<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=us-ascii">
<meta name="Generator" content="Microsoft Word 12 (filtered medium)">
<base href="x-msg://8/"><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;}
@font-face
{font-family:Tahoma;
panose-1:2 11 6 4 3 5 4 4 2 4;}
@font-face
{font-family:Verdana;
panose-1:2 11 6 4 3 5 4 4 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0cm;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Times New Roman","serif";}
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:12.0pt;
font-family:"Times New Roman","serif";}
span.apple-style-span
{mso-style-name:apple-style-span;}
span.EmailStyle18
{mso-style-type:personal-reply;
font-family:"Verdana","sans-serif";
color:#1F497D;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;}
@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:824932507;
mso-list-type:hybrid;
mso-list-template-ids:1664662352 134807553 134807555 134807557 134807553 134807555 134807557 134807553 134807555 134807557;}
@list l0:level1
{mso-level-number-format:bullet;
mso-level-text:\F0B7;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Symbol;}
@list l1
{mso-list-id:1756514882;
mso-list-type:hybrid;
mso-list-template-ids:378146796 134807553 134807555 134807557 134807553 134807555 134807557 134807553 134807555 134807557;}
@list l1:level1
{mso-level-number-format:bullet;
mso-level-text:\F0B7;
mso-level-tab-stop:none;
mso-level-number-position:left;
text-indent:-18.0pt;
font-family:Symbol;}
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" style="word-wrap: break-word;-webkit-nbsp-mode: space;-webkit-line-break: after-white-space">
<div class="WordSection1">
<p class="MsoNormal" style="margin-left:36.0pt">It seems to me that there's only one essential missing language feature, which is appropriately-kinded type-level strings (and, ideally, the ability to reflect these strings back down to the value level). Given
that, template haskell, and the HList bag of tricks, I'm confident that a fair number of elegant records packages can be crafted. Based on that experience, we can then decide what syntactic sugar would be useful to elide the TH layer altogether.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">I think we can do this part without much trouble, once the dust has settled on -XPolyKinds. It certainly fits with all the work we’ve been doing recently on
the kind system. I agree that it’s a fairly basic requirement; for example, it’s also assumed by http://hackage.haskell.org/trac/ghc/wiki/Records/OverloadedRecordFields<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Specifically<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;color:#1F497D"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Allow String as a new kind<br>
<br>
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l1 level1 lfo1"><![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;color:#1F497D"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Now you can define classes or types with kinds like<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:72.0pt"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">MyCls :: String -> a -> Constraint<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:72.0pt"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">T :: String -> *<br>
<br>
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;color:#1F497D"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Provide type-level string literals, so that “foo” :: String<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Open questions:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;color:#1F497D"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Is String (at the kind level) a synonym for [Char]? I’m inclined *<b>not</b>* to do this initially, because it would require us to have promoted
character literals too -- and the implementation of record labels as strings of type-level cons-cells is not going to be efficient.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;color:#1F497D"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">If String is not a kind level synonym for [Char], maybe it should have a different name. For example, “foo” :: Label? Or Atom? After all, if
it doesn’t behave like a Haskell string it probably should not have the same name.<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;color:#1F497D"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Are there any operations over Labels?
<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="font-size:11.0pt;font-family:Symbol;color:#1F497D"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">I don’t know exactly what you have in mean by “the ability to reflect the type-level string at the value level”.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Verdana","sans-serif";color:#1F497D"><o:p> </o:p></span></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 #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Gershom Bazerman [mailto:gershomb@gmail.com]
<br>
<b>Sent:</b> 31 December 2011 19:12<br>
<b>To:</b> Simon Peyton-Jones<br>
<b>Cc:</b> Greg Weber; glasgow-haskell-users@haskell.org<br>
<b>Subject:</b> Re: Records in Haskell<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal">On Dec 31, 2011, at 1:28 PM, Simon Peyton-Jones wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<div style="border:none;border-left:solid blue 1.5pt;padding:0cm 0cm 0cm 4.0pt;border-width:initial;border-color:initial;z-index:auto">
<div>
<div>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt">The trouble is that I just don't have the bandwidth (or, if I'm honest, the motivation) to drive this through to a conclusion. And if no one else does either, perhaps it isn't *that* important to anyone. That
said, it clearly is *somewhat* important to a lot of people, so doing nothing isn't very satisfactory either.<br>
<br>
Usually I feel I know how to move forward, but here I don't.<br>
<span style="color:#888888"><br>
Simon</span><o:p></o:p></p>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<div>
<p class="MsoNormal">It seems to me that there's only one essential missing language feature, which is appropriately-kinded type-level strings (and, ideally, the ability to reflect these strings back down to the value level). Given that, template haskell, and
the HList bag of tricks, I'm confident that a fair number of elegant records packages can be crafted. Based on that experience, we can then decide what syntactic sugar would be useful to elide the TH layer altogether.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Beyond that, it would really help namespacing in general to appropriately extend the module system to allow multiple modules to be declared within a single file -- or, better yet, "submodules". I know that this introduces a few corner cases
that need to be thought through -- what happens with overlapping declarations, for example. But I tend to think the path here is relatively straightforward and obvious, and the added expressive power should make namespacing issues much more tractable. Like
the type-level strings proposal, this isn't about implementing records as such -- rather, it's about generally extending the expressive power of the language so that record systems--among other things--are easier to write.<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Cheers,<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal">Gershom<o:p></o:p></p>
</div>
</div>
</div>
</body>
</html>