#12680: Permit type equality instances in signatures -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: backpack Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by goldfire): I've not followed the Backpack blow-by-blow, but using `data S` to denote an abstract type of any stripe looks bad to me. `type S` is much better, because users can't make any assumptions about a `type` declaration that they might for a `data` declaration. Separately, you could get yourself into deep water here (not that that's necessary bad): {{{ type Nat type family (a :: Nat) + (b :: Nat) instance a + b ~ b + a }}} Good luck implementing this signature! :) It would be super awesome if the user could prove the equality manually. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12680#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler