#10457: Revise/remove custom mapM implementation for lists -------------------------------------+------------------------------------- Reporter: dolio | Owner: Type: task | Status: patch Priority: normal | Milestone: 7.12.1 Component: libraries/base | Version: 7.10.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Revisions: Phab:D1124 -------------------------------------+------------------------------------- Changes (by bgamari): * differential: Phab:D924 => Phab:D1124 Old description:
Recently, Simon Marlow asked why the list instance for Traversable had a custom mapM implementation that used the Monad operations. Having looked a bit, I don't think there's any good reason. The only fusion that the custom mapM can participate in is due to it being written as foldr, but traverse is, as well. So as long as 'mapM = traverse' is able to inline appropriately, there should be no difference.
Further, this can be changed, in principle, for 7.10.2. It doesn't change any types, only the implementation.
mapM = traverse is the class default definition, so this could possibly be completed by just removing the custom definition.
Link to the libraries thread: https://mail.haskell.org/pipermail/libraries/2015-May/025708.html
New description: Recently, Simon Marlow asked why the list instance for `Traversable` had a custom `mapM` implementation that used the `Monad` operations. Having looked a bit, I don't think there's any good reason. The only fusion that the custom `mapM` can participate in is due to it being written as `foldr`, but `traverse` is, as well. So as long as `mapM = traverse` is able to inline appropriately, there should be no difference. Further, this can be changed, in principle, for 7.10.2. It doesn't change any types, only the implementation. `mapM = traverse` is the class default definition, so this could possibly be completed by just removing the custom definition. Link to the libraries thread: https://mail.haskell.org/pipermail/libraries/2015-May/025708.html -- Comment: Phab:D924 proposed a redefinition of `mapM_` in addition to the removal of list's override of `mapM`. This changed the performance characteristics of user code, as seen in #10711. I've extracted the removal of the `mapM` override into Phab:D1124 and forwarded the `mapM_` issue to the Core Libraries Committee. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10457#comment:13 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler