parsing - <|> in Parsec - why do these examples behave differently? -


i think i'm misunderstanding <|> in parsec - have input stream contains either bunch of as in 1 representation or bunch of as in representation. expect following functions equivalent (given input form said, , have verified is):

foo =     ...     a1s <- many $ try $ a1     a2s <- many $ try $ a2     return $ a1s ++ a2s

versus

foo =     ...     <- (many $ try $ a1) <|> (many $ try $ a2)     return

what going wrong? first function works on input, second function fails, saying unexpected a2, expecting a1.

when give sequence of a2 latter parser, first many matches , returns empty list, doesn't try match against second many.

you can use many1 instead.

foo =     ...     <- many1 a1 <|> many a2     return

in case, many1 fails when give sequence of a2, , many matches against input.


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