August 2012 - Haskell-Cafe

Parsec type error with Flexible Contexts
by Marco Túlio Gontijo e Silva 09 Aug '12

09 Aug '12

Hi. I wanted to generalize the definitions in Text.Parsec.Language to be able to use them with Parsec Text instead of Parsec String. I'm getting a type error with the following code snippet: http://hpaste.org/72844 I don't really understand the error, specially with regard to the fact that the if I comment one of the marked lines on the code pasted the program build. Any help is welcome. Greetings. -- marcot http://marcot.eti.br/

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NSF funding available for U.S. graduate students to attend ICFP 2012
by Andy Adams-Moran 09 Aug '12

09 Aug '12

U.S. Graduate Students and Advisors! Funding is now available for U.S. graduate students to attend ICFP 2012, the International Conference on Functional Programming, to be held in Copenhagen, September 9-15 2012. You must be full-time in a U.S. graduate program to apply, but need not have a paper in the conference. Apply here: http://tinyurl.com/8wkh4ww In the comments field, note that you're applying for NSF funding, and provide your supervisor's name and your school. Not attending a U.S. graduate school? Check out the other possibilities for funding your travel at: http://www.icfpconference.org/icfp2012/students.html Looking forward to seeing you in Copenhagen! Cheers, The ICFP 2012 Organizing Team -- Andy Adams-Moran Phone: 503.626.6616, x113 Galois Fax: 503.214.8120 421 SW 6th Ave, Suite #300 http://www.galois.com Portland, OR 97204 adams-moran(a)galois.com

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NSF funding available for U.S. graduate students to attend ICFP 2012
by Andy Adams-Moran 09 Aug '12

09 Aug '12

U.S. Graduate Students and Advisors! Funding is now available for U.S. graduate students to attend ICFP 2012, the International Conference on Functional Programming, to be held in Copenhagen, September 9-15 2012. You must be full-time in a U.S. graduate program to apply, but need not have a paper in the conference. Apply here: http://tinyurl.com/8wkh4ww In the comments field, note that you're applying for NSF funding, and provide your supervisor's name and your school. Not attending a U.S. graduate school? Check out the other possibilities for funding your travel at: http://www.icfpconference.org/icfp2012/students.html Looking forward to seeing you in Copenhagen! Cheers, The ICFP 2012 Organizing Team -- Andy Adams-Moran Phone: 503.626.6616, x113 Galois Fax: 503.214.8120 421 SW 6th Ave, Suite #300 http://www.galois.com Portland, OR 97204 adams-moran(a)galois.com

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Data.Data and OverlappingInstances
by Timo von Holtz 09 Aug '12

09 Aug '12

Hi cafe, in my code I use Data.Data and OverlappingInstances. My problem now is, that I can't use functions using these instances as arguments for fromConstrM. This is what my problem looks like: {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE UndecidableInstances #-} import Data.Data class Test a where foo :: Monad m => m a instance Num a => Test a where foo = return 1 instance Test Int where foo = return 2 test constr = fromConstrM foo constr but when I compile I get: test.hs:15:26: Overlapping instances for Test d arising from a use of `foo' at test.hs:15:26-28 Matching instances: instance [overlap ok] (Num a) => Test a -- Defined at test.hs:9:9-23 instance [overlap ok] Test Int -- Defined at test.hs:12:9-16 (The choice depends on the instantiation of `d' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the first argument of `fromConstrM', namely `foo' In the expression: fromConstrM foo constr In the definition of `test': test constr = fromConstrM foo constr Failed, modules loaded: none. Is there a way out? Right now I use a "case (typeOf x) of" kind of construct, but it gets pretty messy and I even have to unsafeCoerce at one point. - Timo

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mutable arrays of tuples
by David Feuer 09 Aug '12

09 Aug '12

So I was thinking about a mutable array of tuples, but to avoid allocating tuples to modify their fields, I guess I really want an immutable array of tuples of STRefs. Just how much less efficient is this than a plain mutable array? might it even make sense to use parallel mutable arrays? The thought of that is disgusting to me, but at least one of the arrays could likely be made unboxed...

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Haskell Weekly News: Issue 239
by Daniel Santa Cruz 08 Aug '12

08 Aug '12

Welcome to issue 239 of the HWN, an issue covering crowd-sourced bits of information about Haskell from around the web. This issue covers the week of July 29 to August 4, 2012. Quotes of the Week * romm: i thought i knew programming. this is like discovering a new continent. * edwardk: i think the comonads are warm fuzzy, and the monads are warm sticky. its easy to get out of a comonad, its soft and you like wearing it. monads on the other hand stick to you and are icky Top Reddit Stories * Update on GHC's new code generator Domain: hackage.haskell.org, Score: 70, Comments: 39 On Reddit: [1] http://goo.gl/NkmqH Original: [2] http://goo.gl/4bUw1 * The first video in my new series of free Haskell live coding videos Domain: youtube.com, Score: 64, Comments: 61 On Reddit: [3] http://goo.gl/f1tmg Original: [4] http://goo.gl/ihwze * hbro, Haskell based browser, ideologically similar to uzbl Domain: github.com, Score: 48, Comments: 18 On Reddit: [5] http://goo.gl/apILX Original: [6] http://goo.gl/STSwc * Thinking Functionally with Haskell | PragProg Domain: pragprog.com, Score: 32, Comments: 6 On Reddit: [7] http://goo.gl/yL4Qj Original: [8] http://goo.gl/mHss0 * Two ways of representing perfect binary trees : Inside 374 Domain: blog.ezyang.com, Score: 30, Comments: 1 On Reddit: [9] http://goo.gl/1GfdH Original: [10] http://goo.gl/JJAl1 * Hoogle: from the command line Domain: self.haskell, Score: 28, Comments: 8 On Reddit: [11] http://goo.gl/Ob9Mw Original: [12] http://goo.gl/Ob9Mw * Free monad transformers Domain: haskellforall.com, Score: 27, Comments: 11 On Reddit: [13] http://goo.gl/XbQAj Original: [14] http://goo.gl/bJfrK * If someone asked you "why Haskell?", what would you say? Domain: self.haskell, Score: 26, Comments: 73 On Reddit: [15] http://goo.gl/LQUUh Original: [16] http://goo.gl/LQUUh * Feedback from someone who learnt Haskell as his first language Domain: blog.raynes.me, Score: 25, Comments: 50 On Reddit: [17] http://goo.gl/QsnfZ Original: [18] http://goo.gl/wFOLN * Why isn't `length` part of `Foldable`? Domain: self.haskell, Score: 23, Comments: 19 On Reddit: [19] http://goo.gl/L0WWX Original: [20] http://goo.gl/L0WWX Top StackOverflow Questions * Monoidal parsing — what is it? votes: 19, answers: 2 Read on SO: [21] http://goo.gl/bVGBo * Applying multiple functions to the same value point-free style in Haskell votes: 7, answers: 4 Read on SO: [22] http://goo.gl/PifBz * How do I get consistent criterion benchmarks, or interpret results across runs? votes: 7, answers: 1 Read on SO: [23] http://goo.gl/qwT5c * Why is this prime test so slow? votes: 7, answers: 4 Read on SO: [24] http://goo.gl/QehSC * Is there a theory that combines category theory/abstract algebra and computational complexity? votes: 7, answers: 1 Read on SO: [25] http://goo.gl/CeAFw * Haskell Polymorphic Tree Sum votes: 7, answers: 1 Read on SO: [26] http://goo.gl/NqYQW * reasonably efficient pure-functional matrix product in Haskell? votes: 7, answers: 3 Read on SO: [27] http://goo.gl/8H6o9 * Is access to internal strucutre of a monad required for monad transformer? votes: 7, answers: 1 Read on SO: [28] http://goo.gl/Yln18 * Haskell sub-typeclass requires UndecidableInstances? votes: 6, answers: 2 Read on SO: [29] http://goo.gl/u9FQh * Haskell Data.List.Class and syntax votes: 6, answers: 1 Read on SO: [30] http://goo.gl/wvKEk Until next time, Daniel Santa Cruz References 1. http://hackage.haskell.org/trac/ghc/blog/newcg-update 2. http://www.reddit.com/r/haskell/comments/xmfes/update_on_ghcs_new_code_gene… 3. http://www.youtube.com/watch?v=ScS8Q32lMxA 4. http://www.reddit.com/r/haskell/comments/xfcmk/the_first_video_in_my_new_se… 5. https://github.com/k0ral/hbro 6. http://www.reddit.com/r/haskell/comments/xe0w3/hbro_haskell_based_browser_i… 7. http://pragprog.com/magazines/2012-08/thinking-functionally-with-haskell 8. http://www.reddit.com/r/haskell/comments/xmip7/thinking_functionally_with_h… 9. http://blog.ezyang.com/2012/08/statically-checked-perfect-binary-trees/ 10. http://www.reddit.com/r/haskell/comments/xov6n/two_ways_of_representing_per… 11. http://www.reddit.com/r/haskell/comments/xijsb/hoogle_from_the_command_line/ 12. http://www.reddit.com/r/haskell/comments/xijsb/hoogle_from_the_command_line/ 13. http://www.haskellforall.com/2012/07/free-monad-transformers.html 14. http://www.reddit.com/r/haskell/comments/xg75g/free_monad_transformers/ 15. http://www.reddit.com/r/haskell/comments/xlb1g/if_someone_asked_you_why_has… 16. http://www.reddit.com/r/haskell/comments/xlb1g/if_someone_asked_you_why_has… 17. http://blog.raynes.me/blog/2011/11/27/the-clojure-community-and-me/ 18. http://www.reddit.com/r/haskell/comments/xk7fz/feedback_from_someone_who_le… 19. http://www.reddit.com/r/haskell/comments/xo05l/why_isnt_length_part_of_fold… 20. http://www.reddit.com/r/haskell/comments/xo05l/why_isnt_length_part_of_fold… 21. http://stackoverflow.com/questions/11808539/monoidal-parsing-what-is-it 22. http://stackoverflow.com/questions/11709350/applying-multiple-functions-to-… 23. http://stackoverflow.com/questions/11712234/how-do-i-get-consistent-criteri… 24. http://stackoverflow.com/questions/11718067/why-is-this-prime-test-so-slow 25. http://stackoverflow.com/questions/11725899/is-there-a-theory-that-combines… 26. http://stackoverflow.com/questions/11742405/haskell-polymorphic-tree-sum 27. http://stackoverflow.com/questions/11768656/reasonably-efficient-pure-funct… 28. http://stackoverflow.com/questions/11792275/is-access-to-internal-strucutre… 29. http://stackoverflow.com/questions/11707171/haskell-sub-typeclass-requires-… 30. http://stackoverflow.com/questions/11770733/haskell-data-list-class-and-syn…

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foreign import and gmp
by Lars Kuhtz 08 Aug '12

08 Aug '12

Hi all, There have been rumors that recent versions of GHC may allow foreign imports of objects that link against gmp without interfering with the gmp imports in integer-gmp. What is the current state of that issue? What is the currently recommended way to deal with that problem? Thanks, Lars

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'let' keyword optional in do notation?
by Martijn Schrage 08 Aug '12

08 Aug '12

Hi cafe, For a while now, I've been wondering why the 'let' keyword in a do block isn't optional. So instead of do ... let x = exp1 y = exp2 z <- exp3 ... you could simply write do ... x = exp1 y = exp2 z <- exp3 ... Where each sequence of let-less bindings is put in a separate binding group. I'm no parsing wizard, but I couldn't come up with any situations in which this would cause ambiguity. To me, the let-less version is easier on the eyes, more consistent with <- bindings, and also makes it less of a hassle to move stuff around. The above probably also holds for list/monad comprehensions, but the explicit let has never really bothered me there. Cheers, Martijn Schrage -- Oblomov Systems (http://www.oblomov.com)

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Data structure containing elements which are instances of the same type class
by Daniel Trstenjak 07 Aug '12

07 Aug '12

Hi all, it should be possible a call a function on all elements of the data structure, to add and remove elements. What I currently have: the type class: class Foo a where hasId :: a -> Int -> Maybe a a few instances: data A = A deriving Show instance Foo A where hasId a 1 = Just a hasId _ _ = Nothing data B = B deriving Show instance Foo B where hasId a 2 = Just a hasId _ _ = Nothing data C = C deriving Show instance Foo C where hasId a 3 = Just a hasId _ _ = Nothing the data structure holding any instance of Foo, which itself is a instance of Foo: data Foos l r = Foos l r | FooL l | FooR r | NoFoos deriving Show instance (Foo l, Foo r) => Foo (Foos l r) where hasId (Foos l r) id = case (hasId l id, hasId r id) of (Just l, Just r) -> Just $ Foos l r (Just l, _ ) -> Just $ FooL l (_ , Just r) -> Just $ FooR r _ -> Nothing combinator for Foos: (+++) :: l -> r -> Foos l r l +++ r = Foos l r infixr 5 +++ Now I can write: *Main> A +++ B +++ C +++ A Foos A (Foos B (Foos C A)) *Main> (A +++ B +++ C +++ A) `hasId` 1 Just (Foos A (FooR (FooR A))) Doesn't seem that nice. For every operation I would have to extend the type class. After some operations the data structure contains many dummy nodes (FooR, FooL). Is there some nicer way? Greetings, Daniel

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Re: [Haskell-cafe] HUnit and table-driven tests
by Matthew 07 Aug '12

07 Aug '12

On Tue, Aug 7, 2012 at 12:51 AM, Dean Herington <heringtonlacey(a)mindspring.com> wrote: > At 4:30 PM -0700 8/5/12, Matthew wrote: >> >> On Sun, Aug 5, 2012 at 12:32 AM, Henk-Jan van Tuyl <hjgtuyl(a)chello.nl> >> wrote: >>> >>> On Sun, 05 Aug 2012 03:21:39 +0200, Matthew <wonderzombie(a)gmail.com> >>> wrote: >>> >>>> I've got a function which takes in two chars, describing a playing >>>> card and a suit. An example would be 4C or TH for a 4 of Clubs or a >>>> Ten of Hearts. I need to be able to compare the ranks of a card (e.g. >>>> a King is 13), so a Card is a tuple of rank and suit. The function >>>> which parses a Card is type String -> Maybe Card. >>>> >>>> I'm writing unit tests for this using HUnit, and ideally I'd go with a >>>> table-driven[1] approach, where each test case is a tuple of the input >>>> and the expected output. (Possibly I could expand this to a triple, or >>>> simply a list, to allow for an error message for each test case.) Then >>>> all the test function has to do is run through each case and assert as >>>> necessary. Example: [("TH", Just (Hearts, 10)), ("XH", Nothing)]. >>> >>> >>> >>> A simple solution: >>> >>>> parseCard :: String -> Maybe Card >>>> parseCard string = <your function to test> >>>> test :: Bool >>>> test = all testEqual [("TH", Just (Hearts, 10)), ("XH", Nothing)] >>>> where >>>> testEqual (input, output) = parseCard input == output >>> >>> >>> >>> For a description of 'all', see: >>> >>> >>> http://hackage.haskell.org/packages/archive/base/latest/doc/html/Prelude.ht… >> >> >> Thanks for the response. The one problem I have with this is that it >> will not be at all obvious which test case (or cases!) failed. >> >> That said, maybe I could do something similar, with a Writer? A passed >> test writes "", but a failed one writes a test-specific failure >> message. Then the test itself uses this string as the assert message. > > > > Let HUnit tell you about the failing test cases. Here's one way to do it. > > > import Test.HUnit > import Data.Char (isDigit) > > data Suit = Spades | Hearts | Diamonds | Clubs > deriving (Show, Read, Eq, Ord) > type Rank = Int -- 2 .. 14 (jack=11, queen=12, king=13, ace=14) > type Card = (Suit, Rank) > > > parseCard :: String -> Maybe Card > parseCard [rankChar, suitChar] = do suit <- suitFrom suitChar; rank <- > rankFrom rankChar; return (suit, rank) > parseCard _ = Nothing > > suitFrom char = lookup char [('S', Spades), ('H', Hearts), ('D', Diamonds), > ('C', Clubs)] > > rankFrom dig | isDigit dig = let v = read [dig] in if v >= 2 then Just v > else Nothing > rankFrom char = lookup char [('T', 10), ('J', 11), ('Q', 12), ('K', 13), > ('A', 14)] > > makeTest :: (String, Maybe Card) -> Test > makeTest (string, result) = string ~: result ~=? parseCard string > > tests = [("TH", Just (Hearts, 10)), ("XH", Nothing)] > > main = (runTestTT . TestList . map makeTest) tests ...it seems so obvious now. This is *exactly* what I was looking for; clearly I was over-thinking this. Thanks, Dean! > > > Dean

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