[Haskell-cafe] Parsec and type (IO t) error

13 Jan 2005

      This is probably an easy question, but I'm having a problem with
parsec in the IO monad.  The essential parts of my program looks like
this...
...
import Text.ParserCombinators.Parsec
main = do input <- getContents
         putStr $ show $ parse_text shape_parse input
         --(cam, sh) <- parse_text shape_parse input 
         --putStr $ (show cam) ++ "\n" ++ (show sh)
         putStr "\n"
parse_text p input = case (parse p input) of
                   Left err -> error $ "Invalid input"++(show err)
                   Right x  -> x
shape_parse = do cam <- camera_parse
                shapes <- many1 (sphere_parse <|> plane_parse)
                return (cam, shapes)
-- blah, blah, blah, etc.
This works fine in GHC.  The types for parse_text and shape_parse
are...

*Main> :t parse_text
parse_text :: forall a tok. GenParser tok () a -> [tok] -> a
*Main> :t shape_parse
shape_parse :: forall st. GenParser Char st (Camera, [Shape])
*Main> 

Now when I change main to...
...
main = do input <- getContents
         --putStr $ show $ parse_text shape_parse input
         (cam, sh) <- parse_text shape_parse input
         putStr $ (show cam) ++ "\n" ++ (show sh)
         putStr "\n"
I get the following message from GHCi...

p2.hs:38:
    Couldn't match `IO t' against `(Camera, [Shape])'
        Expected type: GenParser Char () (IO t)
        Inferred type: GenParser Char () (Camera, [Shape])
    In the first argument of `parse_text', namely `shape_parse'
    In a 'do' expression: (cam, sh) <- parse_text shape_parse input

I'm probably missing something silly.  Any hint would be appreciated.

Thanks,

Greg Buchholz