[GHC] #13415: Instance declarations don't recognize named wildcards

[GHC]

#13415: Instance declarations don't recognize named wildcards
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #13324            |  Differential Rev(s):  Phab:D3332
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Replying to [comment:10 simonpj]:
 > What do you mean by "properly detect"?

 I'll once again point you to the original example that I reported:

 {{{#!hs
 {-# LANGUAGE NamedWildCards #-}
 module Bug where

 instance _x => Show (Maybe a)
 }}}

 {{{
 Bug.hs:4:10: error:
     • Illegal constraint: _x (Use ConstraintKinds to permit this)
     • In the context: _x
       While checking an instance declaration
       In the instance declaration for ‘Show (Maybe a)’
   |
 4 | instance _x => Show (Maybe a)
   |          ^^^^^^^^^^^^^^^^^^^^
 }}}

 In this example, **`_x` is a named wildcard, and GHC is not detecting
 this.** The error message //should// be that we're using a named wildcard
 without having `-XPartialTypeSignatures` on.

 > How would things be better if we did "properly detect" them?
 Specifically
 >
 > * Why will the error messages get better? Can you give an example?

 Yes. Please refer to [https://phabricator.haskell.org/D3332#e4a3ef42 this
 test case] in Phab:D3332. Instead of the completely misleading error
 message about `-XConstraintKinds` that it currently gives, it now detects
 the use of a named wildcard and says:

 {{{#!hs
     Wildcard ‘_x’ not allowed
       in an instance declaration for ‘Show’
 }}}

 It doesn't suggest turning on `-XPartialTypeSignatures` yet because that's
 the subject of #13324, and as I noted in comment:9, even getting GHC to
 recognize the use of a wildcard in instance declarations is a somewhat
 significant task, which is why I opened a separate ticket for it in the
 first place.

 > * Is that an argument for putting wildcards everywhere? Why should only
 instance declarations get the benefit of this error message improvement?

 That's not at all what I'm trying to advocate for here. The point (which I
 tried to articulate in comment:9, but I'll restate here) is that we have
 identified a place where we'd //like// to have wildcard constraints, so in
 order to accomplish that goal, we need to:

 1. Change the use of `LHsSigType` in instance declarations to
 `LHsSigWcType` so that we can use wildcards there in the first place
 2. Change the typechecker code so that it fills in wildcard constraints
 when typechecking instance contexts

 Doing both in one go would be an enormous change, so I'm trying to do this
 piecemeal and tackle (1) on its own first. That's it.

 (I originally made an appeal to having better error messages from this
 change, but now I thoroughly regret doing so, because it has completely
 derailed the discussion.)

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13415#comment:11>
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