#9334: Implement "instance chains" -------------------------------------+------------------------------------- Reporter: diatchki | Owner: diatchki Type: feature | Status: new request | Milestone: Priority: normal | Version: 7.9 Component: Compiler | Keywords: (Type checker) | Operating System: Unknown/Multiple Resolution: | Type of failure: None/Unknown Differential Revisions: | Test Case: Architecture: | Blocking: Unknown/Multiple | Difficulty: Unknown | Blocked By: | Related Tickets: | -------------------------------------+------------------------------------- Comment (by goldfire): I agree with Iavor -- it would be great to see instance chains for real. There are two further observations I'd like to make, though: 1. Instance chains can, I believe, be simulated accurately with closed type families. The encoding is bulky, and I think having real instance chains is much better than what we have in 7.8, but an eager programmer can use closed type families today. For example, Iavor's example could be written {{{ type family ChooseStateMInstance x y where ChooseStateMInstance (StateT s m) s = 0 ChooseStateMInstance (t m) s = 1 class StateM' (n :: Nat) x y where ... instance Monad m => StateM' 0 (StateT s m) s where ... instance (MonadTrans t, StateM m s) => StateM' 1 (t m) s where ... type StateM s m = StateM' (ChooseStateMInstance s m) s m }}} Like I said, it's not pretty, but I believe it works. 2. This doesn't necessarily mean that we'll never need overlapping instances -- instance chains seem to only work when the overlap would be contained only in one module. Some programs require inter-module overlap (say, for a global "default" instance). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9334#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler