<div dir="ltr"><div>Will do<br><br></div>Alan<br></div><div class="gmail_extra"><br><div class="gmail_quote">On 13 November 2017 at 19:08, Simon Peyton Jones <span dir="ltr"><<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div link="blue" vlink="purple" lang="EN-GB">
<div class="m_-8694704311956144232WordSection1">
<p class="MsoNormal"><span style="font-size:12.0pt">Alan (adding Shayan and ghc-devs)<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">OK. It’s hard to keep this straight in email. Take a look at<u></u><u></u></span></p>
<p class="MsoNormal" style="text-indent:5.25pt"><span style="font-size:12.0pt"><a href="https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/Instances" target="_blank">https://ghc.haskell.org/trac/<wbr>ghc/wiki/<wbr>ImplementingTreesThatGrow/<wbr>Instances</a><u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Please edit and improve it.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><u></u> <u></u></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">From:</span></b><span lang="EN-US"> Alan & Kim Zimmerman [mailto:<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>]
<br>
<b>Sent:</b> 13 November 2017 13:30</span></p><div><div class="h5"><br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>><br>
<b>Subject:</b> Re: Trees that Grow and constraints<u></u><u></u></div></div><p></p>
</div>
</div><div><div class="h5">
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal" style="margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
At the moment, in GHC master, we have<u></u><u></u></p>
<pre>data HsOverLit p<u></u><u></u></pre>
<pre> = OverLit {<u></u><u></u></pre>
<pre> ol_ext :: (XOverLit p),<u></u><u></u></pre>
<pre> ol_val :: OverLitVal,<u></u><u></u></pre>
<pre> ol_witness :: HsExpr p} -- Note [Overloaded literal witnesses]<u></u><u></u></pre>
<pre><u></u> <u></u></pre>
<pre> | XOverLit<u></u><u></u></pre>
<pre> (XXOverLit p)<u></u><u></u></pre>
<pre>deriving instance (DataIdLR p p) => Data (HsOverLit p)<u></u><u></u></pre>
<pre style="margin-bottom:12.0pt"><br><br>And in HsExtension.hs we have an ever-growing constraint type defining DataIdLR<u></u><u></u></pre>
<pre style="margin-bottom:12.0pt">I am trying to remove the need for that.<u></u><u></u></pre>
<pre style="margin-bottom:12.0pt">In the Experiment.hs file, I found that using<br><br>data Experiment p<br> = Experiment {<br> e_ext :: (XEOverLit p),<br> e_val :: Int }<br> | XExperiment (XXOverLit p)<br>deriving instance Data (GhcPass 'Parsed ) => Data (Experiment (GhcPass 'Parsed ))<br>deriving instance Data (GhcPass 'Renamed ) => Data (Experiment (GhcPass 'Renamed ))<br>deriving instance Data (GhcPass 'Typechecked) => Data (Experiment (GhcPass 'Typechecked))<u></u><u></u></pre>
<pre>will compile using GHC 8.2.1, but not with GHC 8.0.2<u></u><u></u></pre>
<pre><u></u> <u></u></pre>
<pre>Alan<u></u><u></u></pre>
</div>
<div>
<p class="MsoNormal"><u></u> <u></u></p>
<div>
<p class="MsoNormal">On 13 November 2017 at 15:13, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<u></u><u></u></p>
<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="margin-bottom:6.0pt;margin-left:36.0pt">
And it looks like it could work, when bootstrapping from GHC 8.2.1, but this is still a long time away.<u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I’m sorry, I still don’t know what “it” is that “could work”. Can you be more precise. I think you must be suggesting some alternative to my code
below?</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></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"> Alan & Kim Zimmerman [mailto:<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>]
<br>
<b>Sent:</b> 13 November 2017 13:09</span><u></u><u></u></p>
<div>
<div>
<p class="MsoNormal"><br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>><br>
<b>Subject:</b> Re: Trees that Grow and constraints<u></u><u></u></p>
</div>
</div>
</div>
</div>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
<div>
<div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt">Yes.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt">If we can solve this, it means we can get rid of the DataId and ForallXXX constraints defined in hsSyn/HsExtension.hs<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt">And also move the type family definitions to the files where they are used.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"> <u></u><u></u></p>
</div>
<p class="MsoNormal" style="margin-bottom:6.0pt">I suspect that typechecking those constraint sets is adding to the GHC compilation time, significantly<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt">And it looks like it could work, when bootstrapping from GHC 8.2.1, but this is still a long time away.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"> <u></u><u></u></p>
</div>
<p class="MsoNormal" style="margin-bottom:6.0pt">Alan<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"> <u></u><u></u></p>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt">On 13 November 2017 at 15:05, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<u></u><u></u></p>
<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"><span style="font-size:12.0pt">That’s not a problem you are trying to solve – it’s just some code
</span><span style="font-size:12.0pt;font-family:"Segoe UI Emoji",sans-serif">😊</span><span style="font-size:12.0pt">.</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Let me hazard a guess. You want a derived instance for</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> Data (OverLit (GhcPass p))</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">But to do that of course we’ll need Data (XEOverLit (GhcPass p)).</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">We can’t possibly have that at the time of writing the instance declaration, because p is universally quantified. So we are stuck with</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> instance (Data (XEOverLit (GhcPass p))) => Data (OverLit (GhcPass p)) where…</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">and the context gets a constraint for every extension field in OverLit, which is painful.</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">So we have a solution bit contexts, but it’s painful.
</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Is that the problem you are trying to solve?</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></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"> Alan & Kim Zimmerman [mailto:<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>]
<br>
<b>Sent:</b> 13 November 2017 13:01<br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span><u></u><u></u></p>
<div>
<div>
<p class="MsoNormal"><b>Subject:</b> Re: Trees that Grow and constraints<u></u><u></u></p>
</div>
</div>
</div>
</div>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
<div>
<div>
<div>
<div>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt">Sorry, I simplified the problem.<u></u><u></u></p>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">The actual one I am trying to solve will be the general case, so e.g.<br>
<br>
type instance XEOverLit (GhcPass 'Parsed ) = RdrName<br>
type instance XEOverLit (GhcPass 'Renamed ) = Name<br>
type instance XEOverLit (GhcPass 'Typechecked) = Id<u></u><u></u></p>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">or more likely, modelling the existing PostTc and PostRn usages.<u></u><u></u></p>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">And I suppose looking into using them in a single definition might work<u></u><u></u></p>
</div>
<p class="MsoNormal" style="margin-bottom:6.0pt">Alan <u></u>
<u></u></p>
<div>
<div>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"> <u></u><u></u></p>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"> <u></u><u></u></p>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt">On 13 November 2017 at 14:55, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<u></u><u></u></p>
<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="margin-left:36.0pt">
Or do I misunderstand your advice?<u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Well, you said
</span><u></u><u></u></p>
<p class="MsoNormal" style="margin-bottom:6.0pt;margin-left:36.0pt">
Where specifying the type instance with a wild card keeps GHC happy (the XXOverLit case), but specifying for each of the three constructors for pass does not (the XEOverLit case)<u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">My question is: why not just use the wildcard, in that case?</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Do you want to re-state the problem you are trying to solve?</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<div style="border:none;border-left:solid windowtext 1.5pt;padding:0cm 0cm 0cm 4.0pt;border-color:currentcolor currentcolor currentcolor blue">
<div>
<div style="border:none;border-top:solid windowtext 1.0pt;padding:3.0pt 0cm 0cm 0cm;border-color:currentcolor currentcolor">
<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Alan & Kim Zimmerman [mailto:<a href="mailto:alan.zimm@gmail.com" target="_blank">alan.zimm@gmail.com</a>]
<br>
<b>Sent:</b> 13 November 2017 12:49<br>
<b>To:</b> Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>><br>
<b>Cc:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Re: Trees that Grow and constraints</span><u></u><u></u></p>
</div>
</div>
<div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
<div>
<p class="MsoNormal" style="margin-bottom:6.0pt"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<div>
<div>
<blockquote style="border:none;border-left:solid windowtext 1.0pt;padding:0cm 0cm 0cm 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0cm;margin-bottom:5.0pt;border-color:currentcolor currentcolor currentcolor rgb(204,204,204)">
<div>
<div>
<p class="MsoNormal"><span style="font-size:12.0pt">So why not use one?
</span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> </span><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon</span><u></u><u></u></p>
</div>
</div>
</blockquote>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<div>
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt">If I do<br>
<br>
instance (Data p) => Data (Experiment p)<u></u><u></u></p>
</div>
<p class="MsoNormal" style="margin-bottom:12.0pt">then GHC does not know that the type instances for
<br>
<br>
<br>
type instance XEOverLit (GhcPass 'Parsed ) = PlaceHolder<br>
type instance XEOverLit (GhcPass 'Renamed ) = PlaceHolder<br>
type instance XEOverLit (GhcPass 'Typechecked) = PlaceHolder<u></u><u></u></p>
</div>
<p class="MsoNormal">apply.<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">Or do I misunderstand your advice?<u></u><u></u></p>
</div>
<div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
<div>
<p class="MsoNormal">Alan<u></u><u></u></p>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<p class="MsoNormal"> <u></u><u></u></p>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
<p class="MsoNormal"><u></u> <u></u></p>
</div>
</div></div></div>
</div>
</div>
</blockquote></div><br></div>