[GHC] #14363: :type hangs on coerce

17 Oct 2017

      #14363: :type hangs on coerce
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1
           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:
-------------------------------------+-------------------------------------
 This works

 {{{
  ghci -ignore-dot-ghci
 GHCi, version 8.2.1: http://www.haskell.org/ghc/  :? for help
 Prelude> import Data.Coerce
 Prelude Data.Coerce> :t [fmap, coerce]

 <interactive>:1:8: error:
     • Occurs check: cannot construct the infinite type: b ~ f b
         arising from a use of ‘coerce’
     • In the expression: coerce
       In the expression: [fmap, coerce]

 }}}

 But doing it with `contra`

 {{{
 Prelude Data.Coerce> contra = undefined :: Functor f => (b -> a) -> (f a
 -> f b)
 }}}

 it hangs

 {{{
 Prelude Data.Coerce> :t [coerce, contra]
 ^CInterrupted.
 Prelude Data.Coerce> :t [contra, coerce]
 ^CInterrupted.
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14363
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