#13957: Allow deriving multiparameter type classes with representationally equal arguments -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: | -------------------------------------+------------------------------------- Comment (by Iceland_jack): We cannot both have `SIEVE I (->)` and `SIEVE Identity (->)` if we assume a functional dependency, similar to #13404 where an analogous example would compile {{{#!hs class SIEVE p where type SIEVE_ty p :: Type -> Type sIEVE :: p a b -> (a -> SIEVE_ty p b) instance SIEVE (->) where type SIEVE_ty (->) = Identity sIEVE :: (a -> b) -> (a -> Identity b) sIEVE f = f >>> Identity newtype ARR a b = ARR (a -> b) deriving newtype SIEVE }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13957#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler