expanding type synonyms in error messages
Ömer Sinan Ağacan
omeragacan at gmail.com
Fri Jun 26 16:40:17 UTC 2015
Update: I have a patch, it's not quite ready for reviews, but I'm now
getting this error message:
Main.hs:17:26: error:
Couldn't match type ‘[Char]’ with ‘()’
Expected type: Proxy () String () X IO ()
(aka. Proxy () [Char] () X IO ())
Actual type: Consumer String IO String
(aka. Proxy () [Char] () X IO [Char])
In the second argument of ‘(>->)’, namely ‘doubleUp’
In the second argument of ‘($)’, namely ‘loop >-> doubleUp’
cabal: Error: some packages failed to install:
ghc-ty-patch-0.1.0.0 failed during the building phase. The exception was:
ExitFailure 1
I'll tidy the code a bit, add a command line flag etc. and submit for reviews.
2015-06-19 10:13 GMT-04:00 Kostiantyn Rybnikov <k-bx at k-bx.com>:
> Great, thanks for doing this! I'm afraid even if you succeed with patch we
> would still need more "real-world examples" that show the need for this
> feature, and I think this is separate from GHC tests, as they don't need to
> be realistic, but of course please continue and hopefully more examples will
> come.
>
> 19 черв. 2015 16:19 "Ömer Sinan Ağacan" <omeragacan at gmail.com> пише:
>
>> Done: https://ghc.haskell.org/trac/ghc/ticket/10547
>>
>> 2015-06-19 9:12 GMT-04:00 Richard Eisenberg <eir at cis.upenn.edu>:
>> > Don't forget to make a Feature Request on Trac
>> > (https://ghc.haskell.org/trac/ghc/newticket) with a link to the wiki page.
>> > Trac is really the only way things like this don't get lost.
>> >
>> > Thanks!
>> >
>> > Richard
>> >
>> >
>> > On Jun 19, 2015, at 9:07 AM, Ömer Sinan Ağacan <omeragacan at gmail.com>
>> > wrote:
>> >
>> >> Great, thanks Kostiantyn! I'm looking for simple examples that we can
>> >> add to GHC testsuite, if I find something I'll update the wiki page
>> >> also.
>> >>
>> >> I made some progress on the patch, I think I can hopefully submit
>> >> something this weekend for reviews.
>> >>
>> >> 2015-06-19 5:16 GMT-04:00 Kostiantyn Rybnikov <k-bx at k-bx.com>:
>> >>> Created some initial wiki-page with just one example, will keep it in
>> >>> mind
>> >>> to add more as seen.
>> >>>
>> >>>
>> >>> https://wiki.haskell.org/Expanding_type_synonyms_in_error_messages_proposal
>> >>>
>> >>> On Fri, Jun 19, 2015 at 10:42 AM, Simon Peyton Jones
>> >>> <simonpj at microsoft.com>
>> >>> wrote:
>> >>>>
>> >>>> On this thread, a representative collection of *reproducible
>> >>>> examples*
>> >>>> with the actual error message and the desired one, would be
>> >>>> tremendously
>> >>>> helpful. Lacking that, it’s hard to know where to begin. (Creating
>> >>>> the
>> >>>> examples takes a bit of work, I know.)
>> >>>>
>> >>>>
>> >>>>
>> >>>> Start a wiki page! Stuff in email threads gets lost
>> >>>>
>> >>>>
>> >>>>
>> >>>> Simon
>> >>>>
>> >>>>
>> >>>>
>> >>>> From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of
>> >>>> Christopher Allen
>> >>>> Sent: 19 June 2015 04:27
>> >>>> To: Ömer Sinan Ağacan
>> >>>> Cc: ghc-devs
>> >>>> Subject: Re: expanding type synonyms in error messages
>> >>>>
>> >>>>
>> >>>>
>> >>>> Just to add my own +1, having this when working with streaming
>> >>>> libraries
>> >>>> (I've needed it less with lens, oddly) would be a tremendous help for
>> >>>> people
>> >>>> learning them I think. I think I've run into the same thing as
>> >>>> Kostiantyn in
>> >>>> the past.
>> >>>>
>> >>>>
>> >>>>
>> >>>> On Thu, Jun 18, 2015 at 9:42 PM, Ömer Sinan Ağacan
>> >>>> <omeragacan at gmail.com>
>> >>>> wrote:
>> >>>>
>> >>>> It's good to see that I'm not the only one who wants this. I'm doing
>> >>>> some GHC hacking nowadays and I'll give it a try.
>> >>>>
>> >>>>
>> >>>> 2015-06-18 4:41 GMT-04:00 Kostiantyn Rybnikov <k-bx at k-bx.com>:
>> >>>>> I wanted to add that synonym expansion would be especially helpful
>> >>>>> in
>> >>>>> error-messages like:
>> >>>>>
>> >>>>> Expected type: <non-expanded, small type, like "Producer a m ()">
>> >>>>> Actual type: <your type, like "Proxy a a' b b' m v">
>> >>>>>
>> >>>>> I would be glad if we could have an expansions enabling flag in GHC,
>> >>>>> and
>> >>>>> could consider turning it on by default if it will look good for
>> >>>>> that.
>> >>>>>
>> >>>>> 16 черв. 2015 22:44 "Richard Eisenberg" <eir at cis.upenn.edu> пише:
>> >>>>>
>> >>>>>> GHC tries hard to preserve type synonyms where possible, but of
>> >>>>>> course,
>> >>>>>> it
>> >>>>>> can't preserve all of them. The general rule it tries to follow is:
>> >>>>>> preserve
>> >>>>>> vanilla type synonyms; expand type families. This is true both in
>> >>>>>> expected
>> >>>>>> types and actual types.
>> >>>>>> If you have a case where you believe that GHC could preserve a type
>> >>>>>> synonym in an expected type, submit a bug report. (Note that
>> >>>>>> constraint
>> >>>>>> synonyms are particularly hard to preserve!)
>> >>>>>>
>> >>>>>> It would be very easy to report both the synonym-preserving form
>> >>>>>> and
>> >>>>>> the
>> >>>>>> expanded form in an error report, at the cost of making error
>> >>>>>> reports
>> >>>>>> even
>> >>>>>> more verbose. You're welcome to submit a feature request, and this
>> >>>>>> would
>> >>>>>> likely make a good first patch to GHC if you want to get your hands
>> >>>>>> dirty.
>> >>>>>> I'd personally prefer the feature to be protected behind a flag (to
>> >>>>>> avoid
>> >>>>>> seeing that `String` expands to `[Char]` everywhere, for example),
>> >>>>>> but
>> >>>>>> others may feel differently here.
>> >>>>>>
>> >>>>>> Richard
>> >>>>>>
>> >>>>>> On Jun 16, 2015, at 11:20 AM, Ömer Sinan Ağacan
>> >>>>>> <omeragacan at gmail.com>
>> >>>>>> wrote:
>> >>>>>>
>> >>>>>>> Hi all,
>> >>>>>>>
>> >>>>>>> While working with complex types with lots of arguments etc.
>> >>>>>>> errors
>> >>>>>>> are
>> >>>>>>> becoming annoying very fast. For example, GHC prints errors in
>> >>>>>>> this
>> >>>>>>> way:
>> >>>>>>>
>> >>>>>>> Expected type: <type without any synonyms>
>> >>>>>>> Actual type: <type with synonyms>
>> >>>>>>>
>> >>>>>>> Now I have to expand that synonym in my head to understand the
>> >>>>>>> error.
>> >>>>>>>
>> >>>>>>> I was wondering if implementing something like this is possible:
>> >>>>>>>
>> >>>>>>> In type error messages, GHC also prints types that are cleaned
>> >>>>>>> from
>> >>>>>>> type
>> >>>>>>> synonyms. Maybe something like this:
>> >>>>>>>
>> >>>>>>> Expected type: <type1>
>> >>>>>>> (without synonyms): <type1, synonyms are expanded>
>> >>>>>>> Actual type: <type2>
>> >>>>>>> (without synonyms): <type2, synonyms are expanded>
>> >>>>>>>
>> >>>>>>> If this is not always desirable for some reason, we can hide this
>> >>>>>>> behavior
>> >>>>>>> behind a flag.
>> >>>>>>>
>> >>>>>>> What do GHC devs think about this? Is this, in theory, possible to
>> >>>>>>> do?
>> >>>>>>> How hard
>> >>>>>>> would it be to implement this?
>> >>>>>>>
>> >>>>>>> Thanks.
>> >>>>>>> _______________________________________________
>> >>>>>>> ghc-devs mailing list
>> >>>>>>> ghc-devs at haskell.org
>> >>>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>> >>>>>>
>> >>>>>> _______________________________________________
>> >>>>>> ghc-devs mailing list
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>> >>>>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>> >>>> _______________________________________________
>> >>>> ghc-devs mailing list
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>> >>>>
>> >>>>
>> >>>>
>> >>>>
>> >>>>
>> >>>> --
>> >>>>
>> >>>> Chris Allen
>> >>>>
>> >>>> Currently working on http://haskellbook.com
>> >>>>
>> >>>>
>> >>>> _______________________________________________
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>> >>>> ghc-devs at haskell.org
>> >>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>> >>>>
>> >>>
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