#13947: GHC 8.2 gives misleading error message for out-of-scope infix type constructor -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Poor/confusing Unknown/Multiple | error message Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- {{{#!hs {-# LANGUAGE TypeOperators #-} module Bug where f :: () -> Int :~: Int f = undefined }}} GHC 8.0.2 gives a reasonable error message: {{{ $ /opt/ghc/8.0.2/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:4:12: error: Not in scope: type constructor or class ‘:~:’ }}} But GHC 8.2.1's error message is much more confusing: {{{ $ /opt/ghc/8.2.1/bin/ghc Bug.hs [1 of 1] Compiling Bug ( Bug.hs, Bug.o ) Bug.hs:4:6: error: Precedence parsing error cannot mix ‘(->)’ [infixr 0] and ‘:~:’ [infixl 0] in the same infix expression | 4 | f :: () -> Int :~: Int | ^^^^^^^^^^^^^^^^^ Bug.hs:4:12: error: Not in scope: type constructor or class ‘:~:’ A data constructor of that name is in scope; did you mean DataKinds? | 4 | f :: () -> Int :~: Int | ^^^^^^^^^^^ }}} I was thrown akilter by the claim that `:~:` was `infixl 0`, since it was displayed before the real cause of the error (that it's out-of-scope). Adding `import Data.Type.Equality` fixes the error, but GHC really shouldn't be making claims about the fixities of out-of-scope type constructors in the first place. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13947 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler