#10928: Refine pattern synonym signatures -------------------------------------+------------------------------------- Reporter: mpickering | Owner: Type: bug | Status: new Priority: normal | Milestone: 8.0.1 Component: Compiler | Version: 7.10.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: Other | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): -------------------------------------+------------------------------------- Comment (by goldfire): Concur with comment:14. Keep current syntax, but with reversed ordering. Specifying only one constraint would now indicate a '''required''' constraint. On a separate note, the parallelism with GADT syntax discussed in comment:12 is already broken. Consider {{{#!hs data G a where MkG :: G Int pattern P :: G Int pattern P = MkG }}} The pattern and the data constructor have '''different''' static semantics. Specifically, {{{#!hs -- this works: ctor :: G a -> a ctor MkG = 5 -- this doesn't: pat :: G a -> a pat P = 5 }}} This is because a non-uniformity in the return type of a pattern is taken as a required equality constraint, not a provided equality constraint. The top set of definitions is equivalent to {{{#!hs data G a where MkG :: a ~ Int => G a pattern P :: {- required -} a ~ Int => {- provided -} () => G a pattern P = MkG }}} This was a design decision made in pattern synonym typing. I don't love this decision, but I can't quite argue against it either. It's one more way in which we must be honest that the things appearing after `MkG ::` and `P ::` are '''not''' types, but type-like specifications with a very specific interpretation. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10928#comment:16 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler