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<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">I think you need a ‘Sub’.<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>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">A cast (e `cast` g) requires a representational coercion. I think you have given it a (stronger) nominal one. Sub gets from one to t’other.<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>
<p class="MsoNormal"><span style="font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">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>
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<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""> Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org]
<b>On Behalf Of </b>Conal Elliott<br>
<b>Sent:</b> 14 April 2014 06:00<br>
<b>To:</b> ghc-devs@haskell.org; glasgow-haskell-users@haskell.org<br>
<b>Subject:</b> Help with coercion & roles?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
I’m working on a GHC plugin (as part of my Haskell-to-hardware work) and running into trouble with coercions & roles. Error message from Core Lint:
<o:p></o:p></p>
<pre style="margin-left:30.0pt"><code>Warning: In the expression:<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code><o:p> </o:p></code></pre>
<pre style="margin-left:30.0pt"><code> LambdaCCC.Lambda.lamvP#<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> @ (GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool)<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> @ (Simple.HasIf GHC.Types.Bool)<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> "tpl"#<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> ((LambdaCCC.Lambda.varP#<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> @ (GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool)<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> "tpl"#)<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> `cast` (<LambdaCCC.Lambda.EP>_N (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N))<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> </code><code><span style="font-family:"Cambria Math","serif"">∷</span> LambdaCCC.Lambda.EP<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> (GHC.Types.Bool<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> → GHC.Types.Bool → GHC.Types.Bool → GHC.Types.Bool)<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> ~#<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> LambdaCCC.Lambda.EP (Simple.HasIf GHC.Types.Bool)))<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code> <o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code>Role incompatibility: expected nominal, got representational<o:p></o:p></code></pre>
<pre style="margin-left:30.0pt"><code>in <LambdaCCC.Lambda.EP>_N (Sym (Simple.NTCo:HasIf[0] <GHC.Types.Bool>_N))</code><o:p></o:p></pre>
<p>Do you see anything inconsistent/incompatible in the coercions or roles above? I constructed the nominal
<code><span style="font-size:10.0pt">EP</span></code> <code><span style="font-size:10.0pt">Refl</span></code> coercion, and applied it (<code><span style="font-size:10.0pt">AppCo</span></code>) an existing coercion of a simpler type.<o:p></o:p></p>
<p>Thanks,<o:p></o:p></p>
<p class="MsoNormal">-- Conal <o:p></o:p></p>
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