Not followed by

[gsnedders]

I've ended up with something like:

header = some(lambda tok: tok.type == "HEADER")
data = some(lambda tok: tok.type == "DATA")
empty_line = some(lambda tok: tok.type == "EMPTY")

body = many(data | empty_line)

segment = header + body
segments = segment + many(skip(empty_line) + segment)

This ends up with an unexpected token error for the second HEADER token with a token stream like HEADER BODY EMPTY HEADER BODY as the EMPTY gets consumed by body and hence it cannot be consumed by segments.

In Haskell I'd solve this with something like body = data <|> (try ( do { empty_line ; notFollowedBy header } )). As far as I can tell, there's nothing comparable to try or notFollowedBy. Is there any sensible way to define such a grammar?

[gsnedders]

Something like:

def notFollowedBy(p):
    @Parser
    def _notFollowedBy(tokens, s):
        try:
            p.run(tokens, s)
        except NoParseError, e:
            return skip(pure(None)).run(tokens, State(s.pos, e.state.max))
        else:
            raise NoParseError(u'is followed by', s)

    _notFollowedBy.name = u'(notFollowedBy %s)' % (p,)
    return _notFollowedBy

Seems to work. I'm sure I'm failing to do something right here, though, with that function! I'll open some PR with that soonish, I guess.

If I'm not mistaken, we have no need for try because | never consumes anything on the LHS, right?