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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">I opened a ticket and replied:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><a href="https://ghc.haskell.org/trac/ghc/ticket/13996#comment:1">https://ghc.haskell.org/trac/ghc/ticket/13996#comment:1</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Conal Elliott<br>
<b>Sent:</b> 19 July 2017 01:01<br>
<b>To:</b> David Feuer <david@well-typed.com><br>
<b>Cc:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Repeated computations under a lambda<o:p></o:p></span></p>
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Here's the code in question, slightly rephrased:<o:p></o:p></p>
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<o:p> </o:p></p>
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> exampleC t = \ x -> x + s where s = sin t<o:p></o:p></p>
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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.<o:p></o:p></p>
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<o:p> </o:p></p>
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-- Conal<o:p></o:p></p>
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<o:p> </o:p></p>
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On Tue, Jul 18, 2017 at 4:52 PM, David Feuer <<a href="mailto:david@well-typed.com" target="_blank">david@well-typed.com</a>> wrote:<o:p></o:p></p>
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On Tuesday, July 18, 2017 3:55:28 PM EDT Conal Elliott wrote:<br>
> Hi Sebastian,<br>
><br>
> Thanks for the reply. It's that I don't want `exampleC` to be eta-expanded.<br>
> Apparently GHC does by default even when doing so moves computation under<br>
> lambda. I've thought otherwise for a very long time.<br>
<br>
GHC really likes to eta-expand, because that can be very good for allocation,<br>
unboxing, and I don't know what else. Do you really need to represent the<br>
intermediate result by a *function*? Would it work just to save the Double<br>
itself? I suspect you could likely convince GHC to leave it alone.<br>
<span style="color:#888888"><br>
<span class="hoenzb">David</span></span><o:p></o:p></p>
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<o:p> </o:p></p>
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