#11835: ApplicativeDo failed to desugar last line with pure $ <expr> -------------------------------------+------------------------------------- Reporter: Cosmia | Owner: simonmar Type: bug | Status: merge Priority: normal | Milestone: 8.0.2 Component: Compiler | Version: 8.0.1-rc2 Resolution: | Keywords: ApplicativeDo Operating System: MacOS X | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: #11607 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonmar): GHC translates {{{ do x <- e return (f x) }}} into {{{ (\x -> f x) <$> e }}} So we have to spot `return`, because the transformation removes it. You could invent more rules, say spot `return $ e` for example, but there would always be more examples that you couldn't handle, it's not possible to be completely general here. Consider `return` (or `pure`) as part of the syntax of the `do` expression, like a keyword. Maybe if we were starting from scratch the syntax would be different. Indeed, in Monad Comprehensions, the `return` is always there implicitly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11835#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler