Thanks very much. It works! I just wonder if you can help me to define a Monad to make "do" notion works :-) ? I know it is bothering, but I just ever tried to define a Monad, failed either. What I did to define a Monad was: instance Monad Parser where return v = (\inp->[(v,inp)]) f >>= g = = (\inp -> case parse p inp of [] -> [] [(v,out)]->parse (f v) out) But it did not compile :-(. Best Regards, Guo-ping 2010/3/17 Michael Snoyman :

Hi, You can only use do notation if you actually create an instance of Monad, which for Parser you haven't done. To continue as is, replace the first line with: import Prelude hiding (return, fail, (>>=)) and the p function with p = item >>= \x -> item >>= \_ -> item >>= \y -> return (x, y) I've basically de-sugared the do-notation you wrote and hid the >>= from Prelude so that the one you declared locally is used. Michael On Tue, Mar 16, 2010 at 9:09 PM, 国平张 wrote:

...

Hi,

I am a beginner for haskell. I was stuck with a sample of "programming in haskell". Following is my code: --------------------------------------------------------------------- import Prelude hiding (return, fail)

type Parser a = (String->[(a,String)])

return :: a -> Parser a return v = (\inp->[(v,inp)])

item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] failure :: Parser a failure = \inp -> []

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) ---------------------------------------------------------------------

But it cannot be loadded by Hug, saying:

Couldn't match expected type `Char' against inferred type `[(Char, String)]' Expected type: [((Char, Char), String)] Inferred type: [(([(Char, String)], [(Char, String)]), String)] In the expression: return (x, y) In the expression: do x <- item item y <- item return (x, y)

-------------------------------------------------------------------

I googled and tried a few days still cannot get it compiled, can someone do me a favor to point out what's wrong with it :-) ? _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Sorry to bother again. I just cannot figure out how it could compile. I got compile errors. Can someone point out what is right code to use a do notion to make a Parser works. Thanks in advance. -------------------------------------------------------------------------------------------- newtype Parser a = P { parse :: (String -> [(a,String)]) } instance Monad Parser where return v = P (\s -> [(v,s)]) p >>= f = P (\s -> case parse p s of [] -> [] [(v,str)] -> parse (f v) str) fail _ = P (\_ -> []) item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] p :: Parser (Char,Char) p = do x <- item item y <- item return (x,y) ---------------------------------------------------------------------------------

Sorry. The same error, This is new stuff. ----------------------------------------------------------------------------------- newtype Parser a = P { parse :: (String -> [(a,String)]) } instance Monad Parser where return v = P (\s -> [(v,s)]) p >>= f = P (\s -> case parse p s of [] -> [] [(v,str)] -> parse (f v) str) fail _ = P (\_ -> []) item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)] p :: Parser (Char,Char) p = do { x <- item ; item ; y <- item ; return (x,y) } ----------------------------------------------------------------------------------- I got following: Prelude> :load c:\b.hs [1 of 1] Compiling Main ( C:\b.hs, interpreted ) C:\b.hs:13:7: The lambda expression `\ inp -> ...' has one argument, but its type `Parser Char' has none In the expression: \ inp -> case inp of { [] -> [] (x : xs) -> [...] } In the definition of `item': item = \ inp -> case inp of { [] -> [] (x : xs) -> ... } Failed, modules loaded: none. 2010/3/19 Stephen Tetley :

On 19 March 2010 04:35, 国平张 wrote:

...

Sorry to bother again. I just cannot figure out how it could compile. I got compile errors. Can someone point out what is right code to use a do notion to make a Parser works.

It looks like the p parser may have the wrong indentation - although this might be due to either your mail client or my client formatting wrongly:

p :: Parser (Char,Char) p = do x <- item item y <- item return (x,y)

Try - with white space all aligned to the start character /x/ of the first statement in the do:

p :: Parser (Char,Char) p = do x <- item item y <- item return (x,y)

Or with braces and semis:

p :: Parser (Char,Char) p = do { x <- item ; item ; y <- item ; return (x,y) }

Best wishes

Stephen _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Sorry :-). I am using Hugs, anything I did wrong ? ------------------------------------------------------------------------------------------------ newtype Parser a = P { parse :: (String -> [(a,String)]) } instance Monad Parser where return v = P (\s -> [(v,s)]) p >>= f = P (\s -> case parse p s of [] -> [] [(v,str)] -> parse (f v) str) fail _ = P (\_ -> []) item :: Parser Char item = Parser (\inp -> case inp of [] -> [] (x:xs) -> [(x,xs)]) p :: Parser (Char,Char) p = do { x <- item ; item ; y <- item ; return (x,y) } -------------------------------------------------- Prelude> :load c:\b.hs [1 of 1] Compiling Main ( C:\b.hs, interpreted ) C:\b.hs:12:7: Not in scope: data constructor `Parser' Failed, modules loaded: none. Prelude> 在 2010年3月19日 下午6:01，Stephen Tetley 写道：

2010/3/19 国平张 :

...

Sorry. The same error, This is new stuff.

Ah indeed - I didn't spot that one as I only read the code rather than ran it.

With the change the parser type to use /newtype/ all the primitive parsers have to be encoded inside the newtype's constructor (primitive parsers being ones that have to look directly at the input stream).

item :: Parser Char item = Parser $ \inp -> case inp of [] -> [] (x:xs) -> [(x,xs)]

Or in a more prosaic style

item :: Parser Char item = Parser (\inp -> case inp of [] -> [] (x:xs) -> [(x,xs)])

This is slightly tiresome. Fortunately once you have defined a small set of primitive parsers, many more parsers can be "derived" by combining the primitives rather than looking at the input stream - this is the power of the monadic style. The p parser you defined with the do ... notation is one such derived parser.

Best wishes

Stephen

Hi I'm sorry about that, I should have check the last message runs, but I typed it from a computer that I don't develop on. The code below should run as I've tested it this time. newtype Parser a = P { parse :: (String -> [(a,String)]) } instance Monad Parser where return v = P (\s -> [(v,s)]) p >>= f = P (\s -> case parse p s of [] -> [] [(v,str)] -> parse (f v) str) fail _ = P (\_ -> []) item :: Parser Char item = P (\inp -> case inp of [] -> [] (x:xs) -> [(x,xs)]) p :: Parser (Char,Char) p = do { x <- item ; item ; y <- item ; return (x,y) } ---------------------------------------- For the record - the error in the last code I sent was that the newtype Parser has a different constructor name /P/ to its type name /Parser/ - I hadn't spotted that in the untested code. Apologies again Stephen