Redux Sagas, TypeScript, and call?

Question

As a TypeScript and redux-thunk user, I am curious about the benefits offered by redux-saga. I'd like to give it a shot but am concerned about the call function and the apparent loss of type safety.

If I do this:

function* invalidateReddit(): SagaIterator {
  while (true) {
    const {reddit} = yield take(actions.INVALIDATE_REDDIT)
    yield call( fetchPosts, reddit )
  }

The compiler will not be able to check calls to fetchPosts. So if I changed the signature to not include the argument...

function fetchPosts() {
  // anything here...
}

The invalidateReddit function, which depends on fetchPosts, should fail to compile but it will not because call evaluates my code for me. Is there an established pattern for using this without sacrificing type safety?

UPDATE: The PR at https://github.com/redux-saga/redux-saga/pull/740 looks like it attempts to solve this problem. I will leave this open until it can be closed with a solution.

Falda

Seems like PR mentioned in UPDATE is not working. I've tried latest code on branch and it's any type. It's probably issue with TypeScript itself, but doesn't look like they are working on enabling custom types on yield statements. – Gurgle

Answer 1

Because TypeScript won't complain about excess variables you passed to a function.

As the type declaration per se, https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/redux-saga/index.d.ts#L75

export function call<T1, T2, T3>(fn: EffectFunction<T1, T2, T3>, arg1?: T1, arg2?: T2, arg3?: T3, ...rest: any[]): Effect;

If you pass a function with arity less than 3 to call, all the type parameter will be inferred to {}, the top type of TS. So basically every thing will be assignable in such call call(zeroArityFunc, anything).

At runtime, excess argument won't cause error, so your code should be fine. If fetchPosts does require argument, then the type parameter for it will be inferred and a compiler error will be raised.

Cobbett

I am not sure I understand. I cannot trigger a compiler error for any code that uses call at the moment, regardless of arity. I just noticed an open PR, github.com/redux-saga/redux-saga/pull/740, that might be intended to solve the problem I am describing. – Falda