On Mon, Jun 04, 2007 at 05:42:35PM +0300, Juozas Gaigalas wrote:
Hello,
I am a somewhat experienced programmer and a complete Haskell newbie, so I hope this is the correct ML for my question.
I have decided to learn Haskell and started with Graham Hutton's book. Everything was going nicely until section 8.4, on sequencing functional parsers. I am trying to write an elementary parser that produces the 1st and 3d elements from a string. I am using the code from the book.
---------------------
type Parser a = String -> [(a, String)]
return :: a -> Parser a return v = \inp -> [(v, inp)]
failure :: Parser a failure = \inp -> []
item :: Parser Char item = \inp -> case inp of [] -> [] (x:xs) -> [(x, xs)]
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) -- LINE 34 --------------------
BUT, when I try to :load parse.hs from hugs I get the following error:
ERROR "parse.hs":34 - Last generator in do {...} must be an expression
I have no idea what I am doing wrong and why Hugs is complaining. I hope this question is not too simply for this mailing list, but I have honestly googled for an answer and had found nothing.
If you wish to use 'do' notation with your parser, you must declare it as an instance of Monad class.
Juozas Gaigalas
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