Re: [GHC] #11509: Incorrect error message in StandaloneDeriving: "The data constructors of <typeclass> are not all in scope"

2 Feb 2016

      #11509: Incorrect error message in StandaloneDeriving: "The data constructors of
<typeclass> are not all in scope"
-------------------------------------+-------------------------------------
        Reporter:  edsko             |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1-rc1
      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 edsko):

 Incidentally, when `Serializable` has a nominal role, that call to
 `coerce` is rejected by ghc 8.0 with

 {{{
 Bug.hs:70:10: error:
     • Couldn't match type ‘[a]’ with ‘MyList a’
         arising from a use of ‘coerce’
     • In the expression:
         coerce (dict :: Static (Dict (Serializable [a])))
       In an equation for ‘dict’:
           dict = coerce (dict :: Static (Dict (Serializable [a])))
       In the instance declaration for ‘SC (Serializable (MyList a))’
     • Relevant bindings include
         dict :: Static (Dict (Serializable (MyList a)))
           (bound at Bug.hs:70:3)
 }}}

 which I found somewhat confusing; "Couldn't match type `[a]` with `MyList
 a` arising from a use of `coerce`" isn't very obvious; ghc 7.10.3 here is
 much much clearer:

 {{{
 Bug.hs:70:10:
     Couldn't match type ‘MyList a’ with ‘[a]’
     arising from trying to show that the representations of
       ‘Static (Dict (Serializable [a]))’ and
       ‘Static (Dict (Serializable (MyList a)))’ are the same
     Relevant role signatures:
       type role [] representational
       type role MyList representational
       type role Dict representational
       type role Static representational
       type role Serializable nominal
     Relevant bindings include
       dict :: Static (Dict (Serializable (MyList a)))
         (bound at Bug.hs:70:3)
     In the expression:
       coerce (dict :: Static (Dict (Serializable [a])))
     In an equation for ‘dict’:
         dict = coerce (dict :: Static (Dict (Serializable [a])))
     In the instance declaration for ‘SC (Serializable (MyList a))’
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11509#comment:3
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