[GHC] #11473: Levity polymorphism checks are inadequate

[GHC]

#11473: Levity polymorphism checks are inadequate
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        Reporter:  simonpj           |                Owner:  goldfire
            Type:  bug               |               Status:  new
        Priority:  highest           |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.3
      Resolution:                    |             Keywords:
                                     |  LevityPolymorphism, TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect         |  Unknown/Multiple
  warning at compile-time            |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by rwbarton):

 It seems to me the most logical place to put these levity polymorphism
 checks is in the typing rules for abstraction and application. After all
 `\(a :: t) -> ...` is really different beasts at runtime depending on
 whether `t :: * = TYPE L`, or `t :: TYPE UW32`, or `t :: TYPE UW64` etc.
 We should insist that the rep (the `r` such that `t :: TYPE r`) of `t` be
 known when type-checking the lambda, and resolve the overloading to `\_L`,
 or `\_UW32`, or `\_UW64`. In short, treat `\` as ad-hoc polymorphic.

 Similar comments apply to application. `f x` is really an overloaded `f
 $_r x`, where the typing rule for `$_r` is
 {{{
 s :: RuntimeRep   a :: TYPE r   b :: TYPE s   f :: a -> b   x :: a
 ------------------------------------------------------------------
                              f $_r x :: b
 }}}
 And this makes sense because later to actually compile an application, we
 will absolutely need to know the rep of the type of `x`.

 Then I don't think we need any special rules about the kind of `->`, which
 can become rep-polymorphic in both arguments.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11473#comment:8>
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