#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: | -------------------------------------+------------------------------------- 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 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler