expanding type synonyms in error messages

Fri Jun 19 14:13:34 UTC 2015

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
> >>>> _______________________________________________
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> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> --
> >>>>
> >>>> Chris Allen
> >>>>
> >>>> Currently working on http://haskellbook.com
> >>>>
> >>>>
> >>>> _______________________________________________
> >>>> ghc-devs mailing list
> >>>> ghc-devs at haskell.org
> >>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
> >>>>
> >>>
> >> _______________________________________________
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> >>
> >
>
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