#12447: Pretty-printing of equality `~` without parentheses -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 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: -------------------------------------+------------------------------------- Based off on [https://github.com/ekmett/constraints/blob/master/src/Data/Constraint/Deferr... Data.Constraint.Deferrable] {{{#!hs {-# Language RankNTypes, ConstraintKinds #-} import Data.Typeable class Deferrable p where deferEither :: proxy p -> (p => r) -> Either String r instance (Typeable a, Typeable b) => Deferrable (a ~ b) where deferEither = undefined }}} `PolyKinds` aren't enabled so `deferEither @(_ ~ _)` is not enough to select the `Deferrable (a ~ b)` instance, but it is displayed without parentheses {{{ $ ghci -XTypeApplications -ignore-dot-ghci tyiS.hs GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help [1 of 1] Compiling Main ( tyiS.hs, interpreted ) Ok, modules loaded: Main. *Main> :t deferEither @(_ ~ _) deferEither @(_ ~ _) :: Deferrable t ~ t1 => proxy t ~ t1 -> (t ~ t1 => r) -> Either String r }}} Instead of a preferable {{{#!hs deferEither @(_ ~ _) :: Deferrable (t ~ t1) => proxy (t ~ t1) -> (t ~ t1 => r) -> Either String r }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12447 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler