[GHC] #9781: Make list monad operations fuse

[David Feuer]

On Nov 11, 2014 6:04 AM, "Simon Peyton Jones" <simonpj at microsoft.com> wrote:
> It’s true that, particularly for fusion, inlining can make a huge
difference.  And GHC really does need help… it’s extremely hard for it to
make the “right” choice all the time.

The inliner does indeed do amazing things, and list fusion does indeed do
lovely things for user code. It's just not the most *reliable* optimization
in the compiler. I don't think there's anything wrong with admitting that
and trying to avoid relying on it too heavily in *library* code. Kim-Ee was
right that expanding out mapM by hand bloated the source. I've since
defined `sequence=mapM id` to resolve that problem, and doing so does not
hurt the benchmarks—it relies only on inlining id (which is quite reliable)
and beta-reducing (which is also quite reliable).

> I strongly agree with Kim-Ee that we should play the game of “optimise by
randomly mutating the program and pick the version that (today) happens to
run faster”.  But I don’t think David is doing that.   There is, at least a
Note: [List comprehensions and inlining].

I've been trying to leave a trail of comments and notes as I go. I may need
to go further.
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