#7828: RebindableSyntax and Arrow -------------------------------------+------------------------------------- Reporter: | Owner: jstolarek AlessandroVermeulen | Status: new Type: bug | Milestone: 7.10.1 Priority: normal | Version: 7.6.2 Component: Compiler | Keywords: (Type checker) | Operating System: Unknown/Multiple Resolution: | Type of failure: GHC rejects Differential Revisions: | valid program Architecture: | Test Case: Unknown/Multiple | Blocking: Difficulty: Unknown | Blocked By: | Related Tickets: #1537, | #3613 | -------------------------------------+------------------------------------- Comment (by jstolarek): Replying to [comment:43 ross]:

Replying to [comment:42 jstolarek]:

...

I'm able to compile

{{{ test :: Arrow a => a i i test = proc n -> do returnA -< n }}}

using the new desugaring, which was not possible earlier.

I wouldn't have expected any change in the handling of this. Hm... I just realized I probably misunderstood one thing. My desugaring is slightly different, but this is easy to fix.

If we're allowing rebinding, that should be type-checked and desugared exactly as if it were written

{{{ test :: Arrow a => a i i test = proc n -> (returnA -< n) `thenA` (returnA -< n) }}} I believe that my implementation is doing exactly this. It's just that types don't match.

For the default `thenA`, {{{ thenA :: Arrow a => a (e,s) b -> a (e,s) c -> a (e,s) c u `thenA` v = arr id &&& u >>> arr fst >>> v }}} we have a1 = a2 = a, s1 = s2 = () and t = i, but with rebinding they'd be taken from the type of `thenA`, which could be anything matching the type in the above inference.

At the moment my implementation does not work even for default `thenA`. I keep looking at the desugaring of monadic do-notation trying to figure out where did I go wrong. I guess the discussion here is more about the details of my implementation. To keep in line with [http://www.haskell.org/pipermail /ghc-devs/2014-July/005562.html recent decisions] I will move it to [https://phabricator.haskell.org/ Phab]. I'll post the link once I've created a new phabricator revision. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:44 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler