[Haskell-cafe] Determine instance method from class method callsite

[Robin Palotai]

Hello Simon - I outlined the approach in
https://github.com/google/haskell-indexer/issues/73.
TLDR is
1) at callsite, indeed the HsWrapper needs to be analysed to get the
instance DFunId
2) at instance declaration site, we need to again take note of the DFunId.

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.

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.

Or I might have missed a way to deconstruct / query a DFunId for the method
Vars.

I'll put augmenting the commentary on my mental TODO list :)

2017-10-05 11:58 GMT+02:00 Simon Peyton Jones <simonpj at microsoft.com>:

> Did you get a reply?
>
>
>
> I’m not 100% certain of your question, but consider the
>
>
>
>     bar = show <expr>
>
> for some expression e.   In the input to the type type checker the syntax
> tree for the RHS will be something like
>
>             HsApp (HsVar “show”) <syntax tree for expr>
>
>
>
> (The “show” isn’t really a string, it’s the Name for the class method.)
>
>
>
> After typechecking the syntax tree is augmented (or “elaborated”) with
> type and dictionary application.  So in concrete form it might look like
>
>             bar = show @Foo dShowFoo <expr>
>
>
>
> Because show :: forall a. Show a => a -> String, so show is apply to the
> type of its argument, and then to the dictionary.
>
>
>
> In HsSyn this part is done with a HsWrapper See TcEvidence.HsWrapper.  The
> elaborated syntax tree look like
>
>
>
>             HsApp (HsWrap <wrapper> (HsVar “show”))
>
>                           <syntax tree for elaborated expr>
>
>
>
> The <wrapper> part expresses the type and dictionary application. In this
> case it’ll look like
>
>             WpEvApp dShowFoo (WpTyApp Foo WpHole)
>
>
>
> See the notes with `HsWrapper` in TcEvidence.
>
>
>
> Does that help?
>
>
>
> It would be great to augment the https://ghc.haskell.org/trac/
> ghc/wiki/Commentary with this sort of info (insofar as it doesn’t have it
> already).  If you augment I can review.  Email is quickly lost.
>
>
>
> Simon
>
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Robin
> Palotai
> *Sent:* 19 September 2017 06:39
> *To:* GHC developers <ghc-devs at haskell.org>; haskell <
> haskell-cafe at haskell.org>
> *Subject:* Determine instance method from class method callsite
>
>
>
> Sorry, I messed up subject and mailing list. Copying to both list now
> after the mistake (wanted only ghc-devs for specificity).
>
>
>
> Thanks!
>
>
>
> 2017-09-19 7:36 GMT+02:00 Robin Palotai <palotai.robin at gmail.com>:
>
> Hello GHC devs,
>
>
>
> 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.
>
>
>
> For example, given a code
>
>
>
>     data Foo Int deriving Show
>
>
>
>     bar = show (Foo 3)
>
>
>
> 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.
>
>
>
> 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..
>
>
>
> Two questions:
>
>
>
> 1) Is there maybe functionality for this?
>
>
>
> 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..
>
>
>
> Any hints welcome && Thank you!
>
> Robin
>
>
>
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