#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): Phab:D2345 Wiki Page: | -------------------------------------+------------------------------------- Comment (by chreekat): Comment 6 provides the rationale for this situation, where the expected behavior isn't really possible because of lower-level concerns. Could that rationale be mentioned in the docs? Something like: {{{ --- a/docs/users_guide/glasgow_exts.rst +++ b/docs/users_guide/glasgow_exts.rst @@ -860,7 +860,9 @@ upon the results ``p1...pn`` with either ``return`` or ``pure``. Note: the final statement really must be of the form ``return E`` or ``pure E``, otherwise you get a ``Monad`` constraint. Using ``$`` as -in ``return $ E`` or ``pure $ E`` is also acceptable. +in ``return $ E`` or ``pure $ E`` is also acceptable. This is because we +must spot the ``return`` before `GHC transforms do-syntax into fmap +https://ghc.haskell.org/trac/ghc/ticket/11835?replyto=6#comment:6`_. }}} I've not had much experience contributing to GHC, so I apologize for the unwieldy format of this suggestion. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11835#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler