[GHC] #14363: :type hangs on coerce

[GHC]

#14363: :type hangs on coerce
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 `Functor` constraint not needed to make it hang

 {{{
 Prelude Data.Coerce> contra = undefined :: (a -> b) -> (f b -> f a)
 Prelude Data.Coerce> :t [contra, coerce]
 ^CInterrupted.
 }}}

 Giving it a concrete type gives an actual failure

 {{{
 Prelude Data.Coerce> contra = undefined :: (a -> b) -> (Maybe b -> Maybe
 a)
 Prelude Data.Coerce> :t [contra, coerce]

 <interactive>:1:10: error:
     • Occurs check: cannot construct the infinite type:
         b ~ Maybe (Maybe b)
         arising from a use of ‘coerce’
     • In the expression: coerce
       In the expression: [contra, coerce]
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14363#comment:2>
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