Add structural type

[ForbesLindesay]

TypeScript is structurally typed, so the current implementation is pretty limited in what it will enforce. e.g. https://github.com/dotansimha/graphql-typed-document-node/blob/master/examples/apollo-client-3/index.ts does not currently enforce the type of the variables.

By adding this optional property, even though no code will ever implement this method, it forces TypeScript to actually verify the types.

[dotansimha]

You are right, thanks @ForbesLindesay !

Maybe it can be something like that instead?

export interface TypedDocumentNode<Result = { [key: string]: any }, Variables = { [key: string]: any }> extends DocumentNode {
  __resultType?: Result;
  __variablesType?: Variables;
}

It will make it a bit more clear, what do you think?

[dotansimha]

Thanks @ForbesLindesay , I updated it in master:
https://github.com/dotansimha/graphql-typed-document-node/blob/master/packages/core/src/index.ts#L3-L6

[ForbesLindesay]

Sorry I didn't respond to this in time. Unfortunately your code, although arguably more readable, does not enforce the correct types. I have opened #38 with a fix and a more detailed explanation.