#14394: Inferred type for pattern synonym has redundant equality constraint -------------------------------------+------------------------------------- Reporter: RyanGlScott | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 (Type checker) | Keywords: | Operating System: Unknown/Multiple PatternSynonyms, TypeInType | Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- Load this file into GHCi: {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} module Bug where import Data.Type.Equality pattern Foo = HRefl }}} {{{ $ /opt/ghc/8.2.1/bin/ghci Bug.hs GHCi, version 8.2.1: http://www.haskell.org/ghc/ :? for help Loaded GHCi configuration from /home/rgscott/.ghci [1 of 1] Compiling Bug ( Bug.hs, interpreted ) Ok, 1 module loaded. λ> :i Foo pattern Foo :: () => (* ~ *, b ~ a) => a :~~: b }}} Notice that the type signature for `Foo` has an entirely redundant `* ~ *` constraint. The same does not happen if `TypeInType` is enabled. {{{#!hs {-# LANGUAGE GADTs #-} {-# LANGUAGE PatternSynonyms #-} {-# LANGUAGE TypeInType #-} module Works where import Data.Type.Equality pattern Foo = HRefl }}} {{{ λ> :i Foo pattern Foo :: forall k2 k1 (a :: k1) (b :: k2). () => (k2 ~ k1, (b :: k2) ~~ (a :: k1)) => a :~~: b }}} -- Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14394> GHC <http://www.haskell.org/ghc/> The Glasgow Haskell Compiler