#3919: Or-patterns as GHC extension -------------------------------------+------------------------------------- Reporter: BjornEdstrom | Owner: Type: feature request | Status: new Priority: normal | Milestone: ⊥ Component: Compiler | Version: Resolution: | Keywords: 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 osa1): If we could come up with a nice syntax for function definitions with or- patterns this would be really useful for GHC's own code too. Often we have to re-compile GHC many times and test on some programs to discover all the places we need to update after adding a new data constructor, because of "catch-all" code like this: {{{#!haskell f A = ... f B = ... f unexpected = pprTrace "f" (ppr unexpected) }}} With or patterns, we could do this instead: {{{#!haskell f A = ... f B = ... f unexpected@(C{} | D{} | E{}) = pprTrace "f" (ppr unexpected) }}} (just made up a syntax) This would be very convenient, because now I get a warning after adding data constructor D - instead of discovering this code after a runtime failure. And what happens if I couldn't discover this code because I couldn't come up with an example program that makes GHC run this function? So it's a bit tricky and annoying right now, and it's costing us time because of recompilations and tests. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/3919#comment:11 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler