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

2005-01-13 Thread Greg Buchholz 

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

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

2005-01-13 Thread Greg Buchholz 
Mike Gunter wrote:
 
 I'd guess that
  let (cam, sh) = parse_text shape_parse input
 is what you want?  (Completely untested ...)


Yep.  That did it. 

Thanks,

Greg Buchholz
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