[GHC] #13610: Unhelpful error messages about lifted and unlifted types

[GHC]

#13610: Unhelpful error messages about lifted and unlifted types
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      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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Changes (by nomeata):

 * priority:  normal => low


Comment:

 I am not sure if printing type prefixes would have improved manners. What
 would it say? Maybe
 {{{
       Expected type: (#,#) VoidRep LiftedRep (State# RealWorld) (Weak# ())
         Actual type: (#,#) VoidRep PrimRep   (State# RealWorld) (Weak# ())
 }}}

 But the “actual type” is not well-kinded; it cannot really be the “actual
 type” of anything. So it seems that something went wrong earlier during
 type inference?

 I guess something along these lines would have been more helpful:
 {{{
 WeakDouble.hs:8:15: error:
     • Couldn't match a lifted type with an unlifted type
       Expected type representation: LiftedRep
         Actual type representation: PrimRep
     • In the second element of the unlifted tuple
     • In the type of the expression: mkWeakNoFinalizer# double () s
 }}}

 Anyways; this was not triggered by writing real code, so I’ll lower the
 priority.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13610#comment:3>
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