 Sep 2012 13:30:52 -0700, Alexander Solla wrote: > What is the problem, exactly? It looks to me like UndecidableInstances and > ScopedTypeVariables (on foo, or its arguments) would be enough. I'm not sure what you mean. I don't see the need for UndecidableInstances, and there is no way I can see to bring the `b' into scope in the type sig for foo, ScopedTypeVariables or not - unless I'm missing something. > Also note that as stated, foo's type "is a bottom" (more specifically, is a > function onto bottom, since c is free in the class, and so foo must be > parametrically polymorphic in its return type, and so is devoid of "real" > Haskell values). Hopefully that is just an artefact of the translation to > Foo's and Bar's. Yeah the type for `foo' is irrelevant, I just needed to put something there. -- Francesco * Often in error, never in doubt

2 6

ticketimer - haskell project on Gruender-Garage contest!
by Anton Kholomiov 28 Sep '12

28 Sep '12

I'd like to announce ticketimer.com -- it's a website that is not done yet. With ticketimer you can choose films for your local cinema. People can buy tickets in advance and promote the films they like. Do we need to eat this blockbuster stuff all the time? See how to vote for a change on http://www.indiegogo.com/ticketimer?a=1301421 We are going to use Yesod as a main weapon. Anton

1 0

GHC 6.13 and GHC 7.6 in parallel on Linux
by spacestation＠venussociety.org 28 Sep '12

28 Sep '12

Hello to everyone, First of all sorry if my question posted to this list is maybe OFF TOPIC. Myself is using Haskell from time to time on WIN XP and OpenSuse system. So far without a problem using RWH book as reference and 6.12 version with a lot of modules installed via cabal. Also a Hugs install from source on my OpenSuse system only caused minor problems.setting a symlink solved the minor problem. But what surprised me totally was that Hugs/Cabal and (or ) GHC are using shared libraries and Hugs install interfered with cabal/ghc. I thought that both systems are totally independent from each other.Hugs a Haskell code interpreter GHV a Haskell code compiler. But now my question : How is it possible to run 2 different versions of GHC on the same computer ( OS , in my case OpenSuse 6.1x and hopefully 7.6 ) Any search for "running GHC versions in parallel only returned results for the DPH extensions wiki but not how to run two GHC versions on the same OS. http://www.haskell.org/haskellwiki/GHC/Data_Parallel_Haskell http://www.haskell.org/haskellwiki/Parallel So any feedback welcome Rgds Gottfried

4 4

An easy way to represent and modify graphs?
by Takayuki Muranushi 28 Sep '12

28 Sep '12

tl;dr: I would like to apply genetic algorithm to directed acyclic graphs that represents arithmetic operations. Thank you for hosts, speakers and all participants in ICFP2012. I had chance to make a lot of questions (c.f. [prize.jpg]) and receive lots of stimulating ideas. I have been working on automated parallel code generation and automated tuning for hydrodynamics http://iopscience.iop.org/1749-4699/5/1/015003 ; one of my next target is model acquisition in robotics: http://creativemachines.cornell.edu/sites/default/files/moriguchi11.pdf ; both deal with computations represented in forms of directed acyclic graphs c.f. [graph-1.png] [graph-2.png]. We would like to describe them easily in Haskell, but also to mutate (add random changes) and crossover (create a new graph by mixing other two graphs.) In my previous work, I used Monad to generate graphs and used the Functional Graph Library to explicitly modify the graphs. Now I have learned that there are more elegant methods to describe the computations with sharing. But are those method also enables easy editing of the graphs? That's my question. In [data-reify.hs] I tried Observed Sharing by Gill09. Apparently, sharing is lost once we try to modify the graph. $ runhaskell data-reify.hs let [(1,GraphMul 2 2),(2,GraphAdd 3 3),(3,GraphMul 4 5),(5,GraphVar "X"),(4,GraphVal 40)] in 1 let [(1,GraphMul 2 9),(9,GraphAdd 10 13),(13,GraphMul 14 15),(15,GraphVar "X"),...,(5,GraphVar "X"),(4,GraphVal 42)] in 1 In [GenericMain.hs] I tried PHOAS by Oliveria & Cook 2012. $ runhaskell GenericMain.hs Mu ( a => Fork "Add"(Fork "Sub"(b) (Empty)) (c) b => Fork "Mul"(c) (c) c => Fork "40"(Empty) (Empty) d => Fork "X"(Empty) (Empty) ) Mu ( a => Fork "Add"(Fork "Sub"(b) (Empty)) (c) b => Fork "Mul"(c) (c) c => Fork "42"(Empty) (Empty) d => Fork "X"(Empty) (Empty) ) This time, I can modify a node without loosing the sharing. How about edges? Can you write operations such as "Choose one of the edges that point to (c) and make it point to (d)" ? Is there any good methods to describe and modify structures that has sharing (if it helps, I don't need loops.)? Or, after all is the best way explicit manipulation of the graph structure? Both information are helpful to me. Thanks in advance, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

2 3

haddock changes between 2.10 and 2.12?
by Edward Kmett 27 Sep '12

27 Sep '12

Was there some significant change/bug introduced to haddock made between 2.10 and 2.12? When I look at the haddocks for kan-extensions 3.1: Data.Functor.Yoneda<http://hackage.haskell.org/packages/archive/kan-extensions/3.1/doc/html/Dat…> which purports to have been built by haddock 2.12 it shows all sorts of spurious superclass constraints on almost every instance that should have simplified out, but going back to kan-extensions-2.7: Data.Functor.Yoneda<http://hackage.haskell.org/packages/archive/kan-extensions/2.7/doc/html/Dat…> none of this extra noise is present -- and the code in question hasn't been changed in the interim, so I'm pretty sure its not on my end. -Edward

2 1

Can not configure haskell platform
by s9gf4ult＠gmail.com 27 Sep '12

27 Sep '12

> 13:53 razor@localhost /home/razor/tmp/haskell-platform-2012.2.0.0 % ./configure --prefix=/home/razor/.haskell > ************************************************** > * > * Haskell Platform Source Installer > * > * For the Haskell Platform 2012.2.0.0 and GHC 7.4.1 > * > ************************************************** > checking build system type... scripts/config.guess: line 156: 34:LIBC=gnu: command not found > Invalid configuration `x86_64-unknown-linux-': machine `x86_64-unknown-linux' not recognized > configure: error: /bin/sh scripts/config.sub x86_64-unknown-linux- failed ghc is available in PATH, google does not google. How to fix ?

1 1

Haskell Weekly News: Issue 245
by Daniel Santa Cruz 27 Sep '12

27 Sep '12

Welcome to issue 245 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of September 16 to September 22, 2012. Quotes of the Week * dmwit: You have your intuition correct, despite all the pedantry in the room. * Cale: THIS CODE IS EXCELLENT CODE, I AM AN EXCELLENT ENTERPRISE PROGRAMMER ACCOMPLISHING REAL WORLD TASKS USING PATTERNS LIKE I READ ABOUT IN ALL THE JAVA BLOGS * byorgey: HASKELL is a pure, functional, strongly typed language featuring a name with all capitals, so it is suitable for beginners * ciaranm: a comonad is just a comonoid in the category of endofunctors. what's the coproblem? * dmwit: ISAAC -- ignore success at all costs * merijn: If you're not gonna use the types [by writing them] you might as well just write python * Cale: The real reason for the installed size of GHC being so large is that in the background it secretly downloads a video of SPJ riding a unicycle. Top Reddit Stories * First commercial Haskell-powered (game logic) computer game? (Nikki and the Robots Story Episodes) Domain: buy.joyridelabs.de, Score: 57, Comments: 27 On Reddit: [1] http://goo.gl/hngXz Original: [2] http://goo.gl/Y5Oax * Common misconceptions about Safe Haskell Domain: blog.ezyang.com, Score: 49, Comments: 23 On Reddit: [3] http://goo.gl/NBSx7 Original: [4] http://goo.gl/43i6n * The MonadTrans class is missing a method Domain: haskellforall.com, Score: 44, Comments: 32 On Reddit: [5] http://goo.gl/okxAX Original: [6] http://goo.gl/GnvzB * The Haskell Implementors' Workshop videos and slides available online Domain: haskell.org, Score: 43, Comments: 4 On Reddit: [7] http://goo.gl/LuORg Original: [8] http://goo.gl/pEvww * Writing javascript games in haskell : breakout Domain: jshaskell.blogspot.com, Score: 39, Comments: 10 On Reddit: [9] http://goo.gl/Dv8QK Original: [10] http://goo.gl/lXxsS * HIW 2012. Simon Peyton Jones: GHC Status Update. - YouTube Domain: youtube.com, Score: 37, Comments: 12 On Reddit: [11] http://goo.gl/3GEZK Original: [12] http://goo.gl/j7al9 * Comparing Snap's and Yesod's Template Languages Heist and Hamlet Domain: gist.github.com, Score: 36, Comments: 33 On Reddit: [13] http://goo.gl/AzaVV Original: [14] http://goo.gl/StRaF * Request: Tutorial on debugging using stack trace features of GHC 7.4 Domain: self.haskell, Score: 31, Comments: 6 On Reddit: [15] http://goo.gl/DcR1C Original: [16] http://goo.gl/DcR1C * A kind for regions Domain: monoidal.blogspot.com, Score: 29, Comments: 27 On Reddit: [17] http://goo.gl/4aBe7 Original: [18] http://goo.gl/tBDDW * [screencast] Quick introduction to Haskell using Gtk and the web Domain: vimeo.com, Score: 28, Comments: 4 On Reddit: [19] http://goo.gl/oi2IB Original: [20] http://goo.gl/LOkcf * Warp optimization - Sending header and body at once Domain: yesodweb.com, Score: 27, Comments: 17 On Reddit: [21] http://goo.gl/wstwx Original: [22] http://goo.gl/opsHb * Runtime Codegeneration in Haskell: Harpy vs. LLVM Domain: blog.wien.tomnetworks.com, Score: 24, Comments: 10 On Reddit: [23] http://goo.gl/aD0hE Original: [24] http://goo.gl/jU3nf * Show off your Vim/Emacs config! Domain: self.haskell, Score: 24, Comments: 7 On Reddit: [25] http://goo.gl/jV6Sa Original: [26] http://goo.gl/jV6Sa * Elm 0.4: Graphics Upgrade! Markdown, better for games, new optimizations, minification, ... Domain: elm-lang.org, Score: 22, Comments: 1 On Reddit: [27] http://goo.gl/SJW29 Original: [28] http://goo.gl/2Q5Fg * Making your zipper disappear with Zippo Domain: brandon.si, Score: 22, Comments: 6 On Reddit: [29] http://goo.gl/MQg77 Original: [30] http://goo.gl/Ua8a9 * [ANNOUNCE] Juicy.Pixels 2.0 - With Gif file decoding Domain: hackage.haskell.org, Score: 20, Comments: 12 On Reddit: [31] http://goo.gl/kEKrE Original: [32] http://goo.gl/EVvBh * Videos from this the CUFP 2012 in Copenhagen are now up Domain: cufp.org, Score: 17, Comments: 2 On Reddit: [33] http://goo.gl/mpe8Z Original: [34] http://goo.gl/aCB9I * Haskell developer jobs at StanChart - London, Singapore and Tianjin Domain: haskellers.com, Score: 17, Comments: 6 On Reddit: [35] http://goo.gl/t7axk Original: [36] http://goo.gl/wZrV5 * New GHCJS Hello World (can you find the hidden game?) Domain: ghcjs.github.com, Score: 17, Comments: 10 On Reddit: [37] http://goo.gl/c4BeA Original: [38] http://goo.gl/Aqh1i Top StackOverflow Questions * “What part of Milner-Hindley do you not understand?” votes: 73, answers: 4 Read on SO: [39] http://goo.gl/Nossl * Matching on type level Nat in GHC 7.6 votes: 17, answers: 1 Read on SO: [40] http://goo.gl/YijoA * Threading extra state through a parser in Scala votes: 16, answers: 1 Read on SO: [41] http://goo.gl/8cYMQ * find all types that are instances of a typeclass votes: 13, answers: 1 Read on SO: [42] http://goo.gl/ywHUN * When to use various language pragmas and optimisations? votes: 13, answers: 2 Read on SO: [43] http://goo.gl/qqtID * What are these explicit “forall”s doing? votes: 12, answers: 1 Read on SO: [44] http://goo.gl/fO0kf * How does the ST monad work? votes: 11, answers: 2 Read on SO: [45] http://goo.gl/xxntD * What are the pros and cons of adding additional functions/combinators to a typeclass definition? votes: 10, answers: 2 Read on SO: [46] http://goo.gl/HhDgT * Type parameters constraints for instances of typeclasses with kind * -> * votes: 9, answers: 2 Read on SO: [47] http://goo.gl/bGEQW * Theoretically, is this a valid comonad instance for a list? votes: 9, answers: 2 Read on SO: [48] http://goo.gl/z4DI4 * In haskell how can I uppercase a unicode character with respect to current locale votes: 9, answers: 1 Read on SO: [49] http://goo.gl/enNwK * Does haskell erase types? votes: 8, answers: 2 Read on SO: [50] http://goo.gl/YP8Nn * Is there an Iteratee-like concept which pulls data from multiple sources? votes: 8, answers: 4 Read on SO: [51] http://goo.gl/AOV0Y Until next time, Daniel Santa Cruz References 1. http://buy.joyridelabs.de/ 2. http://www.reddit.com/r/haskell/comments/106ocf/first_commercial_haskellpow… 3. http://blog.ezyang.com/2012/09/common-misconceptions-about-safe-haskell/ 4. http://www.reddit.com/r/haskell/comments/10242c/common_misconceptions_about… 5. http://www.haskellforall.com/2012/09/the-monadtrans-class-is-missing-method… 6. http://www.reddit.com/r/haskell/comments/106xwf/the_monadtrans_class_is_mis… 7. http://www.haskell.org/haskellwiki/HaskellImplementorsWorkshop/2012#Program… 8. http://www.reddit.com/r/haskell/comments/102x4p/the_haskell_implementors_wo… 9. http://jshaskell.blogspot.com/2012/09/breakout.html 10. http://www.reddit.com/r/haskell/comments/109hsb/writing_javascript_games_in… 11. http://www.youtube.com/watch?v=2Pucbf8_hGQ&feature=plcp 12. http://www.reddit.com/r/haskell/comments/102t49/hiw_2012_simon_peyton_jones… 13. https://gist.github.com/3757918 14. http://www.reddit.com/r/haskell/comments/107v79/comparing_snaps_and_yesods_… 15. http://www.reddit.com/r/haskell/comments/104xa2/request_tutorial_on_debuggi… 16. http://www.reddit.com/r/haskell/comments/104xa2/request_tutorial_on_debuggi… 17. http://monoidal.blogspot.com/2012/09/a-kind-for-regions.html 18. http://www.reddit.com/r/haskell/comments/102v1x/a_kind_for_regions/ 19. https://vimeo.com/49918847 20. http://www.reddit.com/r/haskell/comments/10az3o/screencast_quick_introducti… 21. http://www.yesodweb.com/blog/2012/09/header-body 22. http://www.reddit.com/r/haskell/comments/104sx0/warp_optimization_sending_h… 23. http://blog.wien.tomnetworks.com/2012/09/17/runtime-codegeneration-in-haske… 24. http://www.reddit.com/r/haskell/comments/1014ct/runtime_codegeneration_in_h… 25. http://www.reddit.com/r/haskell/comments/10b20f/show_off_your_vimemacs_conf… 26. http://www.reddit.com/r/haskell/comments/10b20f/show_off_your_vimemacs_conf… 27. http://elm-lang.org/blog/announce/version-0.4.0.elm 28. http://www.reddit.com/r/haskell/comments/102zqr/elm_04_graphics_upgrade_mar… 29. http://brandon.si/code/making-your-zipper-disappear-with-zippo/ 30. http://www.reddit.com/r/haskell/comments/103rhu/making_your_zipper_disappea… 31. http://hackage.haskell.org/package/JuicyPixels-2.0 32. http://www.reddit.com/r/haskell/comments/zziwm/announce_juicypixels_20_with… 33. http://cufp.org/videos 34. http://www.reddit.com/r/haskell/comments/102rer/videos_from_this_the_cufp_2… 35. http://www.haskellers.com/jobs/19 36. http://www.reddit.com/r/haskell/comments/106wat/haskell_developer_jobs_at_s… 37. http://ghcjs.github.com/bin/ghcjs-hello.jsexe 38. http://www.reddit.com/r/haskell/comments/10b6h3/new_ghcjs_hello_world_can_y… 39. http://stackoverflow.com/questions/12532552/what-part-of-milner-hindley-do-… 40. http://stackoverflow.com/questions/12463474/matching-on-type-level-nat-in-g… 41. http://stackoverflow.com/questions/12487827/threading-extra-state-through-a… 42. http://stackoverflow.com/questions/12524034/find-all-types-that-are-instanc… 43. http://stackoverflow.com/questions/12534145/when-to-use-various-language-pr… 44. http://stackoverflow.com/questions/12519878/what-are-these-explicit-foralls… 45. http://stackoverflow.com/questions/12468622/how-does-the-st-monad-work 46. http://stackoverflow.com/questions/12515655/what-are-the-pros-and-cons-of-a… 47. http://stackoverflow.com/questions/12497542/type-parameters-constraints-for… 48. http://stackoverflow.com/questions/12537149/theoretically-is-this-a-valid-c… 49. http://stackoverflow.com/questions/12537377/in-haskell-how-can-i-uppercase-… 50. http://stackoverflow.com/questions/12468722/does-haskell-erase-types 51. http://stackoverflow.com/questions/12496654/is-there-an-iteratee-like-conce…

1 0

Haskell Programmers for Distributed Systems
by Richard Lyman 26 Sep '12

26 Sep '12

Hi folks, Parallel Scientific is looking for Haskell developers for distributed systems work. Parallel Scientific, Inc. is a Boulder, CO based funded startup company working in the area of scalable parallelization. We seek experienced Haskell programmers to build a sophisticated, ultra-scalable high availability framework to be used in conjunction with scientific applications, big data applications and I/O systems. We are primarily seeking candidates in Northern England, central France and/or Germany but candidates in other locations will be considered. Relocation to these places can be discussed. Parallel Scientific was founded by Dr Peter Braam in 2010. Peter formerly taught mathematics and computer science at Oxford and Carnegie Mellon. Then he contributed file systems to Linux and invented Lustre (which provides storage to 9 of the top 10 systems in the world). He ran several successful startups, and Parallel Scientific is run by an very experienced management team and board. Successful candidates will work in a modern virtual environment in their homes. Some travel may be required. We provide training in advanced processes for software design and implementation and domain specific knowledge. Required skills: Capability to design and implement sophisticated distributed algorithms (quorum, liveness, lock management, barriers) Very strong background in computer science or mathematics Experience with Haskell, or network and cluster programming in other functional languages (e.g. Erlang) Knowledge of systems programming and operating systems functionality Experience in one or more of the following areas is desirable: Experience in high availability clustering projects Experience with internals of sophisticated distributed I/O and/or database systems To apply, please send a resume to jobs(a)parsci.com. Thanks... Rich -- Rich Lyman richard.lyman(a)parsci.com

1 0

Partial statical linking
by Jason Dusek 26 Sep '12

26 Sep '12

Some time ago, I wrote to this list about making shared libraries with GHC, in such a way that the RTS was linked and ready to go. Recently, I've been looking a similar but, in a sense, opposite problem: linking Haskell executables with some of their non-Haskell dependencies, for distribution. I tried passing a few different sets of options to the linker through GHC, with -optl: -optl'-Wl,-r' -optl'-Wl,-r,-dy' -optl'-Wl,-static,-lffi,-lgmp,-dy' None of these had the desired effect. In the end, running GHC with -v and carefully editing the linker line produced the desired change (I have linked to and provided the diff below). The effect -optl seems to be to introduce options in the linker line just before -lHSrtsmain, which would seem to prevent one from linking libffi and libgmp differently. Is editing and storing away the linker script the best option at present for partially static linking? -- Jason Dusek () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments https://github.com/solidsnack/arx/commit/90ec5efdb0e991344aa9a4ad29456d466e… #@@ -122,10 +122,8 @@ # -lHSarray-0.3.0.2 \ # -lHSbase-4.3.1.0 \ # -lHSinteger-gmp-0.2.0.3 \ #- -lgmp \ # -lHSghc-prim-0.2.0.0 \ # -lHSrts \ #- -lffi \ # -lm \ # -lrt \ # -ldl \ #@@ -136,4 +134,7 @@ # -lgcc_s --no-as-needed \ # /usr/lib/gcc/x86_64-linux-gnu/4.6.1/crtend.o \ # /usr/lib/gcc/x86_64-linux-gnu/4.6.1/../../../x86_64-linux-gnu/crtn.o \ #+ -static \ #+ -lgmp \ #+ -lffi \

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Re: [Haskell-cafe] Church vs Boehm-Berarducci encoding of Lists
by oleg＠okmij.org 26 Sep '12

26 Sep '12

Dan Doel wrote: > >> P.S. It is actually possible to write zip function using Boehm-Berarducci > >> encoding: > >> http://okmij.org/ftp/Algorithms.html#zip-folds > > If you do, you might want to consider not using the above method, as I > seem to recall it doing an undesirable amount of extra work (repeated > O(n) tail). It is correct. The Boehm-Berarducci web page discusses at some extent the general inefficiency of the encoding, the need for repeated reflections and reifications for some (but not all) operations. That is why arithmetic on Church numerals is generally a bad idea. A much better encoding of numerals is what I call P-numerals http://okmij.org/ftp/Computation/lambda-calc.html#p-numerals It turns out, I have re-discovered them after Michel Parigot (so my name P-numerals is actually meaningful). Not only they are faster; one can _syntactically_ prove that PRED . SUCC is the identity. The general idea of course is Goedel's recursor R. R b a 0 = a R b a (Succ n) = b n (R b a n) which easily generalizes to lists. The enclosed code shows the list encoding that has constant-time cons, head, tail and trivially expressible fold and zip. Kim-Ee Yeoh wrote: > So properly speaking, tail and pred for Church-encoded lists and nats > are trial-and-error affairs. But the point is they need not be if we > use B-B encoding, which looks _exactly_ the same, except one gets a > citation link to a systematic procedure. > > So it looks like you're trying to set the record straight on who actually > did what. Exactly. Incidentally, there is more than one way to build a predecessor of Church numerals. Kleene's solution is not the only one. Many years ago I was thinking on this problem and designed a different predecessor: excerpted from http://okmij.org/ftp/Haskell/LC_neg.lhs One ad hoc way of defining a predecessor of a positive numeral predp cn+1 ==> cn is to represent "predp cn" as "cn f v" where f and v are so chosen that (f z) acts as if z == v then c0 else (succ z) We know that z can be either a numeral cn or a special value v. All Church numerals have a property that (cn combI) is combI: the identity combinator is a fixpoint of every numeral. Therefore, ((cn combI) (succ cn)) reduces to (succ cn). We only need to choose the value v in such a way that ((v I) (succ v)) yields c0. > predp = eval $ > c ^ c > # (z ^ (z # combI # (succ # z))) -- function f(z) > # (a ^ x ^ c0) -- value v {-# LANGUAGE Rank2Types #-} -- List represented with R newtype R x = R{unR :: forall w. -- b (x -> R x -> w -> w) -- a -> w -- result -> w} nil :: R x nil = R (\b a -> a) -- constant type cons :: x -> R x -> R x cons x r = R(\b a -> b x r (unR r b a)) -- constant time rhead :: R x -> x rhead (R fr) = fr (\x _ _ -> x) (error "head of the empty list") -- constant time rtail :: R x -> R x rtail (R fr) = fr (\_ r _ -> r) (error "tail of the empty list") -- fold rfold :: (x -> w -> w) -> w -> R x -> w rfold f z (R fr) = fr (\x _ w -> f x w) z -- zip is expressed via fold rzipWith :: (x -> y -> z) -> R x -> R y -> R z rzipWith f r1 r2 = rfold f' z r1 r2 where f' x tD = \r2 -> cons (f x (rhead r2)) (tD (rtail r2)) z = \_ -> nil -- tests toR :: [a] -> R a toR = foldr cons nil toL :: R a -> [a] toL = rfold (:) [] l1 = toR [1..10] l2 = toR "abcde" t1 = toL $ rtail l2 -- "bcde" t2 = toL $ rzipWith (,) l2 l1 -- [('a',1),('b',2),('c',3),('d',4),('e',5)]

4 6

Help me with space leaks
by Alexey Uimanov 26 Sep '12

26 Sep '12

Hello. I am trying to write some thing in haskell and i need fast storage to store and select this things from storage. https://github.com/s9gf4ult/projs/tree/master/haskell/teststorage I am writing simple testing package to determine my needs and select the fastest storage and i have encountered problems. Firstly i decided to check out postgresql-simple, but when i am doing "executeMany" i see space leaks, here is the picture http://bayimg.com/NAdjHAaeA http://bayimg.com/NADjKAAea I dont fully understand what is happening here, but i belive this is because of lazy consuming of "executeMany" or something. So things im trying to insert do not calculate one by one, but this is creating many thunks for calculate them. I would understand how to narrow the cause of this problem and create more strict function "executeMany" which would work in constant space (or space of data). Here the insertMany from postgresql-simple executeMany :: (ToRow q) => Connection -> Query -> [q] -> IO Int64 executeMany _ _ [] = return 0 executeMany conn q qs = do result <- exec conn =<< formatMany conn q qs finishExecute conn q result And this is not looks like a problem case go to formatMany formatMany :: (ToRow q) => Connection -> Query -> [q] -> IO ByteString formatMany _ q [] = fmtError "no rows supplied" q [] formatMany conn q@(Query template) qs = do case parseTemplate template of Just (before, qbits, after) -> do bs <- mapM (buildQuery conn q qbits . toRow) qs return . toByteString . mconcat $ fromByteString before : intersperse (fromChar ',') bs ++ [fromByteString after] Nothing -> fmtError "syntax error in multi-row template" q [] Here bs is the map of bs and must stay lazy evaluated and here we see mconcat which must be strict i think, and if i am right so problem must disapear whan i replace it with strict mconcat mconcat' :: (Monod a) => [a] -> a mconcat' [] = mempty mconcat' (x:xs) = x `seq` (mappend x $ mconcat' xs) but it doesnt. I assume i must write the same for "deepseq" to realy calculate each parameter up to value. But if so, this must touch several projects such as blaze and maybe bytestring because of the need to make NFData instances. How to solve this problem right - way ? Maybe haskell have elegant solution for this ?

1 0

How to take a minimum sub list that only contain certain number of elements of certain type?
by Magicloud Magiclouds 26 Sep '12

26 Sep '12

Hi, For example, I have an array [0..]. Now I want to take a sub list that starts from index 0, and only contain 4 odds, and is minimum result. The answer should be [0, 1, 2, 3, 4, 5, 6, 7]. How to do that? Combining lazy computing, I cannot figure out an efficient algorithm. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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Re: [Haskell-cafe] [Haskell] ANN: HERMIT for GHC 7.6
by Jay Sulzberger 25 Sep '12

25 Sep '12

[Note that original post was on the Haskell list, and not the Haskell-cafe list.] On Tue, 25 Sep 2012, Andrew Farmer <afarmer(a)ittc.ku.edu> wrote: > A new version of HERMIT (0.1.2.0) that is compatible with GHC 7.6 is now > available on hackage. This release has essentially the same functionality > as the 7.4-only pre-alpha release made before ICFP 2012. > > HERMIT (Haskell Equational Reasoning Model-to-Implementation Tunnel) is a > plugin for GHC that provides an interactive interface for applying > transformations directly to GHC's internal intermediate language, Core. > This plugin is part of a larger HERMIT toolkit, which is being developed at > the University of Kansas with the aims of supporting equational reasoning > and allowing custom optimizations to be applied without modifying either > GHC or the Haskell source code. > > Introduction to HERMIT via Neil Sculthorpe's Haskell Symposium 2012 talk: > > http://www.youtube.com/watch?v=x2QH3jJCJso > > Example transformations can be found in the examples folder, contained in > the cabal source package. > > Hackage: http://hackage.haskell.org/package/hermit > Github (for bug reports): https://github.com/ku-fpg/hermit > > Andrew Farmer Thank you and all the HERMIT Team for this! Does the HERMIT system provide an executable Haskell Core, that is, we can get an ordinary text file which can be executed by some version of GHC+HERMIT? oo--JS.

1 0

Re: [Haskell-cafe] [Haskell] Well-Typed and Skills Matter offer Haskell courses in London in October
by Claude Heiland-Allen 25 Sep '12

25 Sep '12

On 19/09/12 08:52, Andres Löh wrote: > Registration for all these events is open. I hope to see many > of you there. Is there an informal hangout without the £225 price-tag > > Cheers, > Andres >

3 5

in the vector library, why define unbox tuples > 2?
by Jeff Shaw 25 Sep '12

25 Sep '12

I'm looking at storing a data type with 7 fields in an unboxed vector, which means that I'll have to use GenUnboxTuple to create an instance for Unbox (a,b,c,d,e,f,g), but I was thinking that another solution is to use "instance (Unbox a, Unbox b) => Unbox (a,b)" recursively to create "instance Unbox (a,(b,(c,(d,(e,(f,g))))))". Is there any reason I shouldn't say something like, import qualified Data.Vector.Unboxed as Vector data X = X Int Int Int Int Int Int Int toTuple (X a b c d e f g) = (a,(b,(c,(d,(e,(f,g)))))) toVector = Vector.fromList . map toTuple I guess I'm wondering if there is a technical reason why I should use an explicit Unbox instance for 7-tuples, or if it's just a convenience to have Unbox defined for tuples > 2? Thanks, Jeff

1 0

Fwd: [Haskell] ANNOUNCE: MFlow-0.1.5
by Alberto G. Corona 25 Sep '12

25 Sep '12

Oh, I´m stupid. You mean web pages with multiple tabs.... I have not tested it. but each tab can be handled easily by a different server process.. or it can be handled in a single server process, like in a menu. For example, this code present different options, and the process renders different things depending on the response. The last option is a link to a different process, while the others( wlinks) are links that return back to the same process. The operator <|> is the applicative operator. a breakline is prepended to each link: data Ops= Ints | Strings | Actions | Ajax | Opt deriving(Typeable,Read, Show) mainf= do r <- ask $ wlink Ints (bold << "increase an Int") <|> br ++> wlink Strings (bold << "increase a String") <|> br ++> wlink Actions (bold << "Example of a string widget with an action") <|> br ++> wlink Ajax (bold << "Simple AJAX example") <|> br ++> wlink Opt (bold << "select options") <++ (br +++ linkShop) -- this is an ordinary XHtml link case r of Ints -> clickn 0 Strings -> clicks "1" Actions -> actions 1 Ajax -> ajaxsample Opt -> options mainf where linkShop= toHtml $ hotlink "shop" << "shopping" . Alberto 2012/9/18 Alberto G. Corona <agocorona(a)gmail.com>: > Hi Jake > > I don´t know what you mean with multiple tabs. The user management is > simple, anonymous clients are identified with a cookie. if the user > is logged (MFlow has widgets for logging-validation) the user is the > identifier. > > The state of a process is associated to the client identifier and to > the path invoked in the url requested. > > I don´t know if this answer your question.... > > Alberto > > 2012/9/18 Jake McArthur <jake.mcarthur(a)gmail.com>: >> This sounds really cool. >> >> How do you handle users having multiple tabs? >> >> On Tue, Sep 18, 2012 at 11:26 AM, Alberto G. Corona <agocorona(a)gmail.com> wrote: >>> Hi haskellers and specially the web developers. >>> >>> http://hackage.haskell.org/package/MFlow-0.1.5.3 >>> >>> MFlow is a is a Web framework with some unique, and I mean unique, >>> characteristics that I find exciting: >>> >>> - It is a Web application server that start and restart on-demand >>> stateful web server processes (not request.-response) >>> This means that all the page navigation can be coded in a single >>> procedure. This increases readability of the programmer code. I woul >>> call it >>> a anti-node.js. Buit usual request-response (stateless) server >>> processes are also allowed >>> >>> - When the process is invoqued as result of an URL request, the Web >>> app server not only restart the process but also recover its execution >>> state. The enclosing Workflow monad provides the thread state >>> persistence. There are state timeouts and process timeouts defined by >>> the programmer. Processes with no persistent state (transient) are >>> possible. >>> >>> -The user interface is made of widgets. They are formlets with added >>> formatting, attributes, validations, modifiers and callbacks, that >>> are composable, so the pieces are reusable and return type safe >>> responses to the calling process. Even the links are part of widgets >>> and return back type safe inputs at compile time to the calling server >>> process. Tho glue these components, ordinary applicative combinators >>> and other extra combinators are used. >>> >>> - The widgets and the communication don´t make assumptions about the >>> architecture, so it can be adapted to non-web environments. This >>> versions has interface for WAI-warp, Hack, Text.XHtml (xhtml) , and >>> Haskell Server Pages. >>> >>> -The widget rendering can be converted to ByteStrings automatically >>> with special combinators. A mix of widgets with different formats can >>> be combined in the same source file. For example Text.Html and HSP >>> (Haskell server pages) >>> >>> -These widgets can be cached, to avoid widget rendering on every interaction. >>> >>> -To handle the back button, and because the processes are stateful, >>> they can run backwards until the response match. This is transparent >>> for the programmer, thanks to the embedded FlowM monad. >>> >>> -All the code is in pure Haskell. No deployment, special scripts, >>> formats etc are necessary. >>> >>> -Besides automatic state persistence, TCache provides transactions and >>> user data persistence, that can be configured for SQL databases. >>> Default persistence in files permit very rapid prototyping. Just code >>> and run it with runghc. >>> >>> -Has AJAX support >>> >>> All of this sounds very complicated, but really it is simple!. Most of >>> these things are transparent. The resulting code is quite readable and >>> has very little plumbing! >>> >>> There is a non trivial example that some of these functionalities >>> embedded here that you can run: >>> >>> http://hackage.haskell.org/packages/archive/MFlow/0.1.5.3/doc/html/MFlow-Fo… >>> >>> Take a look and tell me your opinion. I hope that you find it as >>> exciting as me. >>> >>> I´m looking for people to collaborate in the development of MFlow. >>> >>> Although still it is experimental, it is being used in at least one >>> future commercial project. So I have te commitment to continue its >>> development. There are many examples in the documentation and in the >>> package. >>> >>> Alberto >>> >>> _______________________________________________ >>> Haskell mailing list >>> Haskell(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell

2 4

Getting started with Shake
by Richard Wallace 24 Sep '12

24 Sep '12

Hey all, I'm going to be working on a webapp in Haskell in the upcoming months and am thinking Shake would be a good fit - I'll need to do js, css, and probably some graphics processing as part of the build and would like to use Shake to automate deployment. I'm not entirely sure how to get started with Shake. I've used to make, jam, maven, sbt and other build tools in the past. All of these keep a build configuration in the project and use that to build the project. In sbt, which is most like Shake, the build configuration is written in Scala and compiled by sbt when you run the build. The result of the compilation is saved and reused unless it detects that the build source files changed. With shake I'm not sure exactly how to get started. Should I have a separate project where I create the build system for the webapp? Or can I setup something similar to sbt? Also, how do I handle dependencies with shake? cabal will pull in packages from hackage and do the needful, is there anything in shake to do the same? If not, how is it normally done? Thanks, Rich

2 1

ANN: New wiki page and mailing list for combinatorics and Haskell
by Brent Yorgey 24 Sep '12

24 Sep '12

Quite a while ago there was a mailing list thread where the idea was brought up of creating a mailing list for those interested in combinatorics in the setting of Haskell. Well, I've finally gotten around to it. If you're interested in (probably extremely low-traffic) discussion and announcements relating to doing combinatorics in Haskell (computing combinatorial/ number-theoretic functions; exhaustive or random generation of combinatorial objects; type algebra; etc.), please join! http://projects.haskell.org/cgi-bin/mailman/listinfo/combinatorics There is also a wiki page for collecting information here: http://www.haskell.org/haskellwiki/Combinatorics -Brent

1 0

Organizaing tests in Haskell
by Jan Stolarek 24 Sep '12

24 Sep '12

Dear list, during many years of Java programming I've been faithful to TDD methology. Recently I've been trying to figure out how to do tests in Haskell. Thanks to RWH and help from great folks at #haskell I've managed to get on my feet. There is however one issue I wasn't able to solve. In Java it is very easy to separate test code from the actual source. A Java project simply contains two folders: src and tests. These two are treated as the source directories. Both these directories have the same subdirectory structure. For example I have a source file src/myPackage/mySubpackage/MyClass.java and test for it are kept in file tests/myPackage/mySubpackage/MyClassTest.java. These two files are considered by Java to be in the same package. Here's the main trick: fileds and methods that are marked as protected in Java are accessible to other classes in the same package. This allows to test internal methods of a class by marking them as protected instead of private. This breaks encapsulation but only within a package, which is acceptable. Now I'd like to achieve something similar in Haskell. I'm using cabal's support for testing. I created separate src and tests directories, both with the same subdirectory structure. I keep tests for each module in a separate file (e.g. I have src/Math/MyModule.hs and tests/Math/MyModuleTest.hs) and I have one file that assembles all the tests into a single test suite (I use test-framework for that). The only problem is that in order to test some function from a module I have to expose that function, which pretty much forces me to give up on encapsulation. Is there any better solution to organize tests in Haskell? Should I just give up on module encapsulation, or should I only test functions exposed by the module and don't worry about internal functions? Perhaps I should use some different approach? Jan

4 8

Darcs on Windows 7
by Vasili I. Galchin 24 Sep '12

24 Sep '12

Hello Haskellers, I installed darcs on a Windows 7 machine. A darcs folder was created under "Program Files(x86)" folder. However, when I pull up "Program..." on the left side, darcs not there for me to run it. Why? Is darcs run only from the CLI? Regard, Vasili

4 3

Platform Versioning Policy: upper bounds are not our friends
by Bryan O'Sullivan 23 Sep '12

23 Sep '12

Hi, folks - I'm sure we are all familiar with the phrase "cabal dependency hell" at this point, as the number of projects on Hackage that are intended to hack around the problem slowly grows. I am currently undergoing a fresh visit to that unhappy realm, as I try to rebuild some of my packages to see if they work with the GHC 7.6 release candidate. A substantial number of the difficulties I am encountering are related to packages specifying upper bounds on their dependencies. This is a recurrent problem, and its source lies in the recommendations of the PVP itself (problematic phrase highlighted in bold): When publishing a Cabal package, you should ensure that your dependencies > in the build-depends field are accurate. This means specifying not only > lower bounds, *but also upper bounds* on every dependency. I understand that the intention behind requiring tight upper bounds was good, but in practice this has worked out terribly, leading to depsolver failures that prevent a package from being installed, when everything goes smoothly with the upper bounds relaxed. The default response has been for a flurry of small updates to packages in which the upper bounds are loosened, thus guaranteeing that the problem will recur in a year or less. This is neither sensible, fun, nor sustainable. In practice, when an author bumps a version of a depended-upon package, the changes are almost always either benign, or will lead to compilation failure in the depending-upon package. A benign change will obviously have no visible effect, while a compilation failure is actually *better* than a depsolver failure, because it's more informative. This leaves the nasty-but-in-my-experience-rare case of runtime failures caused by semantic changes. In these instances, a downstream package should *reactively* add an upper bound once a problem is discovered. I propose that the sense of the recommendation around upper bounds in the PVP be reversed: upper bounds should be specified *only when there is a known problem with a new version* of a depended-upon package.

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[Snap] Argument Substitution in Heist Templates with Splices
by Sebastian Fischer 23 Sep '12

23 Sep '12

Hello, the following program demonstrates that arguments in Heist templates are sometimes not substituted in presence of splices: {-# LANGUAGE OverloadedStrings #-} import Blaze.ByteString.Builder (toByteString) import qualified Data.ByteString.Char8 as BS import Data.Functor ((<$>)) import Data.Maybe (fromJust) import Text.Templating.Heist -- just return input node unchanged testSplice :: Splice IO testSplice = (:[]) <$> getParamNode main = do writeFile "test.tpl" "<arg /><test attr='${arg}'><arg /></test>" state <- either error id <$> loadTemplates "." defaultHeistState (builder,_) <- fromJust <$> renderWithArgs [("arg","42")] state "test" BS.putStrLn $ toByteString builder -- 42<test attr='42'>42</test> let state' = bindSplices [("test",testSplice)] state (builder',_) <- fromJust <$> renderWithArgs [("arg","42")] state' "test" BS.putStrLn $ toByteString builder' -- 42<test attr='42'><arg></arg></test> Without using splices, all occurrences of 'arg' in the template are substituted. When using a splice, 'arg' is not substituted underneath the input node of the splice. It is substituted in an attribute of the input node. Is this intentional? How can I ensure substitution also underneath the input node? Best, Sebastian

2 2

Fwd: ANNOUNCE: hsqml-0.1.1 (and hsqml-morris-0.1.0)
by Matthew West 22 Sep '12

22 Sep '12

Hello, In case anyone is wondering this does not compile on vanilla Mac OS 10.6 because XCode 3.2 doesn't support C++11. I believe that XCode 4.2 does support it however I've not yet paid Apple the $100 they want before I can download it for 10.6. Macports has more recent versions of GCC however this option turned out to be fraught with difficulties[1][2]. Has anybody got HsQML working with XCode 4.2? Matt [1] https://trac.macports.org/wiki/UsingTheRightCompiler [2] http://www.haskell.org/haskellwiki/Mac_OS_X_Strike_Force On 11 Sep 2012, at 08:47, Robin KAY wrote: > Dear All, > > I would like to announce a new package called 'hsqml' [1]. > > HsQML provides a Haskell binding to the Qt Quick framework. It allows you to write graphical applications where the front-end is written in Qt Quick's QML language (incorporating JavaScript) and the back-end is written in Haskell. The two layers are coupled together via a facility to define custom JavaScript objects through which QML code can call into Haskell. > > HsQML requires an installation of Qt 4.7 (including QtDeclarative) or later on your path. It has been tested with GHC 7.4 on both Linux and Windows. > > There is also a simple example program available using HsQML on Hackage called 'hsqml-morris'. It implements a human v computer game of Nine Men's Morris with game logic and AI written in Haskell and the presentation layer in QML. > > The library is currently limited by a lack of support for Qt signals, so calls can only be made from JavaScript to Haskell and not the other way. I plan to add support for these in a future release. Migration over to Qt 5.x is also likely once it's released. > > [1] http://hackage.haskell.org/package/hsqml-0.1.1 > [2] http://hackage.haskell.org/package/hsqml-morris-0.1.0 > > Regards, > > -- > Robin KAY

1 0

Haskell Wiki News
by Colin Adams 22 Sep '12

22 Sep '12

This doesn't seem to be up-to-date. It announces GHC 7.4 released (but 7.6.1 was released a couple of weeks ago).

2 1

Munich Haskell Meeting
by Christian Neukirchen 22 Sep '12

22 Sep '12

Dear all, Heinrich is on holiday next week, thus I'll manage it this time: Once again, it's time for our monthly get-together. Functional programmers from Munich and other cities will meet on Thu, 27th of September, at 19h30 at Cafe Puck in Munich. If you plan to join, please go to http://www.haskell-munich.de/dates and click the button! With hope to meet many of you there, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

1 0

Question about type inference of a GADT term
by Florian Lorenzen 22 Sep '12

22 Sep '12

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hello cafe, I have the following GADT definitions capturing the simply typed lambda calculus with de Bruijn indices for variables and explicitly annotated types for variables: {-# LANGUAGE GADTs #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE DataKinds #-} - -- Typing contexts data Ctx = CtxNil | CtxCons Ctx Type - -- Types data Type = TyInt | TyArrow Type Type - -- Variable indices data Ix (ctx :: Ctx) (ty :: Type) where IxZero :: Ix ctx ty IxSucc :: Ix ctx ty1 -> Ix (CtxCons ctx ty2) ty1 - -- Type representations data TyRep (ty :: Type) where TyRepInt :: TyRep TyInt TyRepArrow :: TyRep ty1 -> TyRep ty2 -> TyRep (TyArrow ty1 ty2) - -- Terms data Term (ctx :: Ctx) (ty :: Type) where TmInt :: Integer -> Term ctx TyInt TmVar :: Ix ctx ty -> Term ctx ty TmAdd :: Term ctx TyInt -> Term ctx TyInt -> Term ctx TyInt TmApp :: Term ctx (TyArrow ty1 ty2) -> Term ctx ty1 -> Term ctx ty2 TmAbs :: TyRep ty1 -> Term (CtxCons ctx ty1) ty2 -> Term ctx (TyArrow ty1 ty2) For the following definition test1 = TmAbs TyRepInt (TmVar IxZero) GHCi infers the type test1 :: Term ctx (TyArrow 'TyInt ty2) I was a bit puzzled because I expected Term ctx (TyArrow TyInt TyInt) Of course, this more special type is an instance of the inferred one, so I can assign it by a type signature. Can someone explain why the inferred type is more general? Terms like test2 = TmAbs TyRepInt (TmAdd (TmVar IxZero) (TmInt 5)) have the type I expected: test2 :: Term ctx (TyArrow 'TyInt 'TyInt) Thank you and best regards, Florian - -- Florian Lorenzen Technische Universität Berlin Fakultät IV - Elektrotechnik und Informatik Übersetzerbau und Programmiersprachen Sekr. TEL12-2, Ernst-Reuter-Platz 7, D-10587 Berlin Tel.: +49 (30) 314-24618 E-Mail: florian.lorenzen(a)tu-berlin.de WWW: http://www.user.tu-berlin.de/florenz/ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://www.enigmail.net/ iEYEARECAAYFAlBcXs4ACgkQvjzICpVvX7b1WQCePiL+SFNj9X+U0V2fnykuatLX pIcAn1VDNRiSR18s7UgctdPeNzFgStbi =LBGb -----END PGP SIGNATURE-----

2 2

Church vs Boehm-Berarducci encoding of Lists
by oleg＠okmij.org 22 Sep '12

22 Sep '12

There has been a recent discussion of ``Church encoding'' of lists and the comparison with Scott encoding. I'd like to point out that what is often called Church encoding is actually Boehm-Berarducci encoding. That is, often seen > newtype ChurchList a = > CL { cataCL :: forall r. (a -> r -> r) -> r -> r } (in http://community.haskell.org/%7Ewren/list-extras/Data/List/Church.hs ) is _not_ Church encoding. First of all, Church encoding is not typed and it is not tight. The following article explains the other difference between the encodings http://okmij.org/ftp/tagless-final/course/Boehm-Berarducci.html Boehm-Berarducci encoding is very insightful and influential. The authors truly deserve credit. P.S. It is actually possible to write zip function using Boehm-Berarducci encoding: http://okmij.org/ftp/ftp/Algorithms.html#zip-folds

6 10

Passing CString array to Haskell shared library
by Alexander Mumme 21 Sep '12

21 Sep '12

Hello everyone! Long time reader, first time poster. Was wondering if someone could give me some direction or hints on how I might go about passing a CString array into an exported Haskell function. What I'm trying to do is augment the RecordLinkage package from R using Haskell. Seems pretty straight forward. With some help from Neil Mitchell's excellent blog post on the subject ( http://neilmitchell.blogspot.com/2011/10/calling-haskell-from-r.html) I've managed to make calls into Haskell and utilize the Text.EditDistance package. This is working rather well, except that I'm trying to calculate the edit distance from each of 3000 strings to each of 780,000 strings. Since I'm calling Haskell from R once for every comparison (and allocating a result record in R for each return) I find myself running out of memory rather quickly. What I'd like to do is send both complete string lists into Haskell to process, and have it pass me back a result vector. Below is how I'm currently making the call using individual strings. I've tried looking through the GHC User's Guide, through the Wiki on usingthe FFI, and a number of other resources, but I seem to have come to an impasse. Could anyone lend some assistance? ------------- Code start ----- {-# LANGUAGE ForeignFunctionInterface #-} module Levenshtein where import Foreign.C.Types import Foreign.C.String import Foreign.Ptr import Foreign.Storable import Text.EditDistance levenshteinWeight :: Ptr Int -> Ptr Int -> Ptr Int -> Ptr CString -> Ptr CString -> Ptr Int -> IO () levenshteinWeight del ins subs str1 str2 result = do del <- peek del ins <- peek ins subs <- peek subs str1' <- peekCString =<< peek str1 str2' <- peekCString =<< peek str2 poke result $ levenshteinDistance EditCosts { deletionCosts = ConstantCost del, insertionCosts = ConstantCost ins, substitutionCosts = ConstantCost subs, transpositionCosts = ConstantCost 1} str1' str2' foreign export ccall levenshteinWeight :: Ptr Int -> Ptr Int -> Ptr Int -> Ptr CString -> Ptr CString -> Ptr Int -> IO () ------------- Code end ----- Thanks in Advance! Alex

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