<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:Wingdings;
panose-1:5 0 0 0 0 0 0 0 0 0;}
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Calibri;
panose-1:2 15 5 2 2 2 4 3 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
h1
{mso-style-priority:9;
mso-style-link:"Heading 1 Char";
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:24.0pt;
font-family:"Calibri",sans-serif;
font-weight:bold;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
a:visited, span.MsoHyperlinkFollowed
{mso-style-priority:99;
color:purple;
text-decoration:underline;}
p.MsoListParagraph, li.MsoListParagraph, div.MsoListParagraph
{mso-style-priority:34;
margin-top:0cm;
margin-right:0cm;
margin-bottom:0cm;
margin-left:36.0pt;
margin-bottom:.0001pt;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
p.Code, li.Code, div.Code
{mso-style-name:Code;
margin-top:0cm;
margin-right:0cm;
margin-bottom:0cm;
margin-left:14.2pt;
margin-bottom:.0001pt;
font-size:10.0pt;
font-family:"Courier New";
font-weight:bold;}
p.msonormal0, li.msonormal0, div.msonormal0
{mso-style-name:msonormal;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
p.m-2251622308724748719code, li.m-2251622308724748719code, div.m-2251622308724748719code
{mso-style-name:m_-2251622308724748719code;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
span.Heading1Char
{mso-style-name:"Heading 1 Char";
mso-style-priority:9;
mso-style-link:"Heading 1";
font-family:"Calibri Light",sans-serif;
color:#2F5496;
mso-fareast-language:EN-GB;}
p.m-2251622308724748719m-3683590175518582062code, li.m-2251622308724748719m-3683590175518582062code, div.m-2251622308724748719m-3683590175518582062code
{mso-style-name:m_-2251622308724748719m-3683590175518582062code;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
p.m-2251622308724748719m-3683590175518582062msolistparagraph, li.m-2251622308724748719m-3683590175518582062msolistparagraph, div.m-2251622308724748719m-3683590175518582062msolistparagraph
{mso-style-name:m_-2251622308724748719m-3683590175518582062msolistparagraph;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:11.0pt;
font-family:"Calibri",sans-serif;}
span.EmailStyle24
{mso-style-type:personal-reply;
font-family:"Calibri",sans-serif;
color:windowtext;}
.MsoChpDefault
{mso-style-type:export-only;
font-family:"Calibri",sans-serif;
mso-fareast-language:EN-US;}
.MsoPapDefault
{mso-style-type:export-only;
margin-top:6.0pt;
margin-right:0cm;
margin-bottom:6.0pt;
margin-left:0cm;}
@page WordSection1
{size:612.0pt 792.0pt;
margin:72.0pt 72.0pt 72.0pt 72.0pt;}
div.WordSection1
{page:WordSection1;}
/* List Definitions */
@list l0
{mso-list-id:262421548;
mso-list-template-ids:1590747564;}
@list l0:level1
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:36.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l0:level2
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:72.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l0:level3
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:108.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l0:level4
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:144.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l0:level5
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:180.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l0:level6
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:216.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l0:level7
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:252.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l0:level8
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:288.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l0:level9
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:324.0pt;
mso-level-number-position:left;
text-indent:-18.0pt;
mso-ansi-font-size:10.0pt;
font-family:Symbol;}
@list l1
{mso-list-id:448279074;
mso-list-type:hybrid;
mso-list-template-ids:-482995234 134807553 134807555 134807557 134807553 134807555 134807557 134807553 134807555 134807557;}
@list l1:level1
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Symbol;}
@list l1:level2
{mso-level-number-format:bullet;
mso-level-text:o;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:"Courier New";}
@list l1:level3
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Wingdings;}
@list l1:level4
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Symbol;}
@list l1:level5
{mso-level-number-format:bullet;
mso-level-text:o;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:"Courier New";}
@list l1:level6
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Wingdings;}
@list l1:level7
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Symbol;}
@list l1:level8
{mso-level-number-format:bullet;
mso-level-text:o;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:"Courier New";}
@list l1:level9
{mso-level-number-format:bullet;
mso-level-text:;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Wingdings;}
ol
{margin-bottom:0cm;}
ul
{margin-bottom:0cm;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="EN-GB" link="blue" vlink="purple">
<div class="WordSection1">
<p class="MsoNormal">OK. I propose:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2">To give atomicModieyMutVarOf# its proper type, with a pair, as in the proposal.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2">To do that I’ll fiddle with genprimopcode, to allow it to parse tuples as well as unboxed tuples; not hard.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2">This would disallow all this stuff about “any type that has a first field looking like a”, restricting to pairs alone. This didn’t form part of the proposal, and was never documented.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo2">Add a bit more clarity to the documentation, so it’d clear what must be forced.<o:p></o:p></li></ul>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Any objections?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt">
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> David Feuer <david.feuer@gmail.com>
<br>
<b>Sent:</b> 12 October 2019 00:00<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> ghc-devs <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: atomicModifyMutVar2<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
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:<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
atomicModifyMutVarQ# :: MutVar# s a -> (q -> a) -> (a -> q)<span style="font-family:"Arial",sans-serif"> -> State# s -> (# State# s, a, q #)</span><o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
<span style="font-family:"Arial",sans-serif">where there's a special rule that (q -> a) is "obviously" a selector that selects a pointer.</span><o:p></o:p></p>
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
On Fri, Oct 11, 2019, 12:56 PM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
</div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-right:0cm">
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;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.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">About pair vs Unit, yes, I can see (just) your point about why a pair might be useful. Here’s a better example:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Suppose mv :: MutVar# Int<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="m-2251622308724748719code">atomicModifyMutVar2# mv $ \a -><o:p></o:p></p>
<p class="m-2251622308724748719code"> let foo = f a<o:p></o:p></p>
<p class="m-2251622308724748719code"> in (g foo, foo)<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Simon<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt">
<div>
<div style="border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> David Feuer <<a href="mailto:david.feuer@gmail.com" target="_blank">david.feuer@gmail.com</a>>
<br>
<b>Sent:</b> 11 October 2019 17:03<br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>><br>
<b>Cc:</b> ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>><br>
<b>Subject:</b> Re: atomicModifyMutVar2</span><o:p></o:p></p>
</div>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<div>
<div>
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">On Fri, Oct 11, 2019, 11:08 AM Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
</div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0cm;margin-bottom:5.0pt">
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">David<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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%7C0b9a79c810d7453ce16008d74e9ebfc1%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637064316038353079&sdata=LGyYiZC9b9g3lZpzTdvQgyEwKCaEsIzFDBcacVE2N5M%3D&reserved=0" target="_blank">
the proposal</a>, and the comments in primops.txt.pp (reproduced below).<o:p></o:p></p>
<h1>Question 1<o:p></o:p></h1>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I think the “real” type of atomicModifyMutVar2 is
<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code">atomicModifyMutVar2# :: MutVar# s a<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code"> -> (a -> (a,b))<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code"> -> State# s<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code"> -> (# State# s, a, (a, b) #)<o:p></o:p></p>
</div>
</div>
</blockquote>
</div>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
</div>
<div>
<div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0cm;margin-bottom:5.0pt">
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Nowhere is this explicitly stated, but I believe that the intended semantics of a call<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code">case (atomicModifyMutVar2# mv f s) of (# s’, x, r #) -> blah<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Then, suppose the old value of the MutVar was ‘<b><span style="font-family:"Courier New"">old’</span></b><o:p></o:p></p>
<ul type="disc">
<li class="m-2251622308724748719m-3683590175518582062msolistparagraph" style="mso-list:l0 level1 lfo1">
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><o:p></o:p></li><li class="m-2251622308724748719m-3683590175518582062msolistparagraph" style="mso-list:l0 level1 lfo1">
The new value of the mutable variable is <b><span style="font-family:"Courier New"">(fst t)</span></b><o:p></o:p></li><li class="m-2251622308724748719m-3683590175518582062msolistparagraph" style="mso-list:l0 level1 lfo1">
The result <b><span style="font-family:"Courier New"">r</span></b> is t<o:p></o:p></li><li class="m-2251622308724748719m-3683590175518582062msolistparagraph" style="mso-list:l0 level1 lfo1">
The result <b><span style="font-family:"Courier New"">x</span></b> is <b><span style="font-family:"Courier New"">old</span></b><o:p></o:p></li></ul>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Question: is that correct? We should state it explicitly.<o:p></o:p></p>
</div>
</div>
</blockquote>
</div>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Yes, that sounds right.<o:p></o:p></p>
</div>
<div>
<div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0cm;margin-bottom:5.0pt">
<div>
<div>
<h1>Question 2<o:p></o:p></h1>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code">atomicModifyMutVar2# :: MutVar# s a<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code"> -> (a -> Box a)<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code"> -> State# s<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code"> -> (# State# s, a, Unit a #)<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">where Unit is defined in Data.Tuple<o:p></o:p></p>
<p class="m-2251622308724748719m-3683590175518582062code"> data Unit a = Unit a<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Now you can force the result of (f old), just as with a pair. But the ‘b’ would no longer complicate matters.
<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Question: is the ‘b’ in the pair significant? Or could we use Unit?<o:p></o:p></p>
</div>
</div>
</blockquote>
</div>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
</div>
<div>
<p><span style="font-family:"Arial",sans-serif">atomicModifyMutVar2# mv $ \a -></span><o:p></o:p></p>
<p><span style="font-family:"Arial",sans-serif"> let foo = expensive_computation a</span><o:p></o:p></p>
<p><span style="font-family:"Arial",sans-serif"> in ([3,foo], foo)</span><o:p></o:p></p>
</div>
<div>
<div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0cm;margin-bottom:5.0pt">
<div>
<div>
<h1>Question 3<o:p></o:p></h1>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">In the comments below you say "but we don't know about pairs here”. Are you sure? What stops you importing Data.Tuple into GHC.Prim? This fancy footwork is one more complication,
if it could be avoided.<o:p></o:p></p>
</div>
</div>
</blockquote>
</div>
</div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">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.<o:p></o:p></p>
</div>
<div>
<div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0cm;margin-bottom:5.0pt">
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
</div>
</div>
</blockquote>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
</div>
</div>
</div>
</div>
</body>
</html>