WIP: infer call return type based on args

types/index.d.ts

@@ -6,7 +6,11 @@
 
 // TODO remove this eslint-disable
 // eslint-disable-next-line import/no-unresolved
-import { ActionMatchingPattern, Buffer } from "@redux-saga/types";
+import {
+  ActionMatchingPattern,
+  Buffer,
+  SagaIterator,
+} from "@redux-saga/types";
 import { Action } from "redux";
 import {
   TakeableChannel,
@@ -45,6 +49,14 @@ import {
   CallEffectDescriptor,
 } from "redux-saga/effects";
 
+export type ExtractReturnValue<ReturnType> = ReturnType extends SagaIterator<
+  infer SagaReturnType
+>
+  ? SagaReturnType
+  : ReturnType extends Promise<infer PromiseReturnType>
+  ? PromiseReturnType
+  : ReturnType;
+
 export type SagaGenerator<RT, E extends Effect = Effect<any, any>> = Generator<
   E,
   RT
@@ -184,22 +196,28 @@ export function putResolve<A extends Action>(
   action: A,
 ): SagaGenerator<A, PutEffect<A>>;
 
-export function call<Args extends any[], Fn extends (...args: Args) => any>(
-  fn: Fn,
+export function call<Args extends unknown[], Return>(
+  fn: (...args: Args) => Return,
   ...args: Args
-): SagaGenerator<SagaReturnType<Fn>, CallEffect<SagaReturnType<Fn>>>;
+): SagaGenerator<
+  ExtractReturnValue<Return>,
+  CallEffect<ExtractReturnValue<Return>>
+>;
+
 export function call<
-  Args extends any[],
+  FnName extends keyof Ctx,
+  Args extends unknown[],
+  Return,
+  Fn extends (this: Ctx, ...args: Args) => Return,
   Ctx extends {
-    [P in Name]: (this: Ctx, ...args: Args) => any;
+    [Key in FnName]: Fn;
   },
-  Name extends string,
 >(
-  ctxAndFnName: [Ctx, Name],
+  ctxAndFnName: [Ctx, FnName],
   ...args: Args
 ): SagaGenerator<
-  SagaReturnType<Ctx[Name]>,
-  CallEffect<SagaReturnType<Ctx[Name]>>
+  ExtractReturnValue<Return>,
+  CallEffect<ExtractReturnValue<Return>>
 >;
 export function call<
   Args extends any[],

types/index.test.ts

@@ -251,4 +251,12 @@ function* mySaga(): Effects.SagaGenerator<void> {
   yield* Effects.spawn([obj, "outer"], emitter, handler);
   yield* Effects.spawn({ context: obj, fn: obj.outer }, emitter, handler);
   yield* Effects.spawn({ context: obj, fn: "outer" }, emitter, handler);
+
+  const identity = <T>(x: T): T => x;
+  // $ExpectType "foo"
+  yield* Effects.call(identity, "foo" as const);
+
+  const ctx = { identity } as const;
+  // $ExpectType "foo"
+  yield* Effects.call([ctx, "identity"], "foo" as const);
 }