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Help with Programming in Haskell example
by Andre Nathan 18 May '07

18 May '07

Hello I've been reading Programming in Haskell, and I'm trying to go through the parser examples in chapter. However, I'm getting a type error when using the "do" notation. Here's the code I'm trying to load in ghci, which is copied from the book: import Prelude hiding ((>>=), return) import Char type Parser a = String -> [(a, String)] return :: a -> Parser a return v = \inp -> [(v, inp)] failure :: Parser a failure = \inp -> [] item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x, xs)] parse :: Parser a -> String -> [(a, String)] parse p inp = p inp (>>=) :: Parser a -> (a -> Parser b) -> Parser b p >>= f = \inp -> case parse p inp of [] -> [] [(v, out)] -> parse (f v) out p :: Parser (Char, Char) p = do x <- item item y <- item return (x, y) The problem is in the definition of "p": Couldn't match expected type `Char' against inferred type `[(Char, String)]' In the expression: x In the first argument of `return', namely `(x, y)' In the expression: return (x, y) Now if I define p as p :: Parser (Char, Char) p = item >>= \x -> item >>= \_ -> item >>= \y -> return (x, y) it works fine, so I'm wondering what else I need to do for the "do" notation to work. Thanks in advance, Andre

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Module dependency graph generator
by Alfonso Acosta 18 May '07

18 May '07

Hi all, I'm searching for a tool which generates a module dependency graph (a graphviz backend would be more than enough). Does anybody know a tool with such functionality? Cheers, Fons

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Quick Sort Algorithm
by PR Stanley 18 May '07

18 May '07

Hi folks qSort (x:xs) = qSort smaller ++ [x] ++ qSort larger where smaller = [a | a . xs, a x ] larger = [b | b . xs, b > x ] Any idea why I can't get this to work? Thanks, Paul

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reversing big list with constant heap space used
by Sergey Perminov 18 May '07

18 May '07

How to solve task of reversing big list with constant heap space used? Amount of heap space used grows exponentially in following examples: 1: main = putStrLn.show.head $reverse [1..10000000] 2 (GHC): import Data.List main = putStrLn.show.head $foldl' (flip (:)) [] [1..10000000] 3 (GHC): import Control.Monad main = foldM (\x y -> return $ y:x) [] [1..10000000] >>= putStrLn.show.head -- Best regards, Sergey Perminov

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Tail Recursion within the IO Monad
by Rob Hoelz 18 May '07

18 May '07

Hello everyone, You may have seen my message about how I'm writing a binding to a C library. This is another question related to that. So, let's say I have a linked list implemented in C. Here's what its definition looks like: struct __linked_list { void *data; struct __linked_list *next; }; typedef struct __linked_list linked_list_t; void *linked_list_getdata(linked_list_t *); linked_list_t *linked_list_next(linked_list_t *); Keep in mind, this is just a segment. So using the Haskell FFI, I import these into my .hsc file: data LinkedList = LL (Ptr linked_list_t) foreign import ccall unsafe "linked_list.h" linked_list_getdata :: Ptr LinkedList -> IO Ptr a foreign import ccall unsafe "linked_list.h" linked_list_next :: Ptr LinkedList -> IO Ptr LinkedList So now that that's done, I attempt to write a Ptr LinkedList -> [String] function (assuming the given LinkedList is holding c strings): linkedListToStringList :: Ptr LinkedList -> IO [String] linkedListToStringList listPtr = if listPtr == nullPtr then return [] else do item <- linked_list_getdata listPtr next <- linked_list_next listPtr cStr <- peek item hStr <- peekCString cStr t <- linkedListToStringList next return (hStr : t) This is just ugly...making the recursive call first, THEN consing the value on? However, this is the only way I could think of doing it. I figure there are three possibilities from here: 1) Leave this code alone, as GHC will optimize it because it's smart. 2) There's a way to more effectively write this code! Change it! 3) Roll my own optimization. I know how to do 3, but I'd rather avoid it. I guess I'm looking for an answer to 2, but if 1 is true, that'd be ok too. Could anyone give me a hand? And as long as I'm asking, is there some kind of monadic function composition operator? I'd like to clean up the above with something like peekCString . peek . linked_list_getdata... Many thanks, Rob Hoelz

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Safe/Unsafe calls with the GHC Foreign Function Interface
by Rob Hoelz 17 May '07

17 May '07

More questions from Rob regarding his mysterious bindings... So I've been reading the docs for the FFI, and it's my understanding that foreign functions imported as "unsafe" are faster, but they've got a problem with callbacks. From what I read, I believe that I should just make a foreign function safe if it sets up a callback; is this correct? Normally I wouldn't bother people with questions like this, but I'd rather be absolutely certain and avoid tracking down a strange bug later. Many thanks as always, Rob

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haskell-lang.org
by Dipen Javia 17 May '07

17 May '07

Hello, I just registered haskell-lang.org a week ago for 1 year. I was wondering if anybody wants it, I am selling it for regular price i got it which is $8 from godaddy. If you have an account with godaddy, tranferring is easy and free. If you dont have an account you can get one for free. Thanks. -- Dipen Javia

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Re: global variables
by Big Chris 17 May '07

17 May '07

On Thu, 17 May 2007, Jason Dagit wrote: > Well, it seems to me that Haskell modules are actually very similar to > singletons. Perhaps all these problems with modules having top level > mutable state could be solved if Haskell modules were parameterizable > at "instantiation"? I'm not saying we should turn the Haskell module > system into an OO system, just that maybe it would be wise to borrow > some ideas from that paradigm. Well, I'm a little unclear as to how the discussion shifted from global variables to modules, but as long as we're here: Rather than looking at OO languages, why not take a lesson from more powerful module systems already in functional programming languages. In particular, the ML module system supports "parameterization at instantiation" with functors. Of course, there is considerable evidence that ML modules and Haskell type classes really do the same thing, but they do seem to be useful in very different situations. --Chris

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IO Monad operation madness
by Rob Hoelz 17 May '07

17 May '07

Hello again, I'm still working on that binding, and I've come to a problem that I just can't figure out. I'm sure it's staring me right in the face, so I think another pair of eyes to take a look. So if you look at my previous message, I defined a linked list data type in C. I'm trying to convert it to a Haskell list of strings, so here's my function: linkedListToHaskellStringList :: LinkedList -> IO [String] linkedListToHaskellStringList listPtr = let convertList' ptr = if listIsNull ptr then [] else let str = peekCString =<< (linked_list_getdata ptr) next <- linked_list_next ptr str : (convertList' next) in sequence $ convertList' listPtr listIsNull :: LinkedList -> Bool listIsNull (LinkedList ptr) = ptr == nullPtr So here's the compile error: Option.hsc:63:14: Couldn't match expected type `[t]' against inferred type `IO LinkedList' In a 'do' expression: next <- linked_list_next ptr In the expression: if listIsNull ptr then [] else do let str = peekCString =<< linked_list_getdata ptr next <- linked_list_next ptr str : (convertList' next) In the definition of `convertList'': convertList' ptr = if listIsNull ptr then [] else do let str = ... next <- linked_list_next ptr str : (convertList' next) Could anyone tell me what I'm doing wrong? Thanks a lot! -Rob

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Darcs annotate denies all knowledge of files...
by Dougal Stanton 17 May '07

17 May '07

I just did a fresh pull of hpodder to see if there was still a bug in it that I just found in the feisty version. I ran > $ darcs get --partial http://darcs.changelog.org/hpodder > $ darcs annotate Commands/SetTitle.hs > > darcs failed: There is no file or directory named 'Commands/SetTitle.hs' There is some mention online that Darcs does this when there are special characters in the filename. That isn't the case here. To be certain I ran it on several other files, all with the same result. I ran it on some of my own repositories and it worked fine. What gives? Cheers, Dougal.

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