[GHC] #16074: Hopelessly confusing error involving runtime-reps

[GHC]

#16074: Hopelessly confusing error involving runtime-reps
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.6.3
      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:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 The reason is that when pretty-printing types, we carefully suppress
 runtime-rep vars (see `IfaceType.defaultRuntimeRepVars`.  But that should
 only apply to ''quantified'' runtime-rep vars like
 {{{
 ($) :: forall (r :: GHC.Types.RuntimeRep) a (b :: TYPE r). (a -> b) -> a
 -> b
 }}}
 where we want to default `r` to `LiftedRep`, and just show the type
 {{{
 forall a b. (a->b) -> a -> b
 }}}
 But in fact `defaultRuntimeRepVars` was also defaulting ''free'' type
 variables, and that means that the skolems `a` and `b` were being printed
 as `LiftedRep`. Gah!

 The fix is probably just to treat free variables differently; I'll try
 that.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16074#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler