fix: update variable type variance to bivariant

[sunnhyn]

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Summary

I've updated the implementation of the __apiType method to shorthand method definition. This modification enables bivariance in the variable type.

Details

Let's consider a function that takes a specific TypedDocumentNode with certain requirements. Assume this function, queryAnimal, receives a Graphql query with AnimalQueryResult as the return type and AnimalQueryVariable as the argument type.

Suppose dogDocumentNode has a narrower scope than AnimalQueryResult for the Return Type ({ name: string; id: string; age: number }) and a narrower scope than AnimalQueryVariable for the Variable Type ({ id: string }). As a result, it should be usable as an argument for queryAnimal. However, due to the Variable Type of TypedDocumentNode being a function argument, TypeScript conducts type checking contravariantly for the VariableType, making it ineligible as an argument.

To create a bivariant function argument in TypeScript, the method needs to be defined as a Shorthand Definition.

type ResultOf<T> = T extends TypedDocumentNode<infer R, any> ? R : never;
type VariablesOf<T> = T extends TypedDocumentNode<any, infer V> ? V : never;

// type TypedDocumentNode<R, V> = { __apiType?: (variables: V) => R }; // AS-IS
type TypedDocumentNode<R, V> = { __apiType?(variables: V): R }; // TO-BE

type AnimalQueryResult = Record<
  string,
  { name: string; id: string; age: number; [key: string]: any }
>;
type AnimalQueryVariable = Record<string, any>;

function queryAnimal<
  DN extends TypedDocumentNode<R, V>,
  R extends AnimalQueryResult,
  V extends AnimalQueryVariable
>(query: DN, variable: VariablesOf<DN>): ResultOf<DN> {
  return "" as any;
}

const dogDocumentNode = "" as TypedDocumentNode<
  { dogQuery: { name: string; id: string; age: number } },
  { id: string }
>;
const dog = queryAnimal(dogDocumentNode, { id: "5172" });

const catDocumentNode = "" as TypedDocumentNode<
  { catQuery: { name: string; id: string } },
  { id: string }
>;
const cat = queryAnimal(catDocumentNode, { id: "5713" });

References

TypeScript FAQ: Why are function parameters bivariant? Typescript Repo: Strict function types #18654

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Latest commit: ab9e8c3eaa0b7b1ef258c50563877662ad184445

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