[GHC] #15077: Suggest NoMonomorphismRestriction or type signature

[GHC]

#15077: Suggest NoMonomorphismRestriction or type signature
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           Reporter:  tomjaguarpaw   |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:
           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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 The following program results in a compile error.  To fix the error one
 can either give an explicit signature or turn on
 `NoMonomorphismRestriction`.  Could the compile error please mention both
 of these options (and provide the exact type signature required)?

 {{{
 type family G a b
 data D a = D (G a ())
 d = D ()
 }}}

 {{{
     • Couldn't match expected type ‘G a0 ()’ with actual type ‘()’
       The type variable ‘a0’ is ambiguous
     • In the first argument of ‘D’, namely ‘()’
       In the expression: D ()
       In an equation for ‘d’: d = D ()
     • Relevant bindings include
         d :: D a0
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15077>
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