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:

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Am Donnerstag 20 August 2009 13:44:15 schrieb Martijn van Steenbergen:

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Goedemiddag café,

Consider the following function, using parsec-3.0.0:

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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.