#8581: Pattern synonym used in an expression context could have different constraints to pattern used in a pattern context -------------------------------------+------------------------------------- Reporter: cactus | Owner: Type: feature request | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: Resolution: | Keywords: | 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): Let's not think about implementation before we have a ''design''. I have not read the entire thread again, but I'm pretty convinced that * We can't have two different types, one for construction and one for pattern matching I think it'll just be too confusing to have two types. It's bad enough to have this provided/required stuff without, in addition, having a completely separate type for construction. Are you seriously proposing to have two signatures for each pattern synonym? (Optionally, I assume.) So: if you give a pattern signature, I think it has to work for both construction and pattern matching. If you need extra constraints for construction, define a smart constructor (that can happen today with regular data constructors). Let's not over-elaborate until we have more experience. There are plenty of [https://ghc.haskell.org/trac/ghc/wiki/PatternSynonyms open tickets to tackle on the pattern-synonym front]. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8581#comment:34 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler