[GHC] #14185: Non-local bug reporting around levity polymorphism

[GHC]

#14185: Non-local bug reporting around levity polymorphism
-------------------------------------+-------------------------------------
        Reporter:  goldfire          |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
                                     |  LevityPolymorphism,
                                     |  TypeErrorMessages
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 simonpj):

 I looked at this in an odd moment.  This instance
 {{{
 instance (Unbox a a', Unbox b b') => Unbox (a,b) (# a', b' #) where
   unbox (a,b) = (# unbox a, unbox b #)
   box (# a, b #) = (box a, box b)
 }}}
 is quantified over four type variables, ''all of which get kind `Type`''.
 But in `testTup` we will need to solve
 {{{
 [W] Unbox (Int, Char) (# Int#, Char# )
 }}}
 That is well-kinded but it doesn't match the instance (which has only
 lifted type variables).

 What ''actually'' happens is that we try to solve
 {{{
 [W] Unbox (Int,Char) alpha
 }}}
 and the fundep says `alpha := (# beta, gamma #)`, but since the fundep
 comes from the tuple instance,
 it insists that `beta::*` and `gamma::*`.

 If you try to make the instance levity-polymorphic:
 {{{
 instance forall a b ar br (a' :: TYPE r1) (b' :: TYPE br).
          (Unbox a a', Unbox b b') => Unbox (a,b) (# a', b' #) where
 }}}
 it rightly fails with
 {{{
 T14185a.hs:23:10: error:
     A levity-polymorphic type is not allowed here:
       Type: a'
       Kind: TYPE r1
     In the type of binder ‘a’
    |
 23 |   box (# a, b #) = (box a, box b)
    |          ^
 }}}
 This does not explain why we get bogus errors in `testInt` but I claim
 there is
 no bug in rejecting `testTup`.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14185#comment:3>
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