#12716: Suggest ScopedTypeVariables when type variable not in scope -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Resolution: duplicate | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): I would love someone to work on this a bit. In Edward's example above, it would be relatively easy to put 'a' in the environment with a flag to say "Not really in scope unless you use `ScopedTypeVariables`. But there is more thinking to do. What if he'd written {{{ f :: forall a. a -> a f x = let g :: a -> a g _ = x in g x }}} Here the programmer probably intended the inner `a -> a` to be scoped by the `forall`; but as it stands, without `ScopedTypeVariables` it means {{{ g :: forall a. a->a }}} which will then be rejected by the type checker. I suggest that * `ExplicitForAll` should imply `ScopedTypeVariables`. (The reverse is already true!) * 'RankNTypes` should imply `ScopedTypeVariables`. (`RankNTypes` already implies `ExplicitForAll`, so the earlier bullet would imply this one.) You could always say `ExplicitForAll, NoScopedTypeVariables` if you really wanted. That still leaves #9244 and friends wanting a better error message/suggestion. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12716#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler