[Haskell-cafe] Re: Parsec lookahead and <|>
daniel.is.fischer at web.de
Thu Aug 20 15:52:45 EDT 2009
Apologies to Christian for the double post, didn't look at the To-field.
Am Thursday 20 August 2009 20:21:30 schrieben Sie:
> Daniel Fischer wrote:
> > Am Donnerstag 20 August 2009 13:44:15 schrieb Martijn van Steenbergen:
> >> Goedemiddag café,
> >> Consider the following function, using parsec-3.0.0:
> >>> la :: Parsec String () (Maybe Char)
> >>> la = lookAhead (optionMaybe anyChar)
> >> *Lookahead> parseTest (char 'a' <|> char 'b') "a"
> >> 'a'
> >> *Lookahead> parseTest (char 'a' <|> char 'b') "b"
> >> 'b'
> >> *Lookahead> parseTest (la *> char 'a' <|> char 'b') "a"
> >> 'a'
> >> *Lookahead> parseTest (la *> char 'a' <|> char 'b') "b"
> >> parse error at (line 1, column 2):
> >> unexpected "b"
> >> expecting "a"
> >> The first three work fine and as expected, but the fourth example fails
> >> where I would expect success. I know <|> won't try the rhs if the lhs
> >> consumed input, but lookAhead's documentation promises not to consume
> >> any input. Is this a bug in Parsec or am I missing something?
> > Bad bug in Parsec (from the beginning, the same happens in parsec-2), I'd
> > say.
> I'd say, its a feature. lookAhead returns whatever its argument returns.
> So in this case it returns "Consumed" without consuming.
In that case, the comment
"lookAhead p parses p without consuming any input."
from the parsec-3 docs is highly misleading.
But lookAhead returning Consumed is not only counterintuitive, try
parseTest (many la) "a"
for a nice memory bomb.
> You can always wrap around a "try" to force the alternative:
> parseTest (try (la >> char 'a') <|> char 'b') "b"
> Cheers Christian
> Maybe it should have been:
> la >> (char 'a' <|> char 'b')
> in the first place.
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