#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: #1537, | #3613 ----------------------------------------------+---------------------------- Comment (by jstolarek): Replying to [comment:34 simonpj]:

So the type checker should be able to construct functions `arr-at-T`, `(>>>)-at-T` (for the ambient arrow T) ''that are polymorphic in the environment argument''.

My concern was what if the `arr` defined by the user cannot be made polymorphic in the environment argument? According to my understanding of this bug report the problem is that during desugaring all calls to user-defined `arr` are instantiated to be the same and thus typechecking fails, because different types are required at different call sites. Why the same thing doesn't happen without rebinding? Replying to [comment:35 ross]:

Not quite. There are three different grammar items here: {{{ exp ::= \ pat -> exp exp ::= proc pat -> cmd cmd ::= \ pat -> cmd }}} In the last one you're already in the command world, and you want to take a value off the stack and give names to its components for use in the subcommand.

Now I see this was even mentioned in "A New Notation for Arrows" paper. Again there's something I don't understand here. So far I though that stack is an implicit mechanism that is used to pass around values bound using `<-`. But here it seems that the user is allowed to explicitly access the stack. Isn't that a bit risky? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:37 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler