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

19 Mar 2010

      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