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

Sun May 14 07:13:02 UTC 2017

The "code in full" link has operators called `#1` and `#2` rather than
`|+|` and `|-|`, but I see one of your test cases fail there too. You're
using quite an old version of megaparsec, 4.3.0 (stackage lts-5.5) and it
looks like something affecting this was fixed in 4.4.0 (e.g. stackage
lts-6.9):

$ stack --resolver lts-5.5 ghci --no-load --no-build
Configuring GHCi with the following packages: Dwt
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Prelude> :l howto/megaparsec/minimal.hs
[1 of 1] Compiling Experim          ( howto/megaparsec/minimal.hs,
interpreted )
Ok, modules loaded: Experim.
*Experim> test
Just (Pair (Var "a") (Var "b"))
Nothing
Just (Pair (Pair (Var "a") (Var "b")) (Pair (Var "c") (Var "d")))
Just (Pair (Pair (Var "a") (Var "b")) (Pair (Var "c") (Var "d")))
*Experim>
Leaving GHCi.
$ stack --resolver lts-6.9 ghci --no-load --no-build
Configuring GHCi with the following packages: Dwt
GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
Prelude> :l howto/megaparsec/minimal.hs
[1 of 1] Compiling Experim          ( howto/megaparsec/minimal.hs,
interpreted )
Ok, modules loaded: Experim.
*Experim> test
Just (Pair (Var "a") (Var "b"))
Just (Pair (Var "a") (Var "b"))
Just (Pair (Pair (Var "a") (Var "b")) (Pair (Var "c") (Var "d")))
Just (Pair (Pair (Var "a") (Var "b")) (Pair (Var "c") (Var "d")))
*Experim>
Leaving GHCi.

On 14 May 2017 at 07:53, Jeffrey Brown <jeffbrown.the at gmail.com> wrote:

> The problem's not solved! Check out this weirdness. |+| and |-| are
> supposed to be identical, mapping to the same function, the Pair
> constructor.
>
>     > mapM_ putStrLn exprs
>     a |+| b
>     a |-| b
>     a |+| b |-| c
>     a |-| b |+| c
>     a |+| b   |-|  c |+| d
>     > mapM_ putStrLn $ map (show . parseMaybe aExpr) exprs
>     Just (Pair (Var "a") (Var "b"))
>     Nothing
>     Just (Pair (Pair (Var "a") (Var "b")) (Var "c"))
>     Nothing
>     Just (Pair (Pair (Var "a") (Var "b")) (Pair (Var "c") (Var "d")))
>
> Why the two failures?
>
> If I change them from InfixN to InfixR or InfixL, then only the first of
> those five expressions parses.
>
> Here are two relevant definitions:
>
>     data AExpr = Var String | Pair AExpr AExpr deriving (Show)
>
>     aOperators :: [[Operator Parser AExpr]]
>     aOperators =
>       [ [ InfixN $ symbol "|+|" *> pure (Pair) ]
>       , [ InfixN $ symbol "|-|" *> pure (Pair) ] -- binds last, I think
>       ]
>
> And here is the code in full[1].
>
> Thanks,
> Jeff
>
>
> [1] https://github.com/JeffreyBenjaminBrown/digraphs-
> with-text/blob/master/howto/megaparsec/minimal.hs
>
>
> On Sun, Oct 23, 2016 at 2:50 PM, Brandon Allbery <allbery.b at gmail.com>
> wrote:
>
>> 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/Tex
>> t-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/t
>>> ree/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
>>> <https://www.facebook.com/mejeff.younotjeff>   |   LinkedIn
>>> <https://www.linkedin.com/in/jeffreybenjaminbrown>(I often miss
>>> messages 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
>>
>
>
>
> --
> Jeff Brown | Jeffrey Benjamin Brown
> Website <https://msu.edu/~brown202/>   |   Facebook
> <https://www.facebook.com/mejeff.younotjeff>   |   LinkedIn
> <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often
> miss messages here)   |   Github <https://github.com/jeffreybenjaminbrown>
>
>
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