#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 vdukhovni): Indeed changing the last line from: {{{ f line locker }}} to {{{ ret <- f line locker return ret }}} resolves the bug for my code example. I hope this generalizes. Don't know how it applies to the original parallel list comprehension example. I did not expect the "where" alternative to generalize, but note that in this case I was able to arrange for all the relevant variables to be in scope by passing them into a closure precisely for that purpose. The idea was mostly to highlight the discrepancy in the behaviour between a "let" and a "where" that should ideally behave identically. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11982#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler