#7828: RebindableSyntax and Arrow ----------------------------------------------+---------------------------- Reporter: AlessandroVermeulen | Owner: Type: bug | jstolarek Priority: normal | Status: new Component: Compiler (Type checker) | Milestone: 7.10.1 Resolution: | Version: 7.6.2 Operating System: Unknown/Multiple | Keywords: Type of failure: GHC rejects valid program | Architecture: Test Case: | Unknown/Multiple Blocking: | Difficulty: Unknown | Blocked By: | Related Tickets: ----------------------------------------------+---------------------------- Comment (by jstolarek): After spending some time on this today I have to admit that arrow desugaring is still a mystery to me. For example this code: {{{ proc n -> do x <- returnA -< n returnA -< x }}} Is desugared to: {{{ arr (\n -> (n, ())) >>> arr (\ds -> case ds of { (ds2, ds3) -> ds2 }) >>> arr (\ds -> ((ds, ()), ())) >>> ((arr (\ds -> case ds of { (ds2, ds3) -> ds2 }) >>> id) *** id >>> arr (\ds -> case ds of { (x, ds2) -> case ds2 of { () -> x } }) ) >>> arr (\ds -> (ds, ())) >>> arr (\ds -> case ds of { (ds2, ds3) -> ds2 }) >>> id }}} This can be simplified to an identity (as expected), but GHC doesn't do it. This definitely seems like an opportunity for improvement. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler