Re: [Haskell-cafe] stuck with a sample of "programming in haskell"

19 Mar 2010


      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
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