#9793: Some as-patterns could be accepted in pattern synonyms -------------------------------------+------------------------------------- Reporter: cactus | Owner: cactus Type: feature request | Status: new Priority: normal | Milestone: 7.10.1 Component: Compiler (Type | Version: 7.8.3 checker) | Operating System: Keywords: pattern synonyms | Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: Difficulty: Easy (less than 1 | None/Unknown hour) | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Currently all as-patterns are rejected in pattern synonym definitions, to avoid situations like {{{ pattern P x y <- x@(y:_) }}} since there's no valid reason to be able to then write something like {{{ f [True] False = ... }}} this would just lead to confusion. However, I think we could accept as-patterns where the body of the as- pattern doesn't contain any variable bindings that are accessible via the pattern synonym. In other words, this should be OK: {{{ pattern P x <- x@(y:_) }}} since it's exactly equivalent to {{{ pattern P x <- x@(_:_) }}} which I don't think is as confusing as the other example. I haven't really made up my mind yet if these should be bidirectional; but they certainly could be, by just ignoring the body of the as-pattern in the wrapper; so the following two would be equivalent: {{{ pattern P1 x = [x@(y:_)] pattern P2 x <- [x@(y:_)] where P2 x = [x] }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9793 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler