[Haskell-cafe] attoparsec and backtracking

[Evan Laforge]

> I think the mistake here is to parse something and then decide if
> its it valid. It should be the parser which decides whether its
> valid. So rather than:
>
>      suffix <- A.option "" ((:"") <$> A.letter_ascii)
>
> try:
>
>      typ <- A.choice [ {- list or valid suffix parsers -} ]
>      return $ Score.Typed typ num

I actually had that originally, but but switched to fail-after for the
better error msg.  It worked with parsec, but then I lost it again
when I switched to attoparsec.  I think Wren is right, I really would
need to refactor the parser to put the decisions in the right spot.

> We you using Parsec as a token parser or as a Char parser. Obviously
> the second is going to be slow in comparison to the first.

It was actually the Text version of parsec, back when that was new.  I
should go do a profile again someday, but since attoparsec and parsec
APIs are almost but not quite the same, it's kind of a pain.  I
actually tried the Text version of attoparsec, back when that was not
yet integrated into attoparsec itself, and bytestring was still
significantly faster.  So I don't know how much was Text vs.
ByteString and how much was parsec vs. attoparsec.