Sorry for the confusion -- it turns out GHC 8.2.2 already optimizes
#8763: forM_ [1..N] does not get fused (10 times slower than go function) -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: infoneeded Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: #7206 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by George): Replying to [comment:36 osa1]: this. Perhaps this can be closed or we may need a new reproducer. I have a very similar program where, with 8.4.1, the forM_ version allocates 50% more than the version with recursive go but both versions run in about the same time . Is it worth giving that code here in this bug or should I enter a new bug referencing this one? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8763#comment:39 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler