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 <da...@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 >
_______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
