#13105: Allow type families in RuntimeReps -------------------------------------+------------------------------------- Reporter: goldfire | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Conal Elliott provided me with this puzzle: {{{ type family RepRep a ∷ RuntimeRep class HasRep a where type Rep a ∷ TYPE (RepRep a) repr ∷ a → Rep a abst ∷ Rep a → a type instance RepRep Int = IntRep instance HasRep Int where type Rep Int = Int# abst n = I# n repr (I# n) = n }}} I think we should accept. However, doing so (even with my solution to #12809 complete) is hard, because we frequently consult a kind in order to determine a runtime representation. When that kind is `TYPE (RepRep Int)`, the code generator throws up its arms in despair. The solution here is either to teach the code generator how to normalise types (requiring propagating the `FamInstEnvs`) or to do a whole-program transformation at some point (zonker? desugarer? maybe we can wait until !CorePrep or even unarisation?) to change ids whose types have kinds like `TYPE (RepRep Int)` into ids with types with kinds like `TYPE IntRep`. But I don't want to let this hold me up at the moment, so I'm posting here as a reminder to revisit this problem. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13105 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler