#12700: Don't warn about redundant constraints for type equalities -------------------------------------+------------------------------------- Reporter: crockeea | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I see * `foo` really does have the type `(RealFrac a, Integral b) => a -> b`. That is, as GHC claims, the constraint can simply be omitted. * But you specifically want to write a less-polymorphic type than the most general type. Another example might be {{{ f :: Ord a => a -> a f x = x }}} The `Ord a` is not needed, but you might want to require it for some software engineering reason. That's your situation here, I believe. * It's worth noting that you can certainly say {{{ f :: [a] -> [a] f x = x }}} thus specifying a less polymorphic type than the most general one, without causing a warning of any kind. This is one of the reasons that `-Wredundant-constraints` is no longer on by default, and is not even enabled by `-Wall`. I'm a bit sad, but the annoyance was too great. The real solution would be a per-function pragma to say "I intend to have a redundant constraint here". If I've understood aright, then we need take no action, except add it as another use-case for the per-function pragma See #11370, #10635 -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12700#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler