spread-retains-output-type

src/operator.test.ts

@@ -1,4 +1,4 @@
-import { between, modulo, prop } from "./operator.ts";
+import { between, modulo, prop, spread } from "./operator.ts";
 
 import { assertEquals } from "https://deno.land/std@0.174.0/testing/asserts.ts";
 
@@ -21,3 +21,10 @@ const assertString = (x: string) => x;
 prop<{ a: number }>()("a")({ a: 1 }) as number;
 // @ts-expect-error: type does not match
 assertString(prop<{ a: number }>()("a")({ a: 1 }));
+
+spread((x: string, y: number) => x + y)(["a", 1]);
+// @ts-expect-error retains input type
+spread((x: string, y: number) => x + y)(["a", "a"]);
+const _1: string = spread((x: string, y: number) => x + y)(["a", 1]);
+// @ts-expect-error retains output type
+const _2: number = spread((x: string, y: number) => x + y)(["a", 1]);

src/operator.ts

@@ -15,7 +15,9 @@ export const greaterEquals = (x: number) => (y: number) => y >= x;
 export const smallerEquals = (x: number) => (y: number) => y <= x;
 export const between = (start: number, end: number) => (x: number) =>
   start <= x && x < end;
-export const unspread = <Inputs extends unknown[]>(...stuff: Inputs): Inputs =>
+// deno-lint-ignore no-explicit-any
+export const unspread = <Inputs extends any[]>(...stuff: Inputs): Inputs =>
   stuff;
-export const spread = <F extends Func>(f: F) => (x: Parameters<F>) => f(...x);
+export const spread = <F extends Func>(f: F) => (x: Parameters<F>) =>
+  f(...x) as ReturnType<F>;
 export const modulo = (y: number) => (x: number) => x % y;