#14700: ApplicativeDo in MonadComprehensions -------------------------------------+------------------------------------- Reporter: lspitzner | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- It appears that a MonadComp (even one that is "Applicative only") is not transformed appropriately in the presence of ApplicativeDo. {{{#!hs {-# LANGUAGE MonadComprehensions #-} {-# LANGUAGE ApplicativeDo #-} value1 = [ f a b c | a <- ma, b <- mb, c <- mc ] value2 = do { a <- ma; b <- mb; c <- mc; return (f a b c) } value3 = f <$> ma <*> mb <*> mc }}} value1 should desugar to value2 via monadcomp and to value3 via appdo. Yet I observe very bad performance in some specific instance (due to a slower Monad instance) when using value1, where value2 and value3 have good performance. I have not looked at desuraged/core dumps but the observation seems clear. Tested on ghc-8.0.2 only. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14700 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler