Add extend to macro defined schemas

[binaryseed]

This PR introduces an extend macro that behaves as the "extend" concept in the GraphQL spec:

• https://graphql.github.io/graphql-spec/draft/#sec-Object-Extensions

We can extend object, input_object, enum, union, and interface.

The GraphQL spec allows extending schema and scalar but that isn't supported here - there isn't a schema macro to extend and scalars can only be extended to have more directives which also isn't supported in macro schemas

For now this works with macro based schemas, but it should be possible to adapt this to SDL based schemas as well (see #772)

[benwilson512]

Your strategy so far looks sound, is there any particular part you'd like feedback on?

[binaryseed]

At first I wasn't sure this would work but I can see it coming together now, I'll let you know if anything arises!

[binaryseed]

So this works for straight forward things like object, input_object, enum just fine

It's a little tricky with union - I got it to merge types which is cool but it seems pretty complicated to handle stuff like resolve_type... I guess in that situation people could use is_type_of

The spec lets you extend just about everything via SDL but we probably don't have to go quite that far to get a lot of value...

[benwilson512]

@binaryseed I need to review in more detail, but from a high level is the actual extension happening in build_types? This approach won't be able to support extending types from other modules, since the attribute stack => blueprint transform is local to a given module.

My sense of how extend would work is that we'd probably create explicit extension blueprint extension structs that would sit under blueprint.extensions. Then there would be a phase in the schema builder pipeline that would apply extensions to the relevant type. Errors like "doesn't exist" could be placed on the extensions as ordinary errors and would fall into the normal error handling we have for schema validity.

[benwilson512]

@binaryseed In master I added https://github.com/absinthe-graphql/absinthe/blob/master/lib/absinthe/schema/notation.ex#L1508 to deal with the fact that we needed two stacks to handle stack stashing for Absinthe Relay, we should unify these helpers.

[GTDev87]

Any progress on this issue? It would be very useful for a dataloader pattern I'm using in my codebase.

[Leeaandrob]

Any news about the issue?

[benwilson512]

This has since been implemented in a different PR.