#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): Here's a different suggestion for rebinding that is perhaps more analogous to the expression case. We could consider do and if commands as sugar for more primitive commands: {{{ do { p <- cmd; stmts } = cmd `bind` \ p -> do { stmts } do { cmd; stmts } = cmd `bind_` do { stmts } do { let defs; stmts } = let defs in do { stmts } do { cmd } = cmd if exp then cmd1 else cmd2 = (| ifThenElseA cmd1 cmd2 |) exp }}} (remember these are commands, not expressions), given the ordinary Haskell definitions {{{ bind :: Arrow a => a (e,s) b -> a (e,(b,s)) c -> a (e,s) c u `bind` f = arr id &&& u >>> arr (\ ((e,s),b) -> (e,(b,s))) >>> f bind_ :: Arrow a => a (e,s) b -> a (e,s) c -> a (e,s) c u `bind_` v = arr id &&& u >>> arr fst >>> v ifThenElseA :: ArrowChoice a => a (e,s) r -> a (e,s) r -> a (e,(Bool,s)) r ifThenElseA thenPart elsePart = arr split >>> thenPart ||| elsePart where split (e, (True, s)) = Left (e, s) split (e, (False, s)) = Right (e, s) }}} Then rebinding would just use whatever definitions of these names are in scope, and they could be associated with {{{BindStmt}}}, etc, in a similar way to expressions. But would this be enough for the people who want rebinding? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:17 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler