[GHC] #11011: Add type-indexed type representations (`TypeRep a`)

[GHC]

#11011: Add type-indexed type representations (`TypeRep a`)
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        Reporter:  bjmprice          |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:
      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 bgamari):

 One bit of trickiness that I've encountered while looking at this is the
 that fact Core Lint disallows unsaturated applications of unlifted types.
 So, for instance, if we have,
 {{{#!hs
 data TTTypeRep (a :: k) where
     TrTyCon :: !Fingerprint -> !(TyCon a) -> TTypeRep (a :: k)
     TrApp   :: forall k1 k2 (a :: k1 -> k2) (b :: k1).
                !Fingerprint
             -> TTypeRep (a :: k1 -> k2)
             -> TTypeRep (b :: k1)
             -> TTypeRep (a b)
 }}}

 And we ask for `Typeable (Array# Int)`, we will attempt to decompose
 `Array# Int` and end up with a subterm,
 {{{#!hs
 array#Rep :: TTypeRep Array#
 array#Rep = TrTyCon @(* -> #) @Array# fprint array#TyCon
 }}}
 in the resulting representation, which currently fails Core Lint due to,
 {{{#!patch
 diff --git a/compiler/coreSyn/CoreLint.hs b/compiler/coreSyn/CoreLint.hs
 index 26e7257..861a9bc 100644
 --- a/compiler/coreSyn/CoreLint.hs
 +++ b/compiler/coreSyn/CoreLint.hs
 @@ -1042,7 +1042,7 @@ lintType ty@(TyConApp tc tys)
    = lintType ty'   -- Expand type synonyms, so that we do not bogusly
 complain
                     --  about un-saturated type synonyms

 -  | isUnliftedTyCon tc || isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
 +  | isTypeSynonymTyCon tc || isTypeFamilyTyCon tc
         -- Also type synonyms and type families
    , length tys < tyConArity tc
    = failWithL (hang (text "Un-saturated type application") 2 (ppr ty))
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11011#comment:7>
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