#14112: bang patterns on pattern synonyms? (left vs right hand sides) -------------------------------------+------------------------------------- Reporter: carter | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1 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 RyanGlScott): Actually, I want to take back my proposal in comment:3. My idea relies on the ability to put "bang patterns" on variable binders, which is something that simply isn't possible anywhere else. (We could change the pattern variable binders to be proper patterns and only allow them to be variables or variables adorned with bangs, but that would be an exceedingly strange design.) In light of this, I would suggest that we endorse explicitly bidirectional pattern synonyms as the only way to achieve a "two-way" strict pattern synonym. That is: {{{#!hs pattern MyPair1 x y <- MkPair !x !y where MyPair1 !x !y = MkPair x y }}} Therefore, the fact that you can currently do this: {{{#!hs pattern MyJust a = JustC !a }}} Should be treated as a bug. `JustC !a` makes no sense as an expression, and moreover, it doesn't even give the strictness behavior that you'd expect, as discovered in comment:2. Does that sound agreeable, carter? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14112#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler