#11982: Typechecking fails for parallel monad comprehensions with polymorphic let (GHC 7.10.3 through 8.6.3) -------------------------------------+------------------------------------- Reporter: simonpj | Owner: josefs Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.10.3 Resolution: | Keywords: ApplicativeDo Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Yes to two tickets. So we have * '''Parallel monad comprehensions''': no difficulty in principle, but `tcSyntaxOp` needs to do impredicative instantiation. See comment:14 * '''ApplicativeDo'''. New ticket needed. I strongly suggest that Step 1 is to take the presentation in the paper and extend it to handle `let` statements. Then it may be clearer what to do. Probably nothing to do with impredicativity. Note also Simon's comment that "I also have a suspicion that we're not treating the last statement of the do in the way that the paper suggests - adding an extra return statement if the last statement is not a return." It would be good to clean this up too. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11982#comment:18 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler