<div dir="auto"><div>I don't remember what documentation, if any, it has. You're right that taking advantage of it is potentially risky. Here's what I think we really want:</div><div dir="auto"><br></div><div dir="auto">atomicModifyMutVarQ# :: MutVar# s a -> (q -> a) -> (a -> q)<span style="font-family:sans-serif">Â </span><font face="sans-serif">Â -> State# s</font><span style="font-family:sans-serif">Â -> (# State# s, a, q #)</span></div><div dir="auto"><font face="sans-serif"><br></font></div><div dir="auto"><font face="sans-serif">where there's a special rule that (q -> a) is "obviously" a selector that selects a pointer.<br></font><br><div class="gmail_quote" dir="auto"><div dir="ltr" class="gmail_attr">On Fri, Oct 11, 2019, 12:56 PM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal" style="margin-left:36.0pt">The result doesn't have to be a pair. It can be a tuple of any size at all. Indeed, it can even be an arbitrary record type whose first pointer field has the appropriate type.<u></u><u></u></p>
<p class="MsoNormal"><u></u>Â <u></u></p>
<p class="MsoNormal">I think that is 100.0% undocumented, in the code, or in the proposal. Are you sure this is a settled consensus among the interested parties?<u></u><u></u></p>
<p class="MsoNormal"><u></u>Â <u></u></p>
<p class="MsoNormal">Adopting it would impose new invariants on the representation of values in GHC that I am deeply reluctant to impose. I would much much prefer to stick with the pair that is (somewhat) documented.<u></u><u></u></p>
<p class="MsoNormal"><u></u>Â <u></u></p>
<p class="MsoNormal">About pair vs Unit, yes, I can see (just) your point about why a pair might be useful. Here’s a better example:<u></u><u></u></p>
<p class="MsoNormal"><u></u>Â <u></u></p>
<p class="MsoNormal">Suppose mv :: MutVar# Int<u></u><u></u></p>
<p class="MsoNormal"><u></u>Â <u></u></p>
<p class="m_-2251622308724748719Code">atomicModifyMutVar2# mv $ \a -><u></u><u></u></p>
<p class="m_-2251622308724748719Code">Â let foo = f a<u></u><u></u></p>
<p class="m_-2251622308724748719Code">Â in (g foo, foo)<u></u><u></u></p>
<p class="MsoNormal"><span><u></u>Â <u></u></span></p>
<p class="MsoNormal"><span>Now, if f is expensive,
<i>and g is not invertible</i>, then sharing foo might be useful. It’s hard to think of a credible example, though. Regardless, we should document it.<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u>Â <u></u></span></p>
<p class="MsoNormal"><span>Simon<u></u><u></u></span></p>
<p class="MsoNormal"><span><u></u>Â <u></u></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> David Feuer <<a href="mailto:david.feuer@gmail.com" target="_blank" rel="noreferrer">david.feuer@gmail.com</a>>
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<b>Sent:</b> 11 October 2019 17:03<br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank" rel="noreferrer">simonpj@microsoft.com</a>><br>
<b>Cc:</b> ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank" rel="noreferrer">ghc-devs@haskell.org</a>><br>
<b>Subject:</b> Re: atomicModifyMutVar2<u></u><u></u></span></p>
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<p class="MsoNormal"><u></u>Â <u></u></p>
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On Fri, Oct 11, 2019, 11:08 AM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank" rel="noreferrer">simonpj@microsoft.com</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">David<u></u><u></u></p>
<p class="MsoNormal">I’m deeply puzzled atomicModifyMutVar2#. I have read
<a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fblob%2Fmaster%2Fproposals%2F0149-atomicModifyMutVar.rst&data=02%7C01%7Csimonpj%40microsoft.com%7C10950014fe9d46a6682508d74e649537%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637064066214164213&sdata=2phccb9r%2Fcg4xs7beKByn%2F1Ivxz7kvN5xkejhOpDfg8%3D&reserved=0" target="_blank" rel="noreferrer">
the proposal</a>, and the comments in primops.txt.pp (reproduced below).<u></u><u></u></p>
<h1>Question 1<u></u><u></u></h1>
<p class="MsoNormal">I think the “real†type of atomicModifyMutVar2 is
<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">atomicModifyMutVar2# :: MutVar# s a<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â -> (a -> (a,b))<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â -> State# s<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â -> (# State# s, a, (a, b) #)<u></u><u></u></p>
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<p class="MsoNormal"><u></u>Â <u></u></p>
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<p class="MsoNormal">Close, but not quite. The result doesn't have to be a pair. It can be a tuple of any size at all. Indeed, it can even be an arbitrary record type whose first pointer field has the appropriate type.<u></u><u></u></p>
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<p class="MsoNormal"><u></u>Â <u></u></p>
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<p class="MsoNormal">Nowhere is this explicitly stated, but I believe that the intended semantics of a call<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">case (atomicModifyMutVar2# mv f s) of (# s’, x, r #) -> blah<u></u><u></u></p>
<p class="MsoNormal">Then, suppose the old value of the MutVar was ‘<b><span style="font-family:"Courier New"">old’</span></b><u></u><u></u></p>
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<li class="m_-2251622308724748719m-3683590175518582062msolistparagraph">
The primop builds a thunk <b><span style="font-family:"Courier New"">t </span></b>=
<b><span style="font-family:"Courier New"">f old</span></b><u></u><u></u></li><li class="m_-2251622308724748719m-3683590175518582062msolistparagraph">
The new value of the mutable variable is <b><span style="font-family:"Courier New"">(fst t)</span></b><u></u><u></u></li><li class="m_-2251622308724748719m-3683590175518582062msolistparagraph">
The result <b><span style="font-family:"Courier New"">r</span></b> is t<u></u><u></u></li><li class="m_-2251622308724748719m-3683590175518582062msolistparagraph">
The result <b><span style="font-family:"Courier New"">x</span></b> is <b><span style="font-family:"Courier New"">old</span></b><u></u><u></u></li></ul>
<p class="MsoNormal">Question: is that correct?  We should state it explicitly.<u></u><u></u></p>
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<p class="MsoNormal">Yes, that sounds right.<u></u><u></u></p>
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<h1>Question 2<u></u><u></u></h1>
<p class="MsoNormal">Next question: Why does f have to return a pair? So far as I can tell, it’s only so that a client can force it.   The ‘b’ part never seems to play a useful role.  So we could
equally well have had<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">atomicModifyMutVar2# :: MutVar# s a<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â -> (a -> Box a)<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â -> State# s<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â -> (# State# s, a, Unit a #)<u></u><u></u></p>
<p class="MsoNormal">where Unit is defined in Data.Tuple<u></u><u></u></p>
<p class="m_-2251622308724748719m-3683590175518582062code">Â Â Â data Unit a = Unit a<u></u><u></u></p>
<p class="MsoNormal">Now you can force the result of (f old), just as with a pair. But the ‘b’ would no longer complicate matters.
<u></u><u></u></p>
<p class="MsoNormal">Question: is the ‘b’ in the pair significant?  Or could we use Unit?<u></u><u></u></p>
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<p class="MsoNormal">Yes, it's somewhat significant. You actually can use Unit with the new primop (it's a tuple of arity 1), so that option is free. But using a pair gets you a bit more: you can build a thunk that's *shared* between the value installed in
the MutVar and the one returned to the outside. Consider<u></u><u></u></p>
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<p class="MsoNormal"><u></u>Â <u></u></p>
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<p><span style="font-family:"Arial",sans-serif">atomicModifyMutVar2# mv $ \a -><u></u><u></u></span></p>
<p><span style="font-family:"Arial",sans-serif">Â let foo = expensive_computation a<u></u><u></u></span></p>
<p><span style="font-family:"Arial",sans-serif">Â in ([3,foo], foo)<u></u><u></u></span></p>
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<h1>Question 3<u></u><u></u></h1>
<p class="MsoNormal">In the comments below you say "but we don't know about pairs hereâ€.  Are you sure? What stops you importing Data.Tuple into GHC.Prim?  This fancy footwork is one more complication,
if it could be avoided.<u></u><u></u></p>
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<p class="MsoNormal">That whole regime came before my time, but since we win a bit by *not* fixing it, o wouldn't jump on it too quick.<u></u><u></u></p>
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