expanding type synonyms in error messages

[Ömer Sinan Ağacan]

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.
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