March 2006 - Haskell-Cafe

Reading files efficiently
by Pete Chown 24 Mar '06

24 Mar '06

I've got another n00b question, thanks for all the help you have been giving me! I want to read a text file. As an example, let's use /usr/share/dict/words and try to print out the last line of the file. First of all I came up with this program: import System.IO main = readFile "/usr/share/dict/words" >>= putStrLn.last.lines This program gives the following error, presumably because there is an ISO-8859-1 character in the dictionary: "Program error: <handle>: IO.getContents: protocol error (invalid character encoding)" How can I tell the Haskell system that it is to read ISO-8859-1 text rather than UTF-8? I now used iconv to convert the file to UTF-8 and tried again. This time it worked, but it seems horribly inefficient -- Hugs took 2.8 seconds to read a 96,000 line file. By contrast the equivalent Python program: print open("words", "r").readlines()[-1] took 0.05 seconds. I assume I must be doing something wrong here, and somehow causing Haskell to use a particularly inefficient algorithm. Can anyone give me any clues what I should be doing instead? Thanks again, Pete

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Define combination of type classes?
by Fritz Ruehr 24 Mar '06

24 Mar '06

What is the easiest way to name a combination of type classes, i.e., to abbreviate the fact that a certain type is an instance of several classes simultaneously? I have a vague sense that this is do-able, but that I am messing up by trying to use an empty class body as below. So in the code below, I try to use FooBar to abbreviate the conjunction of Foo and Bar. But while f (which uses a FooBar constraint) has a valid definition, it can't be used. On the other hand, g (which uses the long-winded constraint), is both a valid defined and useable. (In a real example, imagine that FooBar names a conjunction of a half dozen things, so that the g-like form really is onerous, whereas the f-like form would be sweet and tidy :) .) -- Fritz ------------------- class Foo a where foo :: a -> Int class Bar a where bar :: a -> a class (Foo a, Bar a) => FooBar a f :: FooBar a => a -> Int f a = foo (bar a) g :: (Foo a, Bar a) => a -> Int g a = foo (bar a) instance Foo Char where foo = ord instance Bar Char where bar = id

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how would this be done? type classes? existential types?
by Matthias Fischmann 23 Mar '06

23 Mar '06

hi, this is one of those situations that always make scheme and perl hackers laugh at me: i have written a piece of code that is intuitively clear, and now i am trying to turn it into something that compiles. and here it goes. i have a type class that looks something like this: class Resource a where resourceName :: a -> String resourceAdvance :: a -> a resourceStarved :: a -> Bool resourceSpend :: a -> Int -> a resourceEarn :: a -> Int -> a resource types are rice, crude oil, pizza, software code, and so on. they all have a different internal structure and the same abstract interface, that's why i have defined this type class. now i want to create a list of a type similar to [r1, r2, r3] :: (Resource a) => [a] but with r1 being pizza, r2 being crude oil, and so on. my first idea was this: data Rs = forall a . (Resource a) => Rs a unRs (Rs a) = a rsName :: Rs -> String rsName = resourceName . unRs ... and then export Rs as an abstract data type. this would allow for lists of type [Rs], which is exactly what i want. but what is the type of unRs? or better: can i make it type at all? and isn't this solution a little redundant and verbose? should i do it like in the example for existentially quantified types in the ghc manual? http://www.haskell.org/ghc/docs/latest/html/users_guide/type-extensions.html but wouldnt't the code become really messy? or should i do the type class and instances, and then do Rs the existentially quantified way, with all class methods arguments to the Rs constructor? or is there a completely different way to do this (besides using scheme or perl :-)? thanks, matthias

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Code Review: Sudoku solver
by David F. Place 23 Mar '06

23 Mar '06

Hi All, I really appreciate all the help I received when I asked you to critique my PrefixMap module a few weeks ago. I think I am making good progress in correcting the "lisp" in my Haskell programming. I'll be very grateful to anyone who can take a glance at the attached short program and say if any unidiomatic usages pop out. It solves sudoku puzzles. (What pleasure do people get by doing these in their heads?!?) Cheers, David -------------------------------- David F. Place mailto:d@vidplace.com

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Wheres this going wrong
by Neil Rutland 22 Mar '06

22 Mar '06

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strange ghc program behaviour
by Koen.Roelandt＠mineco.fgov.be 22 Mar '06

22 Mar '06

Hi, I'm a Haskell newbie using HaXml for the conversion of xml files into html files. I'm using GHC 6.4 to compile the program. When I run the program, it will not convert the whole file: the document tree is incomplete and will stop when the limit of e.g. 8k has been reached. E.g. An xml file of 12.3k will result in a file of 8k and will stop at 8k An xml file of 15.7k will result in a file of 16k An xml file of 36k will result in a file of 24k In an output to the screen (and not to a file), the whole document tree is being generated. Am I missing something obvious here (memory issues)? On compilation, ghc issues warnings about the class FiniteMap and Module IOExts being deprecated. Is this the cause? Any ideas? Cheers, K.

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Returning a list element?
by Neil Rutland 22 Mar '06

22 Mar '06

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Re: Associated types and data constructors (was: the MPTC Dilemma (please solve))
by Manuel M T Chakravarty 21 Mar '06

21 Mar '06

David Menendez: > This is something I've been wondering about for a while. Can you do that > sort of thing with associated types? > > As another example, consider this type class for composible continuation > monads: > > class Monad m => MonadCC p sk m | m -> p sk where > newPrompt :: m (p a) > pushPrompt :: p a -> m a -> m a > withSubCont :: p b -> (sk a b -> m b) -> m a > pushSubCont :: sk a b -> m a -> m b > > You can use instances of this class to create backtracking monads, along > these lines: > > data Tree m a = HZero | HOne a | HChoice a (m (Tree m a)) > newtype SR p m a = SR (forall ans. ReaderT (p (Tree m ans)) m a) > > instance MonadCC p sk m => MonadPlus (SR p m) > > With associated types, the MonadCC class becomes: > > class Monad m => MonadCC m where > type Prompt m a > type SubCont m a b > > newPrompt :: m (Prompt m a) > pushPrompt :: Prompt m a -> m a -> m a > withSubCont :: Prompt m b -> (SubCont m a b -> m b) -> m a > pushSubCont :: SubCont m a b -> m a -> m b > > Since |Prompt m| is determined by |m|, can we eliminate the prompt > parameter from SR? > > newtype SR' m a > = SR' (forall ans. ReaderT (Prompt m (Tree m ans)) m a) > > instance MonadCC m => MonadPlus (SR' m) Yes, that's right. > That would presumably lead to SR' having the type > > SR' :: (MonadCC m) => ReaderT (Prompt m (Tree m ans)) m a -> SR' m a > > That doesn't seem like it should be a problem, since it's impossible to > create a value of type |ReaderT (Prompt m (Tree m ans)) m a| unless m is > an instance of MonadCC, but it also puts a context on a newtype data > constructor, which isn't currently allowed. If you go by the rules in the associated types paper, you need to put that context there. However, whether that context is really needed is the topic of an ongoing discussion. Without that sort of context, you can write programs where type errors are raised a bit later than if you have the context, but type safety doesn't seem to be affected by those contexts. The fact that newtype cannot have a context is an argument in favour of not requiring the context. As your example shows, associated types (in comparison to FDs) not only eliminate type arguments from classes, but also from data type declarations. IMHO this improves the readability of programs. Manuel

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Arrows, wxFruit
by Gerbrand van Dieijen 20 Mar '06

20 Mar '06

Hello, I am working on a thesis project which involves for a large part constructing a gui. In short, I want to create a combinator library that gives (a subset of) functionality, reporting libraries as Crystal reports offer. I read about Yampa and Fruit and wxFruit and both seem a very elegant way to construct a gui.Unfortunately, Fruit uses a rather low-level library which makes it of less use. wxFruit looks very nice, but the library does not seem to be maintained anymore. First, is wxFruit still maintained, or are there any plans to further develop it. Or, can you recommend other libraries/frameworks to construct a gui using combinators, that use higher level libraries such as wxHaskell? (An other possibility would be to use wxHaskell only and construct the combinators entirely myself. This might be the most flexible approach.) Second, I can't get wxFruit to compile on my machine. When I compile I use the following command-line: ghci -fglasgow-exts -package data -package wx -i/users/gjhdieij/dowload/afrp-0.4/src WXFruit.hs The packages than almost loads, but I still get the following errors (GHC 6.4, wxHaskell 0.9.4) . Does anyone know a sollution? /users/gjhdieij/dowload/afrp-0.4/src/AFRPUtilities.hs:169:46: Ambiguous occurrence `>>^' It could refer to either `AFRPUtilities.>>^', defined at /users/gjhdieij/dow load/afrp-0.4/src/AFRPUtilities.hs:106:2 or `Control.Arrow.>>^', imported from AFRP at /users/g jhdieij/dowload/afrp-0.4/src/AFRPUtilities.hs:90:0-10 /users/gjhdieij/dowload/afrp-0.4/src/AFRPUtilities.hs:178:48: Ambiguous occurrence `>>^' It could refer to either `AFRPUtilities.>>^', defined at /users/gjhdieij/dow load/afrp-0.4/src/AFRPUtilities.hs:106:2 or `Control.Arrow.>>^', imported from AFRP at /users/g jhdieij/dowload/afrp-0.4/src/AFRPUtilities.hs:90:0-10 (etc.)

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First Question
by Neil Rutland 20 Mar '06

20 Mar '06

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