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do... error
by Ryan Bloor 09 Dec '07

09 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 09 Dec '07

09 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 09 Dec '07

09 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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Re: [Haskell-cafe] parser
by Chris Eidhof 08 Dec '07

08 Dec '07

On 7 dec 2007, at 23:51, Ryan Bloor wrote: > i am using hugs and the isDigit and anything 'is' doesn't work... > they must have forgot to add them in! Does GHC work with them. Yes, it's in base. Alternatively, you could write the functions yourself, they're not that hard. > p.s... that book looks fantastic... bette than the ones i got from > the library; too specific. > also... when and where do i use my predetermined types(EBool) and > how...? I think that is explained in the book. It's also not too hard to figure out yourself. A good strategy for writing Haskell programs in general is to think of the types first: what types should your functions have? Often this will help you in thinking of a good implementation. The first thing you want to do, is parametrize the return-type of your parser. Instead of having type Parser = [(ETerm, String)] you probably want: type Parser a = String -> [(a, String)]. With that type, you could do something like: parseDigit :: Parser Int parseMany :: Parser a -> Parser [a] parseOr :: Parser a -> Parser a -> Parser a You can use these types to come up with the functions. -chris > > > Ryan > > > > > > > CC: haskell-cafe(a)haskell.org > > From: chris(a)eidhof.nl > > To: ryanbloor(a)hotmail.com > > Subject: Re: [Haskell-cafe] parser > > Date: Fri, 7 Dec 2007 23:31:38 +0100 > > > > > > On 7 dec 2007, at 22:55, Ryan Bloor wrote: > > > > > hi > > > > > > The thing is... it will be a simple parser really. The expressions > > > are already defined and we can't use parsec imports. Below is the > > > types I have. > > > I have a function that removes initial spaces from the start of a > > > string. A function that checks if a substring is part of another > > > string, which use the remove space function also. > > > The next function is: which uses the type: > > > > > > type Parser = String -> [(Expr,String)] > > So what does this type really mean? You give it a string, and it > will > > return a list of (Expr, String). I would guess that the list is all > > possible outcomes, and the String in (Expr, String) is probably the > > rest of the parsed data. If you want to find a correct parse, you > > probably want to select the first element from that list that has an > > empty rest. > > > > You could write a parser that parses single digits: > > > > parseDigit :: String -> [(Int, String)] > > parseDigit (x:xs) | isDigit x = [read x] > > > > Of course you have to define the other cases for parseDigit. If you > > had a parseMany function that parses many things of the same type, > you > > could combine that with parseDigit to parse natural numbers. The > other > > thing you are really going to need is a choice-operator. In your > > example, you want to parse terms that are either numbers or "term + > > term": > > > > parseTerm = parseNaturalNumber `parseOr` parseAddition > > > > It's probably best to read a good book or tutorial on parsers. There > > is an excellent textbook on grammars and parsing in Haskell [1], it > > probably explains exactly what you want. > > > > -chris > > > > [1] Johan Jeuring, Doaitse Swierstra: Grammars and Parsing: http://www.cs.uu.nl/docs/vakken/gont/diktaat.pdf > > > > > > > > > > > readExpression :: String -> Expr > > > > > > e.g. readExpression "True" = EBool True > > > e.g. readExpression "(23 + 67)" = EAdd (EInt 23) (EInt 67) > > > > > > > > > ------------------------ > > > Types------------------------------------------ > > > > > > data Type = TNone -- badly typed values > > > | TInt -- integer values > > > | TBool -- boolean values > > > deriving Show > > > data Expr = EInt {vInt :: Int} -- integer values > > > | EBool {vBool :: Bool} -- boolean values > > > | EAdd Expr Expr -- (e1 + e2) > > > | EMin Expr Expr -- (e1 - e2) > > > | EMul Expr Expr -- (e1 * e2) > > > | EAnd Expr Expr -- (e1 && e2) > > > | EOr Expr Expr -- (e1 || e2) > > > | ENot Expr -- not e1 > > > | EComp Expr Expr -- (e1 == e2) > > > | ETest Expr Expr Expr -- if e1 then e2 else e3 > > > | ENone -- badly formed expressions > > > > > > > > > > > > > > > > > > > > > > CC: haskell-cafe(a)haskell.org > > > > From: chris(a)eidhof.nl > > > > To: ryanbloor(a)hotmail.com > > > > Subject: Re: [Haskell-cafe] parser > > > > Date: Fri, 7 Dec 2007 22:17:54 +0100 > > > > > > > > > > > > On 6 dec 2007, at 18:06, Ryan Bloor wrote: > > > > > Can anyone advise me on how to check whether a string contains > > > ints, > > > > > chars, bools, etc.... > > > > > > > > > > "2345 + 6767" shoudl give IntAdd (2345) (6767) > > > > > "2345" should give IntT 2345 > > > > You need to write a parser. There are a lot of libraries that > will > > > > help you write a parser. One library that is often used for > writing > > > > parsers in Haskell is called Parsec [1]. There's good > > > documentation on > > > > that site on how to use it. Parsec is already included in you > > > > distribution. Good luck! > > > > > > > > -chris > > > > > > > > [1]: http://legacy.cs.uu.nl/daan/parsec.html > > > > > > > > > Get closer to the jungle… I'm a Celebrity Get Me Out Of Here! > > > > > Can you guess the film? Search Charades!

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Re: [Haskell-cafe] parser
by Chris Eidhof 07 Dec '07

07 Dec '07

On 7 dec 2007, at 22:55, Ryan Bloor wrote: > hi > > The thing is... it will be a simple parser really. The expressions > are already defined and we can't use parsec imports. Below is the > types I have. > I have a function that removes initial spaces from the start of a > string. A function that checks if a substring is part of another > string, which use the remove space function also. > The next function is: which uses the type: > > type Parser = String -> [(Expr,String)] So what does this type really mean? You give it a string, and it will return a list of (Expr, String). I would guess that the list is all possible outcomes, and the String in (Expr, String) is probably the rest of the parsed data. If you want to find a correct parse, you probably want to select the first element from that list that has an empty rest. You could write a parser that parses single digits: parseDigit :: String -> [(Int, String)] parseDigit (x:xs) | isDigit x = [read x] Of course you have to define the other cases for parseDigit. If you had a parseMany function that parses many things of the same type, you could combine that with parseDigit to parse natural numbers. The other thing you are really going to need is a choice-operator. In your example, you want to parse terms that are either numbers or "term + term": parseTerm = parseNaturalNumber `parseOr` parseAddition It's probably best to read a good book or tutorial on parsers. There is an excellent textbook on grammars and parsing in Haskell [1], it probably explains exactly what you want. -chris [1] Johan Jeuring, Doaitse Swierstra: Grammars and Parsing: http://www.cs.uu.nl/docs/vakken/gont/diktaat.pdf > > > readExpression :: String -> Expr > > e.g. readExpression "True" = EBool True > e.g. readExpression "(23 + 67)" = EAdd (EInt 23) (EInt 67) > > > ------------------------ > Types------------------------------------------ > > data Type = TNone -- badly typed values > | TInt -- integer values > | TBool -- boolean values > deriving Show > data Expr = EInt {vInt :: Int} -- integer values > | EBool {vBool :: Bool} -- boolean values > | EAdd Expr Expr -- (e1 + e2) > | EMin Expr Expr -- (e1 - e2) > | EMul Expr Expr -- (e1 * e2) > | EAnd Expr Expr -- (e1 && e2) > | EOr Expr Expr -- (e1 || e2) > | ENot Expr -- not e1 > | EComp Expr Expr -- (e1 == e2) > | ETest Expr Expr Expr -- if e1 then e2 else e3 > | ENone -- badly formed expressions > > > > > > > > CC: haskell-cafe(a)haskell.org > > From: chris(a)eidhof.nl > > To: ryanbloor(a)hotmail.com > > Subject: Re: [Haskell-cafe] parser > > Date: Fri, 7 Dec 2007 22:17:54 +0100 > > > > > > On 6 dec 2007, at 18:06, Ryan Bloor wrote: > > > Can anyone advise me on how to check whether a string contains > ints, > > > chars, bools, etc.... > > > > > > "2345 + 6767" shoudl give IntAdd (2345) (6767) > > > "2345" should give IntT 2345 > > You need to write a parser. There are a lot of libraries that will > > help you write a parser. One library that is often used for writing > > parsers in Haskell is called Parsec [1]. There's good > documentation on > > that site on how to use it. Parsec is already included in you > > distribution. Good luck! > > > > -chris > > > > [1]: http://legacy.cs.uu.nl/daan/parsec.html > > > Get closer to the jungle… I'm a Celebrity Get Me Out Of Here!

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