#12422: Add decidable equality class -------------------------------------+------------------------------------- Reporter: dfeuer | Owner: Type: feature request | Status: new Priority: normal | Milestone: Component: Core Libraries | 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 goldfire): Shouldn't this go via the libraries list? While what you have there works, I might prefer {{{#!hs type Refuted = a -> Void data Decision a = Yes a | No (Refuted a) class DecideEquality f where -- name lines up with TestEquality decideEquality :: f a -> f b -> Decision (a :~: b) }}} This solution does have one more level of indirection, but it allows you to extract an `a :~: b` from the `Decision`, which will compose better with the other functions from `Data.Type.Equality`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12422#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler