#12534: GHC 8.0 accepts recursive kind signature that GHC 7.10 rejects -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: TypeInType | Operating System: Unknown/Multiple Architecture: | Type of failure: GHC accepts Unknown/Multiple | invalid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC 7.10 rejects this datatype: {{{#!hs {-# LANGUAGE KindSignatures #-} {-# LANGUAGE PolyKinds #-} module Bug where data T (a :: a) }}} {{{ $ /opt/ghc/7.10.3/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:5:1: Kind variable also used as type variable: ‘a’ In the data type declaration for ‘T’ }}} But GHC 8.0 accepts the above code! It appears that GHC implicitly freshens the kind-level `a` so as to have a different name, since the following modification to `T`: {{{#!hs data T a (a :: a) }}} Results in this error: {{{ $ /opt/ghc/8.0.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:5:8: error: Conflicting definitions for ‘a’ Bound at: Bug.hs:5:8 Bug.hs:5:11 Bug.hs:5:16: error: Type variable ‘a’ used in a kind. Did you mean to use TypeInType? the data type declaration for ‘T’ }}} But with this modification: {{{#!hs data T (a :: a) a }}} You won't get an error message about `a` being used as both a kind and type: {{{ $ /opt/ghc/8.0.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:5:9: error: Conflicting definitions for ‘a’ Bound at: Bug.hs:5:9 Bug.hs:5:17 }}} To make things even more confusing, this behavior doesn't seem to carry over to typeclass instances. For example: GHC 8.0 will (rightfully) reject this code: {{{#!hs {-# LANGUAGE KindSignatures #-} {-# LANGUAGE PolyKinds #-} module Bug where data T (a :: a) class C x instance C (T (a :: a)) }}} {{{ $ /opt/ghc/8.0.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:8:16: error: Variable ‘a’ used as both a kind and a type Did you intend to use TypeInType? }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12534 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler