#10457: Revise/remove custom mapM implementation for lists -------------------------------------+------------------------------------- Reporter: dolio | Owner: Type: task | Status: new Priority: normal | Milestone: 7.10.2 Component: libraries/base | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: Phab:D924 -------------------------------------+------------------------------------- Comment (by dolio): If I understand correctly, simonmar has a `Monad` whose `Applicative` instance does batching of remote calls, because you can do that when you use `(<*>)` but not `(>>=)`. So: {{{ f <*> x }}} does one round trip to the server, while: {{{ f >>= \g -> x >>= \y -> return (g y) }}} does two. The custom `mapM` does n trips, where n is the length of the list, while `traverse` does one, because the latter is implemented using: {{{ cons_f x ys = (:) <$> f x <*> ys }}} whereas the custom one is: {{{ cons_f x ys = do { z <- f x ; zs <- ys ; return (z:zs) } }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10457#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler