#10711: Defining mapM_ in terms of traverse_ causes substantial blow-up in ByteCodeAsm -------------------------------------+------------------------------------- Reporter: bgamari | Owner: bgamari Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: Compile-time Unknown/Multiple | performance bug Test Case: ghcirun004 | Blocked By: Blocking: | Related Tickets: Differential Revisions: | -------------------------------------+------------------------------------- Phab:D924 has proposed that we redefine `mapM_`, currently, {{{ mapM_ = foldr ((>>) . f) (return ()) }}} as, {{{ mapM_ = foldr ((*>) . f) (return ()) = traverse_ }}} as part of the AMP proposal. However, this appears to have severe effects on the performance characteristics of the `Assembler` monad defined in `ByteCodeAsm`. In particular, the `mapM_` use in `ByteCodeAsm.assembleBCO` blows up severely, increasing the runtime of the `ghcirun004` testcase from 4 seconds to over 5 minutes. Intriguingly, defining `(*>) = (>>)` in `Assembler`'s `Applicative` instance (as done in Phab:D1097) restores reasonable runtime. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10711 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler