The ScopedTypeVariables extension is one of the few that doesn't recommend itself when it should be used.
I recognize that this may be hard to do, but here is a strawman
#9244: Compiler could warn about type variable shadowing, and hint about ScopedTypeVariables -------------------------------------+------------------------------------- Reporter: stusmith | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.2.1 Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Incorrect | Unknown/Multiple warning at compile-time | Test Case: Blocked By: | Blocking: Related Tickets: #1316, #3691, | Differential Rev(s): #11438,10581 | Wiki Page: | -------------------------------------+------------------------------------- Changes (by thomie): * related: #1316, #3691, #11438 => #1316, #3691, #11438,10581 Comment: @goldfire in #10581: proposal: Always behave as if ScopedTypeVariables is on at binding sites for type variables (in an annotation with forall). Then, at occurrences of type variables, check if the extension is on. If a type variable is in scope but the extension is off, remember that the user probably wants the extension, but then rename the type variable occurrence away from the in- scope one. If a type error ensues, we've remembered that the lack of ScopedTypeVariables may be to blame, and we recommend turning it on. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9244#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler