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<p class="MsoNormal">You can look it up in the class instance environment, which the Simplifier does have access to it. That’s relatively easy when you have a simple dictionary like (Monad IO). But if you want (Eq [Int]) you first of all have to look up the
(Eq [a]) dictionary, then the Eq Int dictionary, and apply the former to the latter. We don’t (yet) have a simple API to do that, although it would not be hard to create one.<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>
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<p class="MsoNormal"><span style="font-size:8.0pt;mso-fareast-language:EN-GB">PS: I am leaving Microsoft at the end of November 2021, at which point
<a href="mailto:simonpj@microsoft.com"><span style="color:#0563C1">simonpj@microsoft.com</span></a> will cease to work. Use
<a href="mailto:simon.peytonjones@gmail.com"><span style="color:#0563C1">simon.peytonjones@gmail.com</span></a> instead. (For now, it just forwards to simonpj@microsoft.com.)<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="mso-fareast-language:EN-GB"> ghc-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Erdi, Gergo via ghc-devs<br>
<b>Sent:</b> 04 October 2021 10:30<br>
<b>To:</b> 'GHC' <ghc-devs@haskell.org><br>
<b>Cc:</b> Montelatici, Raphael Laurent <Raphael.Montelatici@sc.com><br>
<b>Subject:</b> Instantiation of overloaded definition *in Core*<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="msipheader7557e74b" style="margin:0cm"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#317100">PUBLIC</span><o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">I’d like to instantiate Core definitions. For example, suppose I have the following Core definition:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:Consolas">foo :: forall m a b. Monad m => m a -> m b -> m b<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:Consolas">foo = \ @m ($d :: Monad m) @a @b (ma :: m a) (mb :: m b) -> ...<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Now let’s say I’d like to instantiate it for m ~ IO. It is quite straightforward to go from the above to:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:Consolas">foo_IO_0 :: forall a b. Monad IO => IO a -> IO b -> IO b<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:Consolas">foo_IO_0 = \ ($d :: Monad IO) @a @b (ma :: IO a) (mb :: IO b) -> ...<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">However, I would like to go all the way to:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:Consolas">foo_IO :: forall a b. IO a -> IO b -> IO b<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:Consolas">foo_IO = \ @a @b (ma :: IO a) (mb :: IO b) -> ...<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Because instances are coherent, it should be sound to replace all occurrences of
</span><span style="font-size:10.0pt;font-family:Consolas">$d</span><span style="font-size:10.0pt;font-family:"Arial",sans-serif"> with “the” dictionary for
</span><span style="font-size:10.0pt;font-family:Consolas">Monad IO</span><span style="font-size:10.0pt;font-family:"Arial",sans-serif">. However, the places I’ve found for this kind of query seem to live in the typechecker. How do I access this information
while working with Core?<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif"> Gergo<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-GB"><br>
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