#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): I did some more reading and I finally understand Ross' answers :-) Section 3.2 of John Hughes' "Programming with Arrows" was really helpful in understanding the idea of the stack. But I don't yet understand the implementation and I still have problems reading the comments. I figured a good way of improving my understanding will be implementing a prototype solution outlined in [ticket:7828#comment:9 comment 9]. I wanted to begin by implementing it only for selected language constructs and I got stuck on typechecking !BindStmt in !TcMatches. Currently there is only the monadic !BindStmt. In my prototype I want to distinguish between the monadic and arrow !BindStmt. This means I need to write typechecking for the latter (in !TcMatches). I'm not sure how to do that as I don't know what is the type signature for arrow bind. I tried to deduce that based on the comments in !DsArrows, but I'm not sure if this is a correct approach. Can I once again ask for help in reading the comments? {{{ -- D; xs1 |-a c : () --> t -- D; xs' |-a do { ss } : t' xs2 = xs' - defs(p) -- ----------------------------------- -- D; xs |-a do { p <- c; ss } : t' -- -- ---> premap (\ (xs) -> (((xs1),()),(xs2))) -- (first c >>> arr (\ (p, (xs2)) -> (xs'))) >>> ss }}} I'm totally confused by `(\ (xs) -> (((xs1),()),(xs2)))`. Where do `xs1` and `xs2` come from, if we only bind `xs` as the input to `do { p <- c; ss }`? Also, do I understand correctly that bind expression generated in desugaring of arrow do-notation does not have a source-level equivalent? If so is it even correct to represent it using the !BindStmt constructor? In my prototype I'm ignoring Ross' proposal of the new desugaring. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:20 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler