#13680: Can't use TypeApplications with empty list expression -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple TypeApplications | Architecture: | Type of failure: GHC rejects Unknown/Multiple | valid program Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Why can't I do this? {{{#!hs {-# LANGUAGE TypeApplications #-} module Bug where foo :: [Int] foo = [] @Int }}} Compiling this with GHC 8.0.1, 8.0.2, 8.2.1, or HEAD gives me: {{{ $ /opt/ghc/head/bin/ghci Bug2.hs GHCi, version 8.3.20170509: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug2.hs, interpreted ) Bug2.hs:5:7: error: • Cannot apply expression of type ‘[a0]’ to a visible type argument ‘Int’ • In the expression: [] @Int In an equation for ‘foo’: foo = [] @Int | 5 | foo = [] @Int | ^^^^^^^ }}} This seems really strange, since I can use `TypeApplications` with expressions like `Nothing @Int` without issues. According to GHCi: {{{ $ /opt/ghc/head/bin/ghci GHCi, version 8.3.20170509: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci λ> :set -XTypeApplications -fprint-explicit-foralls λ> :type +v [] [] :: forall {a}. [a] λ> :type +v Nothing Nothing :: forall a. Maybe a }}} The type variable for `[]` isn't visible! But I don't see any reason why it shouldn't be, especially since, conceptually, the data type declaration for lists is: {{{#!hs data [] a = [] | a : [a] }}} I suspect that `[]`'s tyvar binder visibility is not wired into GHC correctly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13680 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler