#13512: GHC incorrectly warns that a variable used in a type application is unused -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: | TypeApplications Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple error/warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: TypeApplications, newcomer => TypeApplications Comment: By the way, at one point I looked into how one might fix this bug, but I didn't get very far. There's chicken-and-egg problem: the `Unused quantified type variable` analysis (and subsequent warning) are done entirely within `bindLHsTyVarBndrs`, which is called from `rnValBindsLHS`. However, to account for uses of type variables from type annotations or visible type applications, we'd need the results of `rnValBindsRHS`, but this must be run //after// `rnValBindsLHS`! This leads me to believe that either: * This `Unused quantified type variable` check shouldn't belong in `bindLHsTyVarBndrs`. * We need to somehow make `bindLHsTyVarBndrs` aware of function RHSes as well for warning purposes. Either way, I don't think is quite a newcomer bug :) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13512#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler