#13401: GHCi gives conflicting information about visible type application -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: GHCi | Version: 8.0.1 Resolution: | Keywords: | TypeApplications 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 RyanGlScott): What is happening here is that `:type` is regeneralizing the type of the expression `foo`. In the regeneralized type, `a` is now properly invisible (i.e., not available for type application), which is why `:type foo` will give you `forall {a}. a` regardless of whether you declared foo with `foo = undefined` or `foo :: a; foo = undefined`. If you wish to see the type of `foo` that preserves the visibility of its type parameters, you need to use the `:type +v` GHCi command (which is currently only available in GHC HEAD): {{{ λ> let foo = undefined λ> :type +v foo foo :: forall {a}. a λ> let foo :: a; foo = undefined λ> :type +v foo foo :: forall a. a }}} So I don't think there's a GHC bug here—just slightly misleading documentation at worst. I think including a mention of `:type +v` in the users' guide section on `-XTypeApplications` might dispel this confusion. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13401#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler