In general, the rule for when a do statement incurs a Monad constraint is as follows. If the do-expression has the following form: {{{ do p1 <- E1; ...; pn <- En; return E }}} where none of the variables defined by p1...pn are mentioned in E1...En,
#13511: ApplicativeDo incorrectly requiring Monad -------------------------------------+------------------------------------- Reporter: mnislaih | Owner: (none) Type: feature request | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Resolution: | Keywords: ApplicativeDo Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by mnislaih): The user's guide doesn't explicitly mention let bindings, it only says: then the expression will only require Applicative. Otherwise, the expression will require Monad. The block may return a pure expression E depending upon the results p1...pn with either return or pure. Or were you referring to something else ? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13511#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler