#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 have spent some time on this today and I have some questions. As I understand Simon's comment the idea is to handle the arrow do-notation similarly to the monadic do-notation. Looking at the handling of monadic bind I see that indeed each `BindStmt` stores a unique call to `>>=` and `fail`. These are [https://github.com/ghc/ghc/blob/master/compiler/rename/RnExpr.lhs#L681 generated in the renamer] and then [https://github.com/ghc/ghc/blob/master/compiler/deSugar/DsExpr.lhs#L775 converted to explicit lambda by the desugarer]. For arrow operations - `arr`, `(>>>)`, `first` and possibly `loop`, `(|||)` and `app` - [https://github.com/ghc/ghc/blob/master/compiler/rename/RnExpr.lhs#L416 renamer creates a shared table of names]. [https://github.com/ghc/ghc/blob/master/compiler/deSugar/DsArrows.lhs#L69 Desugarer creates a command environment] that contains identifiers that are shared by all desugared expressions. This basicaly rephrases what Simon wrote. Now I have reached a point where I'm not sure what to do. For monads calls to bind are introduced only (?) when we encounter `BindStmt`. For arrows it is not that simple. For example calls to `>>>` are introduced when we desugar `BodyStmt`, `BindStmt` and `RecStmt` but also by [https://github.com/ghc/ghc/blob/master/compiler/deSugar/DsArrows.lhs#L94 `do_compose`], which does not correspond to any particular source construct but is called in many places of arrow notation desugarer. Another place where we introduce `>>>` is [https://github.com/ghc/ghc/blob/master/compiler/deSugar/DsArrows.lhs#L1038 desugaring of CmdLStmt]. I might be wrong, but it seems to me that adding `SyntaxExpr` to `HsCmd` constructors won't help us here. We need a way of generating fresh calls to desugared arrow operations out of thin air. I also don't understand why is arrow notation [https://github.com/ghc/ghc/blob/master/compiler/rename/RnExpr.lhs#L803 considered not rebindable]? The comment suggests that this is because !DsArrows performs a separate desugaring of arrow notation. Am I right to think that after this change is implemented arrow notation will be marked as rebindable? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler