[Haskell-cafe] Why is Megaparsec treating these two operators differently?

[Brandon Allbery]

Aha. I had forgotten some details.

If you want to have an operator that is a prefix of another operator in the
> table, use the following (or similar) wrapper instead of plain symbol:
> op n = (lexeme . try) (string n <* notFollowedBy punctuationChar)

http://hackage.haskell.org/package/megaparsec-5.1.1/docs/Text-Megaparsec-Expr.html#v:makeExprParser

So you actually need to be a little clever for those two operators to work;
it's not as simple as I had recalled it (which would have been correct for
a basic manual combinator setup). I am going to guess that something in
there is not using `try` and silently consuming the extra "#", but I'd have
to study the `makeExprParser` code in Megaparsec to be certain.

On Sun, Oct 23, 2016 at 5:38 PM, Jeffrey Brown <jeffbrown.the at gmail.com>
wrote:

> Thanks, Brandon! How did you know that?
>
> I changed them to "#1" and "#2" and now it works[1].
>
> But before making that change, why would "a # b ## c # d" evaluate, even
> though "a ## b" would not?
>
>
> [1] https://github.com/JeffreyBenjaminBrown/digraphs-
> with-text/tree/master/howto/megaparsec
> The corrected file is called "experim.hs"; the old one, uncorrected, is
> called "experim.buggy.hs".
>
> On Sun, Oct 23, 2016 at 2:03 PM, Brandon Allbery <allbery.b at gmail.com>
> wrote:
>
>>
>> On Sun, Oct 23, 2016 at 4:15 PM, Jeffrey Brown <jeffbrown.the at gmail.com>
>> wrote:
>>
>>>       [ [ InfixN # symbol "#" *> pure (Pair) ]
>>>       , [ InfixN # symbol "##" *> pure (Pair) ]
>>>       ]
>>>
>>
>> Combinator parsers can't rearrange themselves to do longest token
>> matching. So the ## operator will take the first case, match against
>> `symbol "#"` and aOperator will succeed; the the next token match will hit
>> the unconsumed "#" and fail. If you place "##" first then it will match
>> "##" but not "#", which would the match the second rule.
>>
>> --
>> brandon s allbery kf8nh                               sine nomine
>> associates
>> allbery.b at gmail.com
>> ballbery at sinenomine.net
>> unix, openafs, kerberos, infrastructure, xmonad
>> http://sinenomine.net
>>
>
>
>
> --
> Jeff Brown | Jeffrey Benjamin Brown
> Website <https://msu.edu/~brown202/>   |   Facebook
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> here)   |   Github <https://github.com/jeffreybenjaminbrown>
>



-- 
brandon s allbery kf8nh                               sine nomine associates
allbery.b at gmail.com                                  ballbery at sinenomine.net
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net
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