<div dir="ltr"><div><div><div><div><div>Hello Simon - I outlined the approach in <a href="https://github.com/google/haskell-indexer/issues/73">https://github.com/google/haskell-indexer/issues/73</a>.<br>TLDR is <br>1) at callsite, indeed the HsWrapper needs to be analysed to get the instance DFunId<br></div>2) at instance declaration site, we need to again take note of the DFunId.<br><br></div>It is slightly more complicated than I expected the API. I would have expected that from having the Var of a class method, and the <something> of the instance (here DFunId), it would be easy to resolve the Var of the instance method.<br><br></div>But it seems there's no direct way, one has to build a lookup table from the instance methods' (DFunId + plain stringy method name), and look that up from the callsite.<br><br></div>Or I might have missed a way to deconstruct / query a DFunId for the method Vars.<br><br></div><div>I'll put augmenting the commentary on my mental TODO list :)<br></div></div><div class="gmail_extra"><br><div class="gmail_quote">2017-10-05 11:58 GMT+02:00 Simon Peyton Jones <span dir="ltr"><<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:12.0pt">Did you get a reply?<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt">I’m not 100% certain of your question, but consider the
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bar = show <expr><u></u><u></u></p>
<p class="MsoNormal"><span style="font-size:12.0pt">for some expression e. In the input to the type type checker the syntax tree for the RHS will be something like<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> HsApp (HsVar “show”) <syntax tree for expr><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">(The “show” isn’t really a string, it’s the Name for the class method.)<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt">After typechecking the syntax tree is augmented (or “elaborated”) with type and dictionary application. So in concrete form it might look like<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> bar = show @Foo dShowFoo <expr><u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt">Because show :: forall a. Show a => a -> String, so show is apply to the type of its argument, and then to the dictionary.
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<p class="MsoNormal"><span style="font-size:12.0pt">In HsSyn this part is done with a HsWrapper See TcEvidence.HsWrapper. The elaborated syntax tree look like<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt"> HsApp (HsWrap <wrapper> (HsVar “show”))<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> <syntax tree for elaborated expr><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">The <wrapper> part expresses the type and dictionary application. In this case it’ll look like<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"> WpEvApp dShowFoo (WpTyApp Foo WpHole)<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">See the notes with `HsWrapper` in TcEvidence.<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt">Does that help?<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt">It would be great to augment the
<a href="https://ghc.haskell.org/trac/ghc/wiki/Commentary" target="_blank">https://ghc.haskell.org/trac/<wbr>ghc/wiki/Commentary</a> with this sort of info (insofar as it doesn’t have it already). If you augment I can review. Email is quickly lost.<u></u><u></u></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt">Simon<u></u><u></u></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs [mailto:<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@<wbr>haskell.org</a>]
<b>On Behalf Of </b>Robin Palotai<br>
<b>Sent:</b> 19 September 2017 06:39<br>
<b>To:</b> GHC developers <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>>; haskell <<a href="mailto:haskell-cafe@haskell.org" target="_blank">haskell-cafe@haskell.org</a>><br>
<b>Subject:</b> Determine instance method from class method callsite<u></u><u></u></span></p>
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Sorry, I messed up subject and mailing list. Copying to both list now after the mistake (wanted only ghc-devs for specificity).<u></u><u></u></p>
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Thanks!<u></u><u></u></p>
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2017-09-19 7:36 GMT+02:00 Robin Palotai <<a href="mailto:palotai.robin@gmail.com" target="_blank">palotai.robin@gmail.com</a>>:<u></u><u></u></p>
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Hello GHC devs,<u></u><u></u></p>
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Before inventing the wheel, want to check if there is a GHC API way to look up the (fully) resolved instance method from a class method.<u></u><u></u></p>
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For example, given a code<u></u><u></u></p>
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data Foo Int deriving Show<u></u><u></u></p>
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bar = show (Foo 3)<u></u><u></u></p>
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when inspecting the Typechecked AST for bar's show call, I would like to get to the Name / Id of 'show' of the 'Show' typeclass.<u></u><u></u></p>
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I believe I could use splitHsSigmaTy on the HsType of the function call to get the context, and then evaluate the HsWrapper somehow to find out what instance dictionary is applied to the class restriction in the context, and then look up the instance method
from the dictionary..<u></u><u></u></p>
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Two questions:<u></u><u></u></p>
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1) Is there maybe functionality for this?<u></u><u></u></p>
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2) If not, is there any guarantee about the constraint order in the context, at the method call? So I could more easily determine which constraint's application to look for..<u></u><u></u></p>
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Any hints welcome && Thank you!<u></u><u></u></p>
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<span style="color:#888888">Robin<u></u><u></u></span></p>
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