#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 ross): Replying to [comment:12 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.

The simplifier reduces that to an identity using rules in Control.Arrow (though more rules are probably needed to handle slightly more complex cases). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:14 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler