#16376: GHCi HEAD silently accepts ill-kinded visible type applications with :type -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: highest | Milestone: Component: GHCi | Version: 8.9 Keywords: | 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: -------------------------------------+------------------------------------- If you try something nonsensical like `id @Maybe` in GHCi 8.6.3, it will be properly rejected: {{{ $ ghci -XTypeApplications GHCi, version 8.6.3: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :t id @Maybe <interactive>:1:5: error: • Expecting one more argument to ‘Maybe’ Expected a type, but ‘Maybe’ has kind ‘* -> *’ • In the type ‘Maybe’ In the expression: id @Maybe }}} On GHC HEAD, however, this is seemingly accepted! {{{ $ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive -XTypeApplications GHCi, version 8.9.20190224: https://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :t id @Maybe λ> }}} No error message, just... a newline. Quite bizarre. Even more bizarrely, this phenomenon doesn't appear to happen with `:kind`: {{{ λ> :set -XDataKinds -XPolyKinds λ> type Id (a :: k) = a λ> :k Id @Maybe <interactive>:1:5: error: • Expecting one more argument to ‘Maybe’ Expected a type, but ‘Maybe’ has kind ‘* -> *’ • In the first argument of ‘Id’, namely ‘Maybe’ In the type ‘Id @Maybe’ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16376 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler