Re: Tracking down instances from use-sites

The selector `foo` is then wrapped in a `HsWrapper` which when desugared will apply the type arguments and dictionary arguments.

Nice! I'll give this a try and report back. Thanks.

Finally, you should considering writing this as a source plugin rather than using the GHC API as it will be easier to run in a variety of different scenarios.

It took me a few minutes to find what you meant. For posterity, I think "frontend plugins" is the name: https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/extending_gh... That sounds like a good idea. This is the first time I've seen this feature of GHC. Cheers! On Tue, 26 Jun 2018 at 17:19, Matthew Pickering wrote:

Chris,

I have also considered this question.

1. Look at the `idDetails` of the `Id`. A class selector is a `ClassOpId`. 2,3,

When a class selector `foo` is typechecked, the instance information is of course resolved. The selector `foo` is then wrapped in a `HsWrapper` which when desugared will apply the type arguments and dictionary arguments. Thus, in order to understand what instance has been selected, we need to look into the `HsWrapper`. In particular, one of the constructors is the `WpEvApp` constructor which is what will apply the dictionary argument. In case 2, this will be a type variable. In case 3, this will be the dictionary variable. I'm not sure how to distinguish these two cases easily. Then once you have the dictionary id, you can use `idType` to get the type of the dictionary which will be something like `Show ()` in order to tell you which instance was selected.

You can inspect the AST of a typechecked program using the `-ddump-tc-ast` flag.

Finally, you should considering writing this as a source plugin rather than using the GHC API as it will be easier to run in a variety of different scenarios.

Cheers,

Matt

On Tue, Jun 26, 2018 at 4:40 PM, Christopher Done wrote:

...

Hi all,

Given a TypecheckedModule, what's the most direct way given a Var expression retrieved from the AST, to determine:

1) that it's a class method e.g. `read` 2) that it's a generic call (no instance chosen) e.g. `Read a => a -> String` 3) or if it's a resolved instance, then which instance is it and which package, module and declaration is that defined in?

Starting with this file that has a TypecheckedModule in it: https://gist.github.com/chrisdone/6fcb9f1cba6324148d481fcd4eab6af6#file-ghc-...

I presume at this point that instance resolution has taken place. I'm not sure that dictionaries or chosen instances are inserted into the AST, or whether just the resolved types are inserted e.g. `Int -> String`, where I want e.g. `Read Int`, which might lead me to finding the matching instance from an InstEnv or so.

I'd like to do some analyses of Haskell codebases, and the fact that calls to class methods are opaque is a bit of a road-blocker. Any handy tips? Prior work?

It'd be neat in tooling to just hit a goto-definition key on `read` and be taken to the instance implementation rather than the class definition.

Also, listing all functions that use throw# or functions defined in terms of throw# or FFI calls would be helpful, especially for doing audits. If I could immediately list all partial functions in a project, then list all call-sites, it would be a very convenient way when doing an audit to see whether partial functions (such as head) are used with the proper preconditions or not.

Any tips appreciated,

Chris _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs