Text.Parsec.Combinators.notFollowedBy too restrictive

[bos]

(Imported from Trac #531, reported by nzeh on 2009-03-23)

Say, you want to write something like

aParser :: Parser String
aParser = many1 $ noneOf "a" <|>
                  ( char 'a' >>
                    notFollowedBy $ string "rrgh" >>
                    return 'a'
                  )

This fails because, according to the type signature of notFollowedBy, since the parser parses a stream characters, the parser given to notFollowedBy must have return type Char, which it doesn't. The same happens for something a little more meaningful such as

nonFinalA :: Parser ()
nonFinalA = char 'a' >> notFollowedBy eof

because eof has return type (). This is a request to change the type signature of notFollowedBy to accept a parser with arbitrary return type.

Rationale:

• The return type is ignored anyway.
• The return type has only a loose connection with what part of the input stream the parser consumes.

[bos]

(Imported comment by @dcoutts on 2009-03-23)

Replying to nzeh:

Say, you want to write something like

Looks like you've opened this ticket in the wrong trac. I'm not quite sure where one should report bugs in parsec but it's certainly not here. Sorry. The parsec.cabal file lists a maintainer, you could try emailing them.