[GHC] #14319: Stuck kind families can lead to lousy error messages

[GHC]

#14319: Stuck kind families can lead to lousy error messages
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler (Type    |              Version:  8.3
  checker)                           |             Keywords:  TypeInType,
      Resolution:                    |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Poor/confusing    |  Unknown/Multiple
  error message                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Actually, no. I can get just as bad an error message at the term level:

 {{{#!hs
 f :: forall (s :: Symbol). Proxy s -> F s
 f _ _ = undefined
 }}}

 produces

 {{{
 ArityError.hs:14:1: error:
     • Couldn't match expected type ‘F s’ with actual type ‘p0 -> a0’
       The type variables ‘p0’, ‘a0’ are ambiguous
     • The equation(s) for ‘f’ have two arguments,
       but its type ‘Proxy s -> F s’ has only one
     • Relevant bindings include
         f :: Proxy s -> F s (bound at ArityError.hs:14:1)
    |
 14 | f _ _ = undefined
    | ^^^^^^^^^^^^^^^^^
 }}}

 So we have the same problem there.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14319#comment:1>
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