#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 mpickering): I don't think this is a great idea. The specification about what can appear on the RHS is currently very simple, it just has to be a pattern. The rules for turning this pattern into an expression are also very simple and rely on the fact that a lot of pattern syntax looks like the corresponding expression. Allowing this change complicates things, firstly in the parser, `Vec (x : unVec xs)` is not valid pattern syntax currently. Secondly, the rules for how to perform the inversion require more thought. Looking at Simon's example, I have to think a bit about what should happen.. it seems the corresponding expression should be `Vec (x : Vec xs)` but I'm still not sure that is right! Perhaps the rule is as simple as replace the function with it's inverse but it seems ad-hoc to me. I do have other lingering concerns that the way we designed the feature, the most useful constructs are the least supported. I'm referring to the fact that all interesting uses are explicitly bidirectional pattern synonyms which require view patterns. Such definitions tend to be quite noisy to define -- something more like views would have made things cleaner. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12203#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler