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

Jeffrey Brown jeffbrown.the at gmail.com
Sun May 14 07:38:48 UTC 2017 

Awww yiss! Thanks, David!

On Sun, May 14, 2017 at 12:13 AM, David Turner <
dct25-561bs at mythic-beasts.com> wrote:

> 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>
>>
>> _______________________________________________
>> Haskell-Cafe mailing list
>> To (un)subscribe, modify options or view archives go to:
>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe
>> Only members subscribed via the mailman list are allowed to post.
>>
>
>


-- 
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>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/haskell-cafe/attachments/20170514/bfcda484/attachment.html>

More information about the Haskell-Cafe mailing list