#12142: -Wredundant-constraints warns about constraints introduced via type synonyms. -------------------------------------+------------------------------------- Reporter: scshunt | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Linux Architecture: | Type of failure: Incorrect Unknown/Multiple | warning at compile-time Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I ran into this when using lens, here's a simplied example: {{{#!hs l :: Getter [a] Int l = to length }}} In this case, Getter introduces constraints (Functor f, Contravariant f), but the Functor constraint is not used. This leads to a warning from -Wredundant-constraints: {{{ <interactive>:1:6: warning: [-Wredundant-constraints] • Redundant constraint: Functor f • In the type signature for: l :: (Contravariant f, Functor f) => (Int -> f Int) -> [a] -> f [a] }}} This shouldn't warn, because the constraints were introduced by type synonym and so may well be opaque to the user. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12142 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler