December 2010 - Haskell-Cafe

Data.Typeable TypeRep Ord instance.
by Serguey Zefirov 02 Jan '11

02 Jan '11

Why TypeRep does have equality and doesn't have ordering? It would be good to have that. Right now when I have to order two type representations I convert them to string and then compare. This is somewhat inefficient and not quite straightforward.

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getting last char of String
by Aaron Gray 01 Jan '11

01 Jan '11

Is there an easy Haskell function that gets the last Char of a [Char] or String ? Many thanks in advance, Aaron

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V.I.P.s and the associativity of merge'
by Leon Smith 01 Jan '11

01 Jan '11

Ok, after mulling over the issues that Will Ness has brought up in the last few days [1], I think I have a partial explanation for the apparent tension between Will's observations and Heinrich Apfelmus's Implicit Heaps article [2], which both concern the implementation of mergeAll [3]. The merge' function takes two ordered lists, with the head of the first list less than the head of the second, and merges their contents: merge' [] ys = ys merge' (x:xs) ys = x : merge xs ys The nice thing about this function is we can merge an infinite number of lists by folding right, if assume that the heads of those lists are appropriately ordered. This appears in Richard Bird's code at the end of Melissa O'Neill's "Genuine Sieve of Eratosthenes", though undoubtedly this observation has been independently made by many people. Now, in an ordinary sense, merge' *is* an associative operator, in that given three fully defined ordered lists with ordered heads, merge' xs (merge' ys zs) == merge' (merge' xs ys) zs. This allows us to merge an infinite number of lists using an arbitrary tree of merge' operations, without ever worrying that we will return an incorrect result. (However, we might get stuck in a non-productive loop, for example if we fold left over an infinite list of ordered lists) However, Heinrich's article uses a stronger sense of associativity which includes laziness properties and thus captures some operational characteristics. In this sense, merge' *is not* associative. To remedy this, Heinrich uses VIPs to strengthen the associativity property of merge'. This raises the question, is there some combination of the shape of the merge' tree and some input for which using VIPs dramatically changes the efficiency of a mergeAll operation? I suspect the answer is yes, though I don't know for sure at this point in time. However, I do tacitly believe that the current tree that mergeAll uses doesn't exhibit this property for any input, and so I have simplified the implementations of mergeAll and unionAll in the latest version of data-ordlist-0.4.4 by avoiding the use of VIPs. This has the nice side benefit of modestly improving performance when the elements of the result are highly biased towards the first list. Best, Leon [1] http://permalink.gmane.org/gmane.comp.lang.haskell.cafe/84666 [2] http://apfelmus.nfshost.com/articles/implicit-heaps.html [3] http://hackage.haskell.org/packages/archive/data-ordlist/0.4.4/doc/html/Dat…

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Could someone give me a small code review
by Aaron Gray 31 Dec '10

31 Dec '10

I have attached a tiny parser and lexer. This is really key to getting the lexing correct. The only thing I could not get neatly was to disallow hyphens at the end of identifiers, so have left it off until a good solution arrives. I plan to move over to monads next. Any comments welcome, Aaron

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Haskell Weekly News: Issue 163 - Year End Issue
by Daniel Santa Cruz 31 Dec '10

31 Dec '10

Welcome to issue 163 of the HWN, a newsletter covering developments in the [1]Haskell community. This release of the newsletter contains some summary data for the year 2010. Thanks to everyone involved for making this such a wonderful community! Big cheers to Miran Lipovaa, who got multiple top Reddit stories this year for his work on Learn You a Haskell! Also kudos to Don Stewart, Norman Ramsey, camccann, sepp2k, and KennyTM for taking the time to answer so many questions in StackOverflow. The folks at #haskell never cease to amaze with their friendliness and their helpfulness as well. What did Reddit and StackOverflow think was hot this year? Top Reddit Stories this Year * Galois releases the Haskell Lightweight Virtual Machine (HaLVM): From (galois.com) scored 103 with 26 comments. Read on [2]reddit or the [3]original post. * Try Haskell!: From (self.haskell), scored 105 with 29 comments. Read on [4]reddit or the [5]original post. * GHC 7.0.1 released!: From (haskell.org) scored 100 with 31 comments. Read on [6]reddit or the [7]original post. * Learn You a Haskell: Zippers: From (learnyouahaskell.com) scored 93 with 44 comments. Read on [8]reddit or the [9]original post. * The new LLVM backend to GHC is ready to merge in!: From (haskell.org) scored 89 with 27 comments. Read on [10]reddit or the [11]original post. * Haskell vs. Erlang for bittorent clients: laziness, dynamic typing ..: From (jlouisramblings.blogspot.com) scored 88 with 8 comments. Read on [12]reddit or the [13]original post. * There are more than 4000 people in the Haskell Reddit. Tell us what you're using Haskell for! Work and/or play? What would most help you get the job done?: From (self.haskell), scored 84 with 171 comments. Read on [14]reddit or the [15]original post. * Smoking fast Haskell code using GHC's new LLVM codegen: From (donsbot.wordpress.com) scored 83 with 2 comments. Read on [16]reddit or the [17]original post. * Learn You a Haskell for Great Good! The Book!: From (amazon.com) scored 79 with 29 comments. Read on [18]reddit or the [19]original post. * LYAH: Intro to Monads: From (learnyouahaskell.com) scored 78 with 49 comments. Read on [20]reddit or the [21]original post. Top StackOverflow Answers this Year * [22]Large-scale design in Haskell? | votes: 1 I talk a bit about this in Engineering Large Projects in Haskell and in the Design and Implementation of XMonad. Engineering in the large is about managing complexity. The primary code structuring mechanisms in Haskell for managing complexity are : The type system Use the type system to enforce abstractions, simplifying interactions. Enforce key ... * [23]Curious about the HashTable problem | votes: 1 The problem was that the garbage collector is required to traverse mutable arrays of pointers ("boxed arrays") looking for pointers to data that might be ready to deallocate. Boxed, mutable arrays are the main mechanism for implementing a hashtable, so that particular structure showed up the GC traversal issue. This is common to many languages. The symptom ... * [24]Why are side-effects modeled as monads in Haskell? | votes: 1 Suppose a function has side effects. If we take all the effects it produce as the input and output parameters, then the function is pure to the outside world. So for an impure function f' :: Int -> Int we add the RealWorld to the consideration f :: Int -> RealWorld -> (Int, RealWorld) -- input some states of the whole world, -- modify the ... * [25]Haskell - I am too old for this Švotes: 1 (f . g) x = f (g x) This is true. You concluded from that that (f . g) x y = f (g x y) must also be true, but that is not the case. In fact, the following is true: (f . g) x y = f (g x) y which is not the same. Why is this true? Well (f . g) x y is the same as ((f . g) x) y and since we know that (f . g) x = f (g x) we can reduce that to (f (g x)) ... * [26]Haskell: How is <*> pronounced? | votes: 1 Sorry, I don't really know my math, so I'm curious how to pronounce the functions in the Applicative typeclass Knowing your math, or not, is largely irrelevant here, I think. As you're probably aware, Haskell borrows a few bits of terminology from various fields of abstract math, most notably Category Theory, from whence we get functors and monads. The ... * [27]Haskell Weird Kinds | votes: 1 These are GHC-specific extensions of the Haskell kind system. The Haskell 98 report specifies only a simple kind system: ... type expressions are classified into different kinds, which take one of two possible forms: The symbol * represents the kind of all nullary type constructors. If k1 and k2 are kinds, then k1->k2 is the kind of types ... * [28]Is Haskell really a purely functional language considering unsafePerformIO? | votes: 1 The Languages We Call Haskell unsafePerformIO is part of the Foreign Function Interface specification, not core Haskell 98 specification. It can be used to do local side effects that don't escape some scope, in order to expose a purely functional interface. That is, we use it to hide effects when the type checker can't do it for us (unlike the ST monad, ... * [29]What language to learn after Haskell? | votes: 1 I would like to broaden my knowledge of programming. (...) I thought I would pose the question here, along with several stipulations about the type of language I am looking for. Some are subjective, some are intended to ease the transition from Haskell. Strong type system. (...) It also makes informally reasoning about the correctness of my program ... * [30]Why is writing a compiler in a functional language easier? | votes: 1 Often times a compiler works a lot with trees. The source code is parsed into a syntax tree. That tree might then be transformed into another tree with type annotations to perform type checking. Now you might convert that tree into a tree only containing core language elements (converting syntactic sugar-like notations into an unsugared form). Now you might ... * [31]GHC's RTS options for garbage collection | votes: 1 Generally speaking, garbage collection is a space/time tradeoff. Give the GC more space, and it will take less time. There are (many) other factors in play, cache in particular, but the space/time tradeoff is the most important one. The tradeoff works like this: the program allocates memory until it reaches some limit (decided by the GC's automatic tuning ... About the Haskell Weekly News At this point in time, the HWN has been a bit sporadic. I'm hoping that 2011 will see a much more regular schedule for this channel. I'd like to thank everyone who pitched in and sent stories/comments/quotes. If you have any ideas of how to improve this letter, please don't hesitate to contact me at dstcruz(a)gmail.com. The source code that I use to cullate this newsletter will be made available shortly in github. Here is to wishing you a great 2011! Until next time, Daniel Santa Cruz References 1. http://haskell.org/ 2. http://www.reddit.com/r/haskell/comments/ee4ve/galois_releases_the_haskell_… 3. http://www.galois.com/blog/2010/11/30/galois-releases-the-haskell-lightweig… 4. http://www.reddit.com/r/haskell/comments/b58rk/try_haskell/ 5. file://localhost/r/haskell/comments/b58rk/try_haskell/ 6. http://www.reddit.com/r/haskell/comments/e6o0y/ghc_701_released/ 7. http://haskell.org/ghc/download_ghc_7_0_1.html 8. http://www.reddit.com/r/haskell/comments/e0qqz/learn_you_a_haskell_zippers/ 9. http://learnyouahaskell.com/zippers 10. http://www.reddit.com/r/haskell/comments/b3tby/the_new_llvm_backend_to_ghc_… 11. http://www.haskell.org/pipermail/cvs-ghc/2010-February/052606.html 12. http://www.reddit.com/r/haskell/comments/bwrhz/haskell_vs_erlang_for_bittor… 13. http://jlouisramblings.blogspot.com/2010/04/haskell-vs-erlang-for-bittorent… 14. http://www.reddit.com/r/haskell/comments/bhygx/there_are_more_than_4000_peo… 15. file://localhost/r/haskell/comments/bhygx/there_are_more_than_4000_people_in_the_haskell/ 16. http://www.reddit.com/r/haskell/comments/b4tel/smoking_fast_haskell_code_us… 17. http://donsbot.wordpress.com/2010/02/21/smoking-fast-haskell-code-using-ghc… 18. http://www.reddit.com/r/haskell/comments/dook4/learn_you_a_haskell_for_grea… 19. http://www.amazon.com/Learn-You-Haskell-Great-Good/dp/1593272839/ 20. http://www.reddit.com/r/haskell/comments/d51ao/lyah_intro_to_monads/ 21. http://learnyouahaskell.com/monads 22. http://stackoverflow.com/questions/3077866/large-scale-design-in-haskell/30… 23. http://stackoverflow.com/questions/3058529/curious-about-the-hashtable-prob… 24. http://stackoverflow.com/questions/2488646/why-are-side-effects-modeled-as-… 25. http://stackoverflow.com/questions/3884121/haskell-i-am-too-old-for-this/38… 26. http://stackoverflow.com/questions/3242361/haskell-how-is-pronounced/324285… 27. http://stackoverflow.com/questions/3034264/haskell-weird-kinds/3034295#3034… 28. http://stackoverflow.com/questions/3124591/is-haskell-really-a-purely-funct… 29. http://stackoverflow.com/questions/3770774/what-language-to-learn-after-has… 30. http://stackoverflow.com/questions/2906064/why-is-writing-a-compiler-in-a-f… 31. http://stackoverflow.com/questions/3171922/ghcs-rts-options-for-garbage-col…

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Deep concatenation [Was: Incorrectly inferring type [t]]
by oleg＠okmij.org 31 Dec '10

31 Dec '10

William Murphy wrote: > I've spent a lot of time trying to write a version of concat, which > concatenates lists of any "depth": It is a little bit more involved, but quite possible. The code is not much longer than the one you wrote (essentially, three lines: one class and two instance declarations). Here is the complete code: {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE TypeFamilies, FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} module DeepFlat where class DeepFlat a b | a -> b where dflat :: [a] -> [b] -- If we flatten a list of lists instance DeepFlat a b => DeepFlat [a] b where dflat = concatMap dflat -- If we are given a list of non-lists instance a ~ b => DeepFlat a b where dflat = id test1 = dflat "abracadabra" -- "abracadabra" test2 = dflat ["abra","cadabra"] test3 = dflat [["ab","ra"],["cad","abra"]] test4 = dflat [[["a","b"],["ra"]],[["cad","abra"]]]

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Many broken links
by Henk-Jan van Tuyl 31 Dec '10

31 Dec '10

L.S., I find, quite often, links to haskell.org/xxx pages that have disappeared; isn't it possible to just copy all of these pages, to the original location, before the old server disappears? I used the W3C link checker[0] to check links from the GUI libraries page[1]; the following pages were reported missing: - http://haskell.org/FranTk - http://www.haskell.org/fruit/ - http://haskell.org/ObjectIO - http://haskell.org/graphics/ Other (sets of) pages that are missing: - http://haskell.org/gtk2hs/ - http://haskell.org/hdirect/ - http://www.haskell.org/~simonmar/ - http://www.haskell.org/sitewiki/ - http://www.haskell.org/all_about_monads - http://darcs.haskell.org/hfuse/ - http://darcs.haskell.org/yaht - http://haskell.org/haskore/ - http://www.haskell.org/yampa/ - http://darcs.haskell.org/~lemmih/ and probably many more. I suppose some of the maintainers have left, but the pages might still have some value. Some pages might be copied to the HaskellWiki, if the author gives permission to do so. It would also be good, to know which of these pages are obsolete, so that the links to these pages can be removed. Regards, Henk-Jan van Tuyl [0] http://validator.w3.org/checklink/ [1] http://www.haskell.org/haskellwiki/Applications_and_libraries/GUI_libraries -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Happy Parser problem
by Aaron Gray 31 Dec '10

31 Dec '10

I am trying to get a grammar where keywords are also valid identifiers. Been messing round with the following Happy grammar :- %token 'let' { TokenIdent "let" } 'in' { TokenIdent "in" } ident { TokenIdent $$ } int { TokenInt $$ } '=' { TokenEq } '+' { TokenPlus } '-' { TokenMinus } '*' { TokenTimes } '/' { TokenDiv } '(' { TokenOB } ')' { TokenCB } %% Exp : 'let' Var '=' Exp 'in' Exp { Let $2 $4 $6 } | Exp1 { Exp1 $1 } Exp1 : Exp1 '+' Term { Plus $1 $3 } | Exp1 '-' Term { Minus $1 $3 } | Term { Term $1 } Term : Term '*' Factor { Times $1 $3 } | Term '/' Factor { Div $1 $3 } | Factor { Factor $1 } Factor :: { Factor } : int { Int $1 } | ident { Var $1 } | '(' Exp ')' { Brack $2 } Var :: { Factor } : ident { Var $1 } | 'let' { Var "let" } Here 'Var' should be able to represent a 'let' identifier, as well as a keyword. Happy accepts the grammar but it does not parser the expected 'let let = x in let'. I have attached the full Happy grammar. I don't think this is an LR thing, but could be wrong. Many thanks in advance, Aaron

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A question regarding cmdargs package
by Ben Franksen 31 Dec '10

31 Dec '10

How can I disable the standard arguments 'help' and 'version'? Cheers

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Newbie Question
by Chionidis Ioannis 31 Dec '10

31 Dec '10

Hello everybody! I am quite new to haskell programming. I am trying to make a gui for a drumming project made in euterpea and I want to do something like this : > b <- button f [text := "Run!" , on command := (test mambo_orig)] "test mambo_orig" is executed from ghci but I want It to be able to execute each time the user presses the button. Hope I didn't mess it up in the explaination, Giannis Chionidis

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