We are typechecking `pat <- rhs; body` as if it was `(>>=) rhs (\x ->

#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 simonpj): I wrote body)`. So we instantiate `(>>=)`, expecting its type to have the shape

{{{ rhs_ty -> (pat_ty -> body_ty) }}} That's what `tcSyntaxOp` does.

Then we check that `pat` really does have type `pat_ty` and `rhs` really does have type `rhs_ty`.

The last sentence is terribly important! We are not just checking that `(>>=)` has a type of the right shape (which is, I gather from your question, what you think is going on), ''but also that it's type can be instantiated to match the particular pattern `pat` and the particular rhs `rhs`''. You aren't doing that with `(>>>)` which is why you have all those uninstantiated type variables. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7828#comment:29 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler