[GHC] #10711: Defining mapM_ in terms of traverse_ causes substantial blow-up in ByteCodeAsm

[GHC]

#10711: Defining mapM_ in terms of traverse_ causes substantial blow-up in
ByteCodeAsm
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        Reporter:  bgamari           |                   Owner:  bgamari
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:
      Resolution:                    |                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:
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Comment (by nomeata):

 BTW, here are the relevant definitions that add up to three `>>=` for one
 `*>`:

 {{{#!hs
 a1 *> a2   = (id <$ a1) <*> a2
 (<$)       = fmap . const
 fmap       = liftM
 liftM f m1 = do { x1 <- m1; return (f x1) } -- here is one
 (<*>)      = ap
 ap m1 m2   = do { x1 <- m1; x2 <- m2; return (x1 x2) } -- here are two
 }}}

 It seems that performance-worrying code should simply ''not'' implement
 `Functor` and `Applicative` via the `Monad`-derived methods.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10711#comment:3>
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