[GHC] #12393: Poor error message with equational type constraints

[GHC]

#12393: Poor error message with equational type constraints
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        Reporter:  laneb         |                Owner:
            Type:  bug           |               Status:  closed
        Priority:  normal        |            Milestone:
       Component:  GHCi          |              Version:  8.0.1
      Resolution:  invalid       |             Keywords:
Operating System:  Linux         |         Architecture:  x86_64 (amd64)
 Type of failure:  None/Unknown  |            Test Case:
      Blocked By:                |             Blocking:
 Related Tickets:  #11469        |  Differential Rev(s):
       Wiki Page:                |
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Changes (by thomie):

 * status:  new => closed
 * component:  Compiler => GHCi
 * resolution:   => invalid
 * related:   => #11469


Comment:

 If you turn on `GADTs` or `TypeFamilies` in GHCi, you'll get the error
 message you were hoping for.

 {{{
 *Test> :set -XGADTs
 *Test> myFunc "z"

 <interactive>:3:1: error:
     • Couldn't match expected type ‘Char’
                   with actual type ‘FooInner a0’
       The type variable ‘a0’ is ambiguous
     • In the first argument of ‘print’, namely ‘it’
       In a stmt of an interactive GHCi command: print it
 }}}

 I'm closing this ticket. I agree that the original error message is poor,
 but #11469 (the ticket referred to in comment:2) will fix the problem.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12393#comment:3>
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