Re: expanding type synonyms in error messages

19 Jun 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"  пише:
...
Done: https://ghc.haskell.org/trac/ghc/ticket/10547
...
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 
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 :
...
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@microsoft.com>
...
wrote:
...
On this thread, a representative collection of *reproducible examples*
with the actual error message and the desired one, would be
...
...
...
...
helpful.  Lacking that, it’s hard to know where to begin.   (Creating
...
...
...
...
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@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
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...
...
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(I've needed it less with lens, oddly) would be a tremendous help for
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...
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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@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 :
...
I wanted to add that synonym expansion would be especially helpful in
error-messages like:
Expected type: 
Actual type: 
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
2015-06-19 9:12 GMT-04:00 Richard Eisenberg :
tremendously
the
libraries
people
that.
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16 черв. 2015 22:44 "Richard Eisenberg"  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@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): 
>>          Actual type: <type2>
>>   (without synonyms): 
>>
>> 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.
>> _______________________________________________
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>> ghc-devs@haskell.org
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>
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--
Chris Allen
Currently working on http://haskellbook.com
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