[Haskell-cafe] Parsing problem

Hi all, I'm using the Parsec library to parse the following grammar expr = atom+ atom = integer | var | (expr) The input to the parser is a list of (Token, SourcePos). I have the following code for atom: atom = try variable <|> try integerr <|> do{sat(== Reserved "("); (e,pos) <- expression; sat(==Reserved ")"); return (e,pos)} sat p = do (t,pos) <- item if p t then return(t,pos) else pzero When I run the code on the input "(_ineg_ 0)" the parser fails, but removing sat(==Reserved ")") and it succeeds. Can any one see why? E.