#11451: Inconsistent warnings for unused binders -------------------------------------+------------------------------------- Reporter: simonpj | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Consider compiling this with `-Wunused-matches`: {{{ class C a where op :: a -> a instance C (Maybe a) where op x = x }}} We get no warnings, even though `a` is patently unused. But suppose we add an associated type {{{ class C a where type T a op :: a -> a instance C (Maybe a) where type T (Maybe a) = Int -- Warning on this line op x = x }}} Now we get a warning for an unused binding for `a` on the `type instance`. Edward complained about this inconsistent behaviour in [https://mail.haskell.org/pipermail/glasgow-haskell- users/2016-January/026114.html this email thread]. My thoughts: * Currently GHC does not warn about type variables bound in the instance head but unused in the `where` part. Fixing that might be a good idea, but would be a new feature. * However, given that we don't warn about them, we should definitely not warn about instance type variables being unused in an associated type. But we could warn about ones specific to the associated type itself. Eg {{{ class C2 a where type T2 a b instance C2 (Maybe a) where type T2 (Maybe a) x = Int -- Line XXX }}} Here, on line `XXX`, we might reasonably warn about the unused `x`, but not about the unused `a`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11451 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler