Determine instance method from class method callsite

Simon Peyton Jones simonpj at
Thu Oct 5 09:58:00 UTC 2017

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 with this sort of info (insofar as it doesn’t have it already).  If you augment I can review.  Email is quickly lost.

From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Robin Palotai
Sent: 19 September 2017 06:39
To: GHC developers <ghc-devs at>; haskell <haskell-cafe at>
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).


2017-09-19 7:36 GMT+02:00 Robin Palotai <palotai.robin at<mailto:palotai.robin at>>:
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!

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