GHC does this transformation not nilly-willy, .... If that code would call some unknown function, or allocate memory, then GHC would certainly preserve your intention.
Wonderful. Thanks for the clarification.
-- Conal
On Tue, Jul 18, 2017 at 6:51 PM, Joachim Breitner
Hi,
Am Dienstag, den 18.07.2017, 17:01 -0700 schrieb 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.
usually it is very effective. GHC does this transformation not nilly- willy, but only when its heuristics indicate that it is ok, usually because the operation (here sin) is known to be very cheap. If that code would call some unknown function, or allocate memory, then GHC would certainly preserve your intention.
Greetings, Joachim _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs