I opened a ticket and replied: https://ghc.haskell.org/trac/ghc/ticket/13996#comment:1
Simon From: ghc-devs [mailto:[email protected]] On Behalf Of Conal Elliott Sent: 19 July 2017 01:01 To: David Feuer <[email protected]> Cc: [email protected] Subject: Re: Repeated computations under a lambda 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 <[email protected]<mailto:[email protected]>> 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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