- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] A question about "monad laws"
by askyle 15 Mar '08

15 Mar '08

> My favorite presentation of the monad laws is associativity of Kliesli > composition: > (a1 >=> a2) x = a1 x >>= a2 -- predefined in 6.8 control.monad > -- The laws > return >=> a = a > a >=> return = a > a >=> (b >=> c) = (a >=> b) >=> c If you use this presentation you also need the following law: (a . b) >=> c = (a >=> c) . b that is, compatibility with ordinary function composition. I like to call this "naturality", since it's instrumental in proving return and bind to be natural transformations. The law looks less alien if we use a flipped version of (>=>): (<=<) = flip (>=>) {- Monad Laws in terms of (<=<) -} return <=< f = f <=< return = f -- Identity f <=< (g <=< h) = (f <=< g) <=< h -- Associativity f <=< (g . h) = (f <=< g) . h -- Naturality (which happens to be my favorite presentation of the laws, followed by the derivations that lead to the 'do' notation, which lead to various 'ah' moments from unsuspecting FP-challenged friends) ----- Ariel J. Birnbaum -- View this message in context: http://www.nabble.com/A-question-about-%22monad-laws%22-tp15411587p15975734… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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Implementing fixed-sized vectors (using datatype algebra?)
by Alfonso Acosta 15 Mar '08

15 Mar '08

Hi, The EDSL implementation (system design) I'm working on would really benefit from an implementation of fixed-sized vectors. I thought this would be a generally desired ADT but it turned out I wasn't able to find an implementation. I was thinking about using datatype algebra plus GADTs to implement a type-level parameter indicating the size of the vector. I'm a total noob with regard to GADTs and type-level algebra trickery. So my questions are: 1) Do you think it is feasible? Can you think about a better approach? 2) An implementation of type-level Naturals would really help. What has already been done? Thanks in advance, Fons

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Gtk2hs
by Andrew Coppin 15 Mar '08

15 Mar '08

Just a short one... gtk2hs won't build on my [Linux] laptop. What's the best channel for seeking help with this?

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Type system
by Andrew Coppin 15 Mar '08

15 Mar '08

Haskell has an expressive and powerful type system - which I love. It also has a seemingly endless list of weird and obscure type system extensions. And there are various things you can do in Haskell which *require* some pretty serious type system hackery. And yet, none of this happens in any other programming language I've met. Take Java for example. In Java, a "type" is just an identifier. It has no further structure than that. It does have the added semantics that it refers to that class *or any subclass*, but that's about it. Even taking an extreme example like Eiffel [which supports generics], complex types are extremely rare. And yet they commonly pop up in Haskell. Can anybody put their finger on precisely why that is? Is it because Haskell is used by more PhDs? Is it because Haskell actually allows you to implement constructs that are impossible in other languages? Is it because Haskell really provides greater type safety? Is it something else? Not trying to suggest that either system is "superior", I'm just curios as to the origin of the difference...

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thanks for your feedback concerning tech documentation.
by michael＠schmong.org 14 Mar '08

14 Mar '08

Thanks. I was encouraged by this response I got. I'm ready to go. Since I'm trapped in the space-time continuum like most people, I can't do it all at once. I would like to. Anything that supports haskell is okay by me. My first area of interest is HAppS. I wrote some e-mail to them yesterday, but have yet to hear anything. Should I belt out some e-mail to specific people? By the looks of things, the traffic on that list is pretty slow right now. Michael Litchard

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Re: [Haskell-cafe] Gtk2hs
by Andrew Coppin 14 Mar '08

14 Mar '08

Don Stewart wrote: > andrewcoppin: > >> Just a short one... gtk2hs won't build on my [Linux] laptop. What's the >> best channel for seeking help with this? >> > > Discuss it on the gtk2hs list, with a full error log. > Thanks. I'll go look at that. (Who knows, maybe somebody already solved this one...)

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Ackermann Function Memoization, GHC Weird Output or Bug?
by Donnie Jones 14 Mar '08

14 Mar '08

Hello, I'm learning Haskell, so I was attempting memoization based upon the Fibonacci examples but for the Ackermann function. In my tests, I found what seems to be truncated output. See my comments at the end of the code for the test cases and output. ### Begin Code ### module Main where import Data.Array main = do let m = 3 n = 1 a = ackermann_mem m n putStrLn("Ackermann-mem " ++ show m ++ " " ++ show n ++ " = " ++ show a) -- Functions. -- Based upon examples from: -- http://reddit.com/r/programming/info/16ofr/comments) <http://reddit.com/r/programming/info/16ofr/comments%29> tabulate bounds f = array bounds [(i, f i) | i <- range bounds] dp bounds f = (memo!) where memo = tabulate bounds (f (memo!)) -- Trying to apply memoization function to Ackermann. ackermann_mem m n = dp ((0,0), (30, 1000)) ack (m, n) where ack rec (0, n) = n + 1 ack rec (m, 0) = rec (m - 1, 1) ack rec (m, n) = rec (m - 1, rec (m, n - 1)) {- Test cases: ackermann_mem 4 1 = 533 -- when using (30, 1000) as upper bound. ackermann_mem 4 1 = 5533 -- when using (30, 10000) as upper bound. ackermann_mem 4 1 = 65533 -- when using (30, 100000) as upper bound. <--- correct answer! -} ### End Code ### It seems if I don't choose an upper bound pair for (m,n) that is large enough I get truncated output for the answer, instead of GHC giving me an array index exception... This behavior seems very odd to me, can someone explain? Or is this a bug? Thank you. __ Donnie Jones

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File I/O question
by Andrew Coppin 14 Mar '08

14 Mar '08

Hi Cafe. There's good news and there's bad news. The bad news is... I'm back. [Did I miss anything good?] The good news is... I have an actual question to ask as well. When I write to a file using System.IO, the file is locked for exclusive access. I gather this is as specified in the Haskell Report. Which is nice, but... I'd actually prefer the file to *not* be locked. Anybody know how to do that?

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Space leak - help needed
by Krzysztof Kościuszkiewicz 14 Mar '08

14 Mar '08

Dear Haskellers, Another story from an (almost) happy Haskell user that finds himself overwhelmed by laziness/space leaks. I'm trying to parse a large file (>600MB) with a single S-expression like structure. With the help of ByteStrings I'm down to 4min processing time in constant space. However, when I try to wrap the parse results in a data structure, the heap blows up - even though I never actually inspect the structure being built! This bugs me, so I come here looking for answers. Parser follows: > module Main where > > import qualified Data.ByteString.Lazy.Char8 as B > import Text.ParserCombinators.Parsec > import Text.ParserCombinators.Parsec.Pos > import System.Environment > import System.Exit > import qualified Data.Map as M > import Lexer > > type XdlParser a = GenParser Token XdlState a > > -- Parser state > type XdlState = Counts > > type Counts = M.Map Count Integer > > data Count = ListCount | SymbolCount > deriving (Eq, Ord, Show) > > emptyXdlState = M.empty > > incCount :: Count -> (Counts -> Counts) > incCount c = M.insertWith' (+) c 1 > > -- handling tokens > myToken :: (Token -> Maybe a) -> XdlParser a > myToken test = token showTok posTok testTok > where > showTok = show > posTok = const (initialPos "") > testTok = test > > -- Syntax of expressions > data Exp = Sym !B.ByteString | List ![Exp] > deriving (Eq, Show) > > expr = list <|> symbol > > rparen = myToken $ \t -> case t of > RParen -> Just () > other -> Nothing > > lparen = myToken $ \t -> case t of > LParen -> Just () > other -> Nothing > > name = myToken $ \t -> case t of > Name n -> Just n > other -> Nothing > > list = do > updateState $ incCount ListCount > lparen > xs <- many1 expr > rparen > return () > -- return $! (List xs) > > symbol = do > updateState $ incCount SymbolCount > name >> return () > -- Sym `fmap` name > > -- Top level parser > top :: XdlParser XdlState > top = do > l <- many1 list > eof > getState > > main = do > args <- getArgs > case args of > [fname] -> do > text <- B.readFile fname > let result = runParser top emptyXdlState fname (tokenize text) > putStrLn $ either show show result > _ -> putStrLn "usage: parse filename" >> exitFailure And the Lexer: > module Lexer (Token(..), tokenize) where > > import qualified Data.ByteString.Lazy.Char8 as B > import Control.Monad > import Data.Char > import Data.List > import System.Environment > import System.Exit > > data Token = LParen > | RParen > | Name B.ByteString > deriving (Ord, Eq, Show) > > type Input = B.ByteString > > -- Processor returns Nothing if it can't process the Input > type Processor = Input -> Maybe ([Token], Input) > > -- Tokenize ends the list when all processors return Nothing > tokenize :: Input -> [Token] > tokenize = concat . unfoldr processAll > where > processors = [doSpaces, doComment, doParens, doName] > processAll :: Processor > processAll bs = if B.null bs > then Nothing > else foldr mminus Nothing $ map ($ bs) processors > mminus a@(Just _) _ = a > mminus Nothing b = b > > doSpaces :: Processor > doSpaces bs = > if B.null sp > then Nothing > else Just ([], nsp) > where > (sp, nsp) = B.span isSpace bs > > doComment :: Processor > doComment bs = > if B.pack "# " `B.isPrefixOf` bs > then Just ([], B.dropWhile (/= '\n') bs) > else Nothing > > doParens :: Processor > doParens bs = case B.head bs of > '(' -> Just ([LParen], B.tail bs) > ')' -> Just ([RParen], B.tail bs) > _ -> Nothing > > doName :: Processor > doName bs = > if B.null nsp > then Nothing > else Just ([Name nsp], sp) > where > (nsp, sp) = B.span (not . isRest) bs > isRest c = isSpace c || c == ')' || c == '(' Regards, -- Krzysztof Kościuszkiewicz Skype: dr.vee, Gadu: 111851, Jabber: kokr(a)jabberpl.org "Simplicity is the ultimate sophistication" -- Leonardo da Vinci

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FFI newbie question
by Verma Anurag-VNF673 14 Mar '08

14 Mar '08

I am trying to figure out how to pass array of String (char **) from C to Haskell? I have read the FFI examples, but most of them are centered on calling C from Haskell. I have read in the mailing list, it is rare to call Haskell from C, but my requirement is such that I am going to write Haskell library that needs to be called from C. Also, I read that the linking in FFI needs to be done through ghc. However, I have an existing Make system that uses gcc to do the linking, so what extra steps I need to do make gcc do the linking instead? Anurag

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