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

[Robin Palotai]

About "Var" - that is just a Name with additional info, but happens in the
typechecked tree (according to API docs).

2017-10-05 18:00 GMT+02:00 Simon Peyton Jones <simonpj at microsoft.com>:

> well the HsWrapper might have something much more complicated than a
> DFunId.  It might need to construct the dictionary for Eq [a] from the DFun
> for Eq [a] and a dictionary for Eq a.
>
>
>
> it would be easy to resolve the Var of the instance method.
>
> I don’t know what “the var of the instance method” is.
>
>
>
> I feel I’m missing the point.  maybe others can help?
>
>
>
> Simon
>
>
>
> *From:* Robin Palotai [mailto:palotai.robin at gmail.com]
> *Sent:* 05 October 2017 11:17
> *To:* Simon Peyton Jones <simonpj at microsoft.com>
> *Cc:* GHC developers <ghc-devs at haskell.org>; haskell <
> haskell-cafe at haskell.org>
> *Subject:* Re: Determine instance method from class method callsite
>
>
>
> Hello Simon - I outlined the approach in https://github.com/google/
> haskell-indexer/issues/73
> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fgoogle%2Fhaskell-indexer%2Fissues%2F73&data=02%7C01%7Csimonpj%40microsoft.com%7C6798607dd45a4865d5ef08d50bda3727%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636427954212256346&sdata=HkxeXUYlbyGJnHu2%2BM17Z5aE7XXAZCWCnWwGLMV1%2BEQ%3D&reserved=0>
> .
> 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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