#12203: Allow constructors on LHS of (implicit) bidirectional pattern synonym -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: feature request | Status: new Priority: low | Milestone: Component: Compiler (Type | Version: 8.0.1 checker) | Keywords: Resolution: | PatternSynonyms Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Edward and I discussed this. What if we wrote this: {{{ pattern VCons :: a -> Vec a n -> Vec a (n + 1) pattern VCons x xs = Vec (x : unVec xs) }}} What makes it OK to use bidirectional "=" is that the RHS is invertible. Constructors are invertible. '''But so is `unVec`''', because it's inverse is just `Vec`. So maybe we should just expand the class of invertible RHSs a little? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12203#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler