[GHC] #11712: Very confusing error message with -fprint-explicit-kinds

16 Mar 2016

      #11712: Very confusing error message with -fprint-explicit-kinds
-------------------------------------+-------------------------------------
           Reporter:  bgamari        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  7.10.3
  (Type checker)                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Incorrect
  Unknown/Multiple                   |  warning at compile-time
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 While trying to work out a type error I encountered the following,

 {{{
 compiler/utils/Binary.hs:607:59: error:
     • Could not deduce: (~) * * arg *
       from the context: (~) * k (arg -> res)
     ...
 }}}

 Note how in the first line `(~)` is applied to four arguments whereas on
 the second it is applied to three.

 In general, it would be nice if we could make equalities a bit more
 readable, perhaps special-casing them in the pretty-printer as,
 {{{
 (a :: k) ~ (b :: k')
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11712
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