<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 15 (filtered medium)">
<style><!--
/* Font Definitions */
@font-face
{font-family:Helvetica;
panose-1:2 11 6 4 2 2 2 2 2 4;}
@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: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;
mso-margin-top-alt:auto;
margin-right:0cm;
mso-margin-bottom-alt:auto;
margin-left:0cm;
font-size:12.0pt;
font-family:"Times New Roman","serif";}
p.Code, li.Code, div.Code
{mso-style-name:Code;
margin-top:0cm;
margin-right:0cm;
margin-bottom:0cm;
margin-left:36.0pt;
margin-bottom:.0001pt;
font-size:12.0pt;
font-family:"Courier New";}
span.apple-converted-space
{mso-style-name:apple-converted-space;}
span.EmailStyle20
{mso-style-type:personal-reply;
font-family:"Calibri","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;}
--></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" style="margin-left:36.0pt">I’m not sure when this ”feature” was added, but I’m pretty sure that my original implementation of associated types was exactly what you describe in the solution. Or did I miss anything?<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">I think you are right. I think I added the new stuff in a fit of enthusiasm one day, a fit that I am now regretting! But I’m just checking that
no one has meanwhile become addicted to it.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><br>
Simon <o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><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 #E1E1E1 1.0pt;padding:3.0pt 0cm 0cm 0cm">
<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""> Manuel Chakravarty [mailto:mchakravarty@me.com]
<br>
<b>Sent:</b> 24 June 2014 08:54<br>
<b>To:</b> Simon Peyton Jones<br>
<b>Cc:</b> GHC List; ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Associated type instances<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon,<o:p></o:p></p>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">I’m not sure when this ”feature” was added, but I’m pretty sure that my original implementation of associated types was exactly what you describe in the solution. Or did I miss anything?<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Manuel<o:p></o:p></p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal">Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>>:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">Friends</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">I want to make withdraw (or, rather, simplify) a little-known feature in GHC, but before I do so I want to check that no one is going to have a heart attack.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">Relevant bits of the user manual:<span class="apple-converted-space"> </span><a href="http://www.haskell.org/ghc/docs/latest/html/users_guide/type-families.html#assoc-decl"><span style="color:#954F72">http://www.haskell.org/ghc/docs/latest/html/users_guide/type-families.html#assoc-decl</span></a></span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">All of this arose when thinking about fixing Trac #9063.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">I believe that this change will affect essentially nobody, and I propose to implement forthwith in HEAD (and hence 7.10). </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">Does anyone object?</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">Thanks</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">Simon</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<b><span style="font-size:16.0pt;font-family:"Calibri","sans-serif"">The issue</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">Consider this:</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New"">class C a where<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type T a b :: *<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> <o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New"">instance C [x] where<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type T [x] b = x -> b<o:p></o:p></span></p>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">That is just what you’d expect. But currently this is allowed too:</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New"">instance C [x] where<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type T [x] Int = x -> Int<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type T [x] Bool = Bool -> x<o:p></o:p></span></p>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">That is,<span class="apple-converted-space"> </span><b>GHC 7.8 allows many associated type instances</b>, provided they don’t overlap. But, oddly you can’t further instantiate the instance pattern. This would
make just as much sense, but isn’t allowed:</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New"">instance C [x] where<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type T [Int] b = b -> Int<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type T [Bool] b = Bool -> b<o:p></o:p></span></p>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">Moreover, as the user manual says, for an open kind like *, none of this really makes sense. It really only makes sense for a closed kind. Something like</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New"">class D a where<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type S (b :: Bool) a :: *<o:p></o:p></span></p>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">Now<span class="apple-converted-space"> </span><i>this</i><span class="apple-converted-space"> </span>would make some kind of sense:</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New"">instance D [x] where<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type S True [x] = x -> x<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type S False [x] = x<o:p></o:p></span></p>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">But for closed kinds, you really want a<span class="apple-converted-space"> </span><i>closed</i><span class="apple-converted-space"> </span>type family. So this would be better:</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New"">instance D [x] where<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> type S b [x] = SHelp x b<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> <o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New"">type family SHelp x b where<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> SHelp x True = x -> x<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> SHelp x False = x<o:p></o:p></span></p>
</div>
<div style="margin-left:36.0pt">
<p class="MsoNormal"><span style="font-family:"Courier New""> <o:p></o:p></span></p>
</div>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">So yes, you do have to declare a named helper type, but you get something much more perspicuous and explicit in exchange.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">All of this also applies to the default declaration(s) which you can supply for an associated type (see 7.7.3.2 in the link above), only it’s a bit more complicated and indirect.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<b><span style="font-size:16.0pt;font-family:"Calibri","sans-serif"">My solution</span></b><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif"">I propose to simplify substantially, as follows:</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt;text-indent:-18.0pt">
<span style="font-family:Symbol">·</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-family:"Calibri","sans-serif"">The “shared arguments” of an associated type are the argument positions that mention
a type variable from the class header. So in class C above, the first argument position of T is “shared”; and in class D, the second argument position of S is shared.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt;text-indent:-18.0pt">
<span style="font-family:Symbol">·</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-family:"Calibri","sans-serif"">A instance for an associated type (in a class instance declaration) cf 7.7.3.1
must have</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:72.0pt;text-indent:-18.0pt">
<span style="font-family:"Courier New"">o</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-family:"Calibri","sans-serif"">type variables in the non-shared argument positions, and</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:72.0pt;text-indent:-18.0pt">
<span style="font-family:"Courier New"">o</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-family:"Calibri","sans-serif"">an exact copy of the corresponding instance header type in the shared positions</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt;text-indent:-18.0pt">
<span style="font-family:Symbol">·</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-family:"Calibri","sans-serif"">For each associated type you can have</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:72.0pt;text-indent:-18.0pt">
<span style="font-family:"Courier New"">o</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-family:"Calibri","sans-serif"">at most one default declaration in the class declaration</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoListParagraph" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:72.0pt;text-indent:-18.0pt">
<span style="font-family:"Courier New"">o</span><span style="font-size:7.0pt"> <span class="apple-converted-space"> </span></span><span style="font-family:"Calibri","sans-serif"">at most one type instance declaration in the class instance declaration</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Calibri","sans-serif""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Helvetica","sans-serif"">_______________________________________________<br>
ghc-devs mailing list<br>
<a href="mailto:ghc-devs@haskell.org"><span style="color:#954F72">ghc-devs@haskell.org</span></a><br>
<a href="http://www.haskell.org/mailman/listinfo/ghc-devs"><span style="color:#954F72">http://www.haskell.org/mailman/listinfo/ghc-devs</span></a><o:p></o:p></span></p>
</div>
</blockquote>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
</div>
</div>
</body>
</html>