Thanks for your solution. However, when I try this,
> str1 :: Parser String
> str1 = do str <- many anyToken
> notFollowedBy' semi
> return str
> notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st ()
> notFollowedBy' p = try $ join $ do a <- try p
> return (unexpected
> (show a))
> <|>
> return (return ())
> run:: Show a => Parser a -> String -> IO()
> run p input
> = case (parse p "" input) of
> Left err -> do {putStr "parse error at " ;print err}
> Right x -> print
When I compile, it still displays ";" at the end of the string.
Parser> run str1 "Hello ;"
"Hello ;"
The reason, as I think, because anyToken accepts any kind of token, it
considers ";" as token of its string. Thus, it does not understand
notFollowedBy' ???
Do you have any ideas about this ??? Thanks.
On 11/19/05, Andrew Pimlott <[EMAIL PROTECTED]> wrote:
> On Sat, Nov 19, 2005 at 06:43:48PM -0500, Sara Kenedy wrote:
> > str1 :: Parser String
> > str1 = do {str <- many anyToken; notFollowedBy semi; return str}
> >
> > However, when I compile, there is an error.
> >
> > ERROR "Test.hs":17 - Type error in application
> > *** Expression : notFollowedBy semi
> > *** Term : semi
> > *** Type : GenParser Char () String
> > *** Does not match : GenParser [Char] () [Char]
>
> The problem is that notFollowedBy has type
>
> notFollowedBy :: Show tok => GenParser tok st tok -> GenParser tok st ()
>
> ie, the result type of the parser you pass to notFollowedBy has to be
> the same as the token type, in this case Char. (The reason for this
> type is obscure.) But semi has result type String. You could fix the
> type error by returning a dummy Char:
>
> str1 = do {str <- many anyToken
> ; notFollowedBy (semi >> return undefined)
> ; return str}
>
> I think this will even work; however notFollowedBy is a pretty
> squirrelly function. There was a discussion about it:
>
> http://www.haskell.org/pipermail/haskell/2004-February/013621.html
>
> Here is a version (which came out of that thread) with a nicer type,
> that probably also works more reliably (though I won't guarantee it):
>
> notFollowedBy' :: Show a => GenParser tok st a -> GenParser tok st ()
> notFollowedBy' p = try $ join $ do a <- try p
> return (unexpected (show a))
> <|>
> return (return ())
>
> Andrew
>
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