Repeated computations under a lambda

[Conal Elliott]

Here's the code in question, slightly rephrased:

> exampleC t = \ x -> x + s where s = sin t

I wrote it this way so that `sin t` would be computed once per `t` and
reused for each value of `x`. The intermediate result `s` has type
`Double`---not a function. Without `-fno-do-lambda-eta-expansion`, phase 2
of `ghc -O` causes the `s = sin t` binding to be moved under the `\ x ->
...`. I've been using this programming style for this purpose for longer
than I can remember, and apparently I've been mistaken about its
effectiveness at least part of that time.

-- Conal

On Tue, Jul 18, 2017 at 4:52 PM, David Feuer <david at well-typed.com> wrote:

> On Tuesday, July 18, 2017 3:55:28 PM EDT Conal Elliott wrote:
> > Hi Sebastian,
> >
> > Thanks for the reply. It's that I don't want `exampleC` to be
> eta-expanded.
> > Apparently GHC does by default even when doing so moves computation under
> > lambda. I've thought otherwise for a very long time.
>
> GHC really likes to eta-expand, because that can be very good for
> allocation,
> unboxing, and I don't know what else. Do you really need to represent the
> intermediate result by a *function*? Would it work just to save the Double
> itself? I suspect you could likely convince GHC to leave it alone.
>
> David
>
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