December 2007 - Haskell-Cafe

AW: ST Monad - what's wrong?
by Nicu Ionita 09 Dec '07

09 Dec '07

Ok, solved, the use of ($) was the problem. I changed it > primes :: Set Integer > primes = runST (getPrimes "primes10h7.txt") and it compiles (and works too). Nicu > -----Ursprüngliche Nachricht----- > Von: Nicu Ionita [mailto:nionita@lycos.de] > Gesendet: Sonntag, 9. Dezember 2007 12:12 > An: 'haskell-cafe(a)haskell.org' > Betreff: ST Monad - what's wrong? > > > Hi, > > I'm trying to use the ST monad in order to turn an almost > pure function into a pure one: reading a precalculated list > of primes into a prime set. But the following code brings an error: > > > primes :: Set Integer > > primes = runST $ getPrimes "primes10h7.txt" > > > getPrimes :: String -> (forall s. ST s (Set Integer)) > getPrimes file = > > do cont <- unsafeIOToST (readFile file) > > let set = fromList $ map read $ lines cont > > return set > > And here is the error: > > Couldn't match expected type `forall s. ST s a' > against inferred type `ST s (Set Integer)' > In the second argument of `($)', namely > `getPrimes "primes10h7.txt"' > In the expression: runST $ (getPrimes "primes10h7.txt") > In the definition of `primes': > primes = runST $ (getPrimes "primes10h7.txt") > > Compiled with GHC 6.6.1 with extensions activated. > > Nicu >

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Re: [Haskell-cafe] do... error
by Felipe Lessa 09 Dec '07

09 Dec '07

On Dec 9, 2007 1:01 AM, Ryan Bloor <ryanbloor(a)hotmail.com> wrote: > But what is the right way to indent...? It is so annoying, why does it > matter so much! :( You may read http://en.wikibooks.org/wiki/Haskell/Indentation which tries to explain in a very simple language. -- Felipe.

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do... error
by Ryan Bloor 08 Dec '07

08 Dec '07

hi test :: Parser (Char,Char) test = do x <- item item y <- item return (x,y) How come this brings an error saying that after do {....} it must end with an expression. Ryan _________________________________________________________________ Get free emoticon packs and customisation from Windows Live. http://www.pimpmylive.co.uk

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Re: [Haskell-cafe] Type error in final generator
by Loganathan Lingappan 08 Dec '07

08 Dec '07

Thanks Bryan and Derek. This works! Logo ----- Original Message ---- From: Derek Elkins <derek.a.elkins(a)gmail.com> To: Bryan O'Sullivan <bos(a)serpentine.com> Cc: Loganathan Lingappan <loganathan.lingappan(a)yahoo.com>; haskell-cafe(a)haskell.org Sent: Saturday, December 8, 2007 4:53:54 PM Subject: Re: [Haskell-cafe] Type error in final generator On Sat, 2007-12-08 at 16:39 -0800, Bryan O'Sullivan wrote: > Loganathan Lingappan wrote: > > > main = do > > hSetBuffering stdin LineBuffering > > numList <- processInputs > > foldr (+) 0 numList > > The type of main is understood to be IO (), so it can't return anything. > You could work around this by rewriting the last line above as follows: > > print (foldr (+) 0 numList) > > This prints the number, which is presumably what you want, and print has > type IO (), so it works out nicely here. Nitpicking: Actually, as the error message says, the type of main is IO a so it can 'return' -anything- and that will be discarded as the type makes clear. The issue is that foldr (+) 0 numList :: Integer and that's not IO a for any a. Either way, Bryan's suggestion is probably what you want.

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Type error in final generator
by Loganathan Lingappan 08 Dec '07

08 Dec '07

Hi, I am new to Haskell. I wrote the following code: module Main where import IO main = do hSetBuffering stdin LineBuffering numList <- processInputs foldr (+) 0 numList processInputs = do putStrLn "Enter a number:" strNum <- getLine let num = read strNum if num == 0 then return [] else do rest <- processInputs return (num : rest) And am getting the following errors: ERROR "Ex310.hs":6 - Type error in final generator *** Term : foldr (+) 0 numList *** Type : Integer *** Does not match : IO a Any pointers to the source of this error would be appreciated. Thanks, Logo

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general-revised
by Ryan Bloor 08 Dec '07

08 Dec '07

hi I have four functions below: What I want to do is have a way to parse more than one digit or more than one string head in ParseTrue. Any ideas... removeSpace:: String -> StringremoveSpace = dropWhile (`elem` space) where space = [' '] match :: String -> String -> (Bool, String)match word str | ((isPrefixOf) (removeSpace word) (removeSpace str)) = (True,rest) | otherwise = (False,str) where rest = drop (length (removeSpace word)) (removeSpace str) parseDigit :: String -> [(Int, String)]parseDigit (x:xs) | isDigit x = [(read [x],xs)] | otherwise = [] parseTrue :: String -> (Bool, String)parseTrue x = match "True" x Ryan _________________________________________________________________ Celeb spotting – Play CelebMashup and win cool prizes https://www.celebmashup.com

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Over-allocation
by Gracjan Polak 08 Dec '07

08 Dec '07

Hi, My program is eating too much memory: copyfile source.txt dest.txt +RTS -sstderr Reading file... Reducing structure... Writting file... Done in 20.277s 1,499,778,352 bytes allocated in the heap 2,299,036,932 bytes copied during GC (scavenged) 1,522,112,856 bytes copied during GC (not scavenged) 17,846,272 bytes maximum residency (198 sample(s)) 2860 collections in generation 0 ( 10.37s) 198 collections in generation 1 ( 8.35s) 50 Mb total memory in use INIT time 0.00s ( 0.00s elapsed) MUT time 1.26s ( 1.54s elapsed) GC time 18.72s ( 18.74s elapsed) EXIT time 0.00s ( 0.00s elapsed) Total time 19.98s ( 20.28s elapsed) %GC time 93.7% (92.4% elapsed) Alloc rate 1,186,898,778 bytes per MUT second Productivity 6.3% of total user, 6.2% of total elapsed The source.txt is 800kb, and I expect files of size 100 times more, say 80MB, so using -H800M will not help here much. The profile -p says: Wed Nov 21 14:23 2007 Time and Allocation Profiling Report (Final) copyfile +RTS -p -RTS source.txt dest.txt total time = 4.48 secs (224 ticks @ 20 ms) total alloc = 1,500,359,340 bytes (excludes profiling overheads) COST CENTRE MODULE %time %alloc xparse PdfModel 78.1 95.0 followed by a lot of 0.0 numbers. Then the faulty xparse: xparse uniq borders body bin = do parseOperatorEq "xref" p <- P.many (parseXRefSection uniq) parseOperatorEq "trailer" e <- parseDict let entries = IntMap.fromList (map (\(a,b,c) -> (a,c)) (concat p)) return (Body e entries) where the P is Text.ParserCombinators.ReadP made to work with [Word8]. How do I know WHAT is making so much allocation in this function? My files are xml-like in structure. The task currently is to read, parse, write the file. I can provide more details for interested souls. How do I make my program make less allocations? -- Gracjan

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general
by Ryan Bloor 08 Dec '07

08 Dec '07

hi I have a problem. Function A is a function that passes its input into B Function B is a function that does something once. How do I make it so function A is done multiple times without adding a third function? Ryan _________________________________________________________________ Who's friends with who and co-starred in what? http://www.searchgamesbox.com/celebrityseparation.shtml

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do notation strangeness
by Felipe Lessa 08 Dec '07

08 Dec '07

Hello! I see from http://www.haskell.org/haskellwiki/Monads_as_computation#Do_notation that do { v <- x ; <stmts> } = x >>= \v -> do { <stmts> } However, look at this GHCi session: Prelude> let return' = return :: a -> Maybe a Prelude> do {1 <- return 1; return' "ok"} Just "ok" Prelude> return 1 >>= \1 -> return' "ok" Just "ok" Prelude> do {1 <- return 3; return' "ok"} Nothing Prelude> return 3 >>= \1 -> return' "ok" *** Exception: <interactive>:1:13-30: Non-exhaustive patterns in lambda Hmmm... let's try with IO: Prelude> let return' = return :: a -> IO a Prelude> do {1 <- return 1; return' "ok"} "ok" Prelude> return 1 >>= \1 -> return' "ok" "ok" Prelude> do {1 <- return 3; return' "ok"} *** Exception: user error (Pattern match failure in do expression at <interactive>:1:4) Prelude> return 3 >>= \1 -> return' "ok" *** Exception: <interactive>:1:13-30: Non-exhaustive patterns in lambda Oh! What about lists? Prelude> let return' = return :: a -> [a] Prelude> do {1 <- return 1; return' "ok"} ["ok"] Prelude> return 1 >>= \1 -> return' "ok" ["ok"] Prelude> do {1 <- return 3; return' "ok"} [] Prelude> return 3 >>= \1 -> return' "ok" *** Exception: <interactive>:1:13-30: Non-exhaustive patterns in lambda Something seems wrong to me here. What am I missing? Thanks! -- Felipe.

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a Char->Char function?
by Galchin Vasili 08 Dec '07

08 Dec '07

Hello, I am writing a function(actually much more than this): bozo :: Char -> Char bozo 0x02 = 'a' ... However, I get complaints from ghc suggesting that I should add an "instance declaration (Num, Char). I (mistaking) thought I understood the Haskell class hierarchy and the associated constraints .. but apparently not .. => help please. Should Kind regards, Vasya

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