[GHC] #12563: Bad error message around lack of impredicativity

2 Sep 2016

      #12563: Bad error message around lack of impredicativity
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           Reporter:  goldfire       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 (Broken out from #12557.)

 Consider

 {{{
 foo :: ((forall a. f a) -> f r) -> f r
 foo g = undefined

 x = \g -> foo g
 }}}

 Compiling this code produces

 {{{
 Scratch.hs:24:15: error:
     • Couldn't match expected type ‘(forall (a :: k). f a) -> f r’
                   with actual type ‘t’
       ‘t’ is a rigid type variable bound by
         the inferred type of x :: t -> f r at Scratch.hs:24:1-15
     • In the first argument of ‘foo’, namely ‘g’
       In the expression: foo g
       In the expression: \ g -> foo g
     • Relevant bindings include
         g :: t (bound at Scratch.hs:24:6)
         x :: t -> f r (bound at Scratch.hs:24:1)
 }}}

 The code should be rejected. But, really, the problem is that the type of
 a lambda-bound variable can't be a polytype (unless the type is obvious at
 the binding site). We should improve this error message.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12563
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