January 2008 - Haskell-Cafe

C# parser written in Haskell
by Joel Reymont 27 Jan '08

27 Jan '08

Does anyone have a C# parser written in Haskell? Thanks, Joel -- http://wagerlabs.com

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Testing one's .ghci
by gwern0＠gmail.com 27 Jan '08

27 Jan '08

So I was recently going over my config files (which are version-controlled in Darcs, of course), and I was adding tests for recording patches. For some of the files, it was easy enough - the application generally provided some mean of loading in the rc file and then it would error or not based on whether the file was at least syntactically correct. For example, to test my .emacs, I have 'emacs -batch -f batch-byte-compile ~/.emacs'. If the .emacs won't byte-compile, it probably won't run! My question is, how many I do the same thing with GHCi? No options specifically for this seem to be listed, and the normal trick doesn't work - I tried: > ghci -read-dot-ghci -e 'let main = return() in main' && echo foo And regardless of whether the .ghci had hideous crippling errors in it, at the end "foo" was still merrily being printed out. Does haskell-cafe have any ideas or should I just go file a feature request? -- gwern chameleon Vale RSA Recon 1080H beanpole advisors Harvard Texas PARKHILL

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ANNOUNCE: HStringTemplate -- An Elegant, Functional, Nifty Templating Engine for Haskell
by Sterling Clover 26 Jan '08

26 Jan '08

HStringTemplate is a port of Terrence Parr’s lovely StringTemplate (http://www.stringtemplate.org) engine to Haskell. It is available, cabalized, at: darcs get http://code.haskell.org/HStringTemplate/ As interest has grown in using Haskell for web applications, there has been an increasing buzz about the need for a good templating engine in Haskell. Why might we need this? After all, Haskell has lovely combinator libraries for generating HTML programmatically, enforcing good form through its type system. But sometimes, we don’t want well-formed HTML. We want the ugly stuff that floats around to deal with eight varieties of browser incompatibilities and the latest silly ajax trick. Or sometimes we’re working with a team of graphic designers, and they want to work in almost-HTML. Or sometimes we just want to be able to change the design of a web application on the fly, completely independent of our program logic, and of, heavens forbid, recompiling and possibly messing with a live application. So template engines are popular, and indeed, considered a key part of most web frameworks out there. One problem — they’re mainly awful, imperatively-conceived behemoths that capriciously mix program logic with display and, consequently, entail a great deal of overhead. Enter StringTemplate, a nifty and fairly-well developed template format that’s both pure and functional, and therefore pretty much the only one of its kind. Indeed, it also seems to be getting heavy use in code generation because its paradigm maps neatly to traversing parse-trees. HStringTemplate is not feature-complete, and indeed is only at version 0.1. But it should implement pretty much everything in the standard StringTemplate 3.0 grammar, only nicer, because it’s in Haskell. There are scads of different recursive constructs and ways to handle inclusion and inheritance. Furthermore, HStringTemplate handles conditionals, and sports also a very Haskellish implementation of custom rendering. Templates can be constructed that return strings, ShowSs, bytestrings, or even pretty printer Docs that handle wrapping, indentation, and fill elegantly. Even better, these templates are parsed and compiled only once, after which point there isn't a syntax tree anymore, just a function that operates on the environment of attributes that have been passed to it. Where I take it from here depends in part on what sort of response I get, so patches, gripes, API comments and feature requests are all more than welcome. Please note that I'm still working in 6.6.1. Everything should be in place to compile properly with the base split, but if it isn't, again, patches more than welcome. Full announcement at: http://fmapfixreturn.wordpress.com/ --SC

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Generic permutations
by Cetin Sert 26 Jan '08

26 Jan '08

How can I make a generic permutations function? -- boolean permutations bpms :: Int → [[Bool]] bpms 0 = [[]] bpms n = map (False:) bss ++ map (True:) bss where bss = bpms (n - 1) -- generic permutations pms a :: Int → [[a]] Best Regards, Cetin Sert

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Re: [Haskell-cafe] Re: Generic permutations with high performance
by Cetin Sert 26 Jan '08

26 Jan '08

Hello again Ryan, I have found out where to import those stuff from and tested your more elegant suggestion and my original performance. -- print ((length ∘ pmsO [0,1]) 24) 9~ seconds -- print ((length ∘ pmsE [0,1]) 24) 23~ seconds -- print ((length ∘ pmsU [0,1]) 24) 23~ seconds -- O: original, E: elegant, U: unreadable I prefer performance over elegance (and for me using monads almost feels like adding an unnecessary dependency to the definition of pms. Though I know I may be dead wrong on that. I just don't quite understand monads yet.) I would love to have you and/or others suggest more performant versions of pms (and maybe also come up with a better name for it). mnt :: [a] → [[a]] → [[a]] mnt [] _ = [] mnt _ [] = [] mnt (x:xs) yss = map (x:) yss ++ mnt xs yss pms :: [a] → Int → [[a]] pms [] _ = [[]] pms _ 0 = [[]] pms xxs n = mnt xxs (pms xxs (n - 1)) I generalized 'pms' from the 'bools' function on page 108 of Programming in Haskell (Hutton, 2007) -- Cetin Sert On 26/01/2008, Ryan Ingram <ryani.spam(a)gmail.com> wrote: > > When you say permuations, I think of reorderings of a list, for example: > > permutations [1,2,3] = > [ [1,2,3], > [1,3,2], > [2,1,3], > [2,3,1], > [3,1,2], > [3,2,1] ] > > Here's an implementation: > > -- split [1,2,3] => [ > -- ( 1, [2,3] ), > -- ( 2, [1,3] ), > -- ( 3, [1,2] ) ] > split :: [a] -> [(a, [a])] > split [] = error "split: empty list" > split [a] = [(a, [])] > split (a:as) = (a, as) : map prefix (split as) > where prefix (x, xs) = (x, a : xs) > > permutations :: [a] -> [[a]] > permutations [] = return [] > permutations xs = do > (first, rest) <- split xs > rest' <- permutations rest > return (first : rest') > > The problem you solved can be solved much more elegantly: > > pms : [a] -> Int -> [[a]] > pms xs n = foldM combine [] (replicate n xs) where > combine rest as = liftM (:rest) as > > or, for the unreadable version: > pms xs n = foldM (map . flip (:)) [] $ replicate n xs > > (note that, in the list monad, liftM = map). > > -- ryan >

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Problem with HGL
by jia wang 26 Jan '08

26 Jan '08

Hi, I'm trying to use HGL-3.2.0.0 with ghc-6.8.2 on my windows system. When I run command: runhaskell Setup.hs build it shows an error D:\HGL-3.2.0.0\HGL-3.2.0.0>runhaskell Setup.hs build Preprocessing library HGL-3.2.0.0... Building HGL-3.2.0.0... Graphics/HGL/Key.hs:57:7: Could not find module `Graphics.Win32': it is a member of package Win32-2.1.1.0, which is hidden so, how to solve this problem, thanks

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running into an unresolved linker error on RHEL 5
by Galchin Vasili 25 Jan '08

25 Jan '08

Hello, I am building the unix-2.2.0.0. The linker is failing with -lgmp. I set "export LD_LIBRARY_ATH=/bin:/usr/lib". I did a "ls -l /usr/lib/*gmp*" and I can see: libgmp.so.3 libgmp.so.3.3.3 libgmpxx.so.3 libgmpxx.so.3.0.5, i.e. either gmp shared objects and whatever the gmp "xx" shared objects are. So why is my link of unix-2.2.0.0 failing in DynamicLinker? Kind regards, Vasili

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ANN: bytestringparser-0.2, a Parsec-like parser for lazy ByteStrings
by Bryan O'Sullivan 25 Jan '08

25 Jan '08

Some time ago, Jeremy Shaw wrote a Parsec clone for lazy ByteStrings. I've been using it for a while, and have made substantial changes to it along the way. It's very fast, using the same manual unpacking trick as the binary package to keep performance nippy. It also integrates with the latest flava in parsing combinators, Control.Applicative, providing instances of the Applicative and Alternative typeclasses. The API is rather smaller than that of Parsec, but it's more than adequate for parsing e.g. text-based network protocols. Download: http://hackage.haskell.org/packages/archive/bytestringparser/0.2/bytestring… Haddocks: http://darcs.serpentine.com/bytestringparser/dist/doc/html/bytestringparser… Darcs repository: darcs get http://darcs.serpentine.com/bytestringparser <b

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HList error with hFoldr
by Denis Bueno 25 Jan '08

25 Jan '08

Hello all, I'm doing some machine learning in Haskell and have run into a problem. I have a generic distance function (declare in the MetricSpace) typeclass that returns the distance between two things as a number. I frequently will be working with heterogeneous collections of data, and if possible I'd like to write the distance function (overloaded) once for all. At the moment, I'm using HList to represent the collections of data uniformly. I wrote the following as a sanity check but I can't even get it to compile. The effect I'd like is that I can call `dist` on any two HLists which have the correct, corresponding element types in the correct positions, and `dist` will be recursively invoked to calculate the typical euclidean distance (squared sum of individual element distances, and at the and taking the square root). My attempt at doing this follows. For the moment my HLists only contain Ints. I'm using GHC 6.8.2 and HList 0.1. import HList class (Num i) => MetricSpace e i where dist :: e -> e -> i instance Num i => MetricSpace Int i where x `dist` y = fromIntegral $ abs (y - x) data ApplyDistSum = ApplyDistSum instance (MetricSpace e r) => Apply ApplyDistSum ((e, e), r) r where apply _ (p, v) = v + uncurry dist p -- Why should the following generate an error? testApplyDistSum = hFoldr ApplyDistSum 0 ((4,4) .*. hNil) {- The error: /Users/denbuen/edu/cornell/meng/classes/cs678/code/practice/Knn.hs:53:19: No instance for (Apply ApplyDistSum ((t, t1), r1) r) arising from a use of `hFoldr' at /Users/denbuen/edu/cornell/meng/classes/cs678/code/practice/Knn.hs:53:19-56 Possible fix: add an instance declaration for (Apply ApplyDistSum ((t, t1), r1) r) In the expression: hFoldr ApplyDistSum 0 ((4, 4) .*. hNil) In the definition of `testApplyDistSum': testApplyDistSum = hFoldr ApplyDistSum 0 ((4, 4) .*. hNil) -} Thanks in advance. -- Denis

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Evolutionary Computing
by PR Stanley 25 Jan '08

25 Jan '08

Hi What does the list think of EC? Genetic algorithm is apparently the latest thing. Is EC mathematically reasonable? Paul

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