[GHC] #7862: Could not deduce (A) from the context (A, ...)

[GHC]

#7862: Could not deduce (A) from the context (A, ...)
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              Reporter:  alang9      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  x86_64 (amd64)
      Operating System:  Linux       |       Difficulty:  Unknown
       Type of failure:  GHC         |       Blocked By:
  rejects valid program              |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 The problem in comment:5 is unrelated to the original bug report -- it is
 a !PolyKinds issue. If you turn on `-fprint-explicit-kinds`, you get

 {{{
     Could not deduce (Category k2 (Morphisms (k -> k -> *) k2 * c))
       arising from a use of ‘Control.Category.id’
     from the context (Category k c,
                       Category k1 (Morphisms (k -> k -> *) k1 * c))
       bound by the instance declaration at Scratch.hs:30:10-64
     or from (Objects (k -> k -> *) k2 c a)
       bound by the type signature for
                  id :: (Objects (k -> k -> *) k2 c a) =>
                        Morphisms (k -> k -> *) k2 * c a a
       at Scratch.hs:33:5-6
     In the expression: Control.Category.id
     In an equation for ‘id’: id = Control.Category.id
     In the instance declaration for ‘SmallCategory (k -> k -> *) c’
 }}}

 and you can see a more sensible error message.

 What would be nice here is a note suggesting to turn on `-fprint-explicit-
 kinds`.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7862#comment:7>
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