#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:20 jstolarek]:

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 }`?

They are subsets of `xs`: `xs1` is the variables used by `c`, while `xs2` is the variables used by `ss` but not bound in `p`. So the command 'c' gets local environment `xs1` and stack `()`. The translations of statements are arrows that take an environment (here `xs2`) as input.

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.

I don't understand that question - I don't see a bind expression in the generated code. Do you mean the `BindStmt` emitted by the type checker? No, that wouldn't correspond to something one could write in the command sublanguage. Part of the motivation for my proposal is that the desugaring of do notation would have a source-level equivalent, albeit still in the command sublanguage, but it could be checked using the existing rules for commands. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:21 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler