#15661: Nullary constraint in GHCi breaks `:t` command -------------------------------------+------------------------------------- Reporter: taktoa | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: GHCi | Version: 8.4.3 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- If you create a value whose type has a nullary constraint (i.e.: a constraint that does not reference any of the type variables in scope) and then try to run `:t` on it, GHCi attempts to run instance resolution and fails before printing the type. Expected output: {{{ GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> :set -XFlexibleContexts Prelude> data Foo = Foo Prelude> let x :: (Show Foo) => () ; x = () Prelude> :t x x :: Show Foo => () }}} Actual output: {{{ GHCi, version 8.4.3: http://www.haskell.org/ghc/ :? for help Prelude> :set -XFlexibleContexts Prelude> data Foo = Foo Prelude> let x :: (Show Foo) => () ; x = () Prelude> :t x <interactive>:1:1: error: No instance for (Show Foo) arising from a use of ‘x’ }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15661 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler