#14238: `:kind` suppresses visible dependent quantifiers by default in GHCi 8.2.1 -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 (Type checker) | Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: Incorrect result Unknown/Multiple | at runtime Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Load this program into GHCi on 8.2.1 or later: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE TypeInType #-} import Data.Kind data Foo (k :: Type) :: k -> Type where MkFoo :: Foo (k1 -> k2) f -> Foo k1 a -> Foo k2 (f a) }}} And ask it what the kind of `Foo` is: {{{ GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Ok, 1 module loaded. λ> :k Foo Foo :: k -> * }}} This is just plain wrong: the actual kind of `Foo` should be `forall k -> k -> *`. Normally, one can omit `forall`s from a kind signature, but this is a special case where we're using a //visible// forall. In other words, `forall k -> k -> *` is not the same as `k -> *`, as the former takes two arguments, whereas the latter takes one. A workaround is to force GHCi to come to its senses by explicitly enabling `-fprint-explicit-foralls`: {{{ λ> :set -fprint-explicit-foralls λ> :k Foo Foo :: forall k -> k -> * }}} This is actually a regression since GHC 8.0.2, since GHCi did the right thing by default then: {{{ GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Main ( Bug.hs, interpreted ) Ok, modules loaded: Main. λ> :k Foo Foo :: forall k -> k -> Type }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14238 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler