[Haskell-cafe] newbie question on Parsers from "Programming In Haskell"

[Ilya Tsindlekht]

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

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