Support multiple values in Function.apply (#4705)

Author: thewilkybarkid

.changeset/wise-adults-give.md

@@ -0,0 +1,5 @@
+---
+"effect": minor
+---
+
+Support multiple values in Function.apply

packages/effect/dtslint/Function.tst.ts

@@ -1,4 +1,4 @@
-import { flow, identity, Option, pipe } from "effect"
+import { flow, Function, identity, Option, pipe } from "effect"
 import { describe, it } from "tstyche"
 
 describe("Function", () => {
@@ -27,4 +27,27 @@ describe("Function", () => {
         )
     })
   })
+
+  it("apply", () => {
+    const apply1 = Function.apply("a")
+    const apply2 = Function.apply("a", 1)
+
+    const countArgs = (...args: Array<unknown>) => args.length
+    const arg1 = (a: string) => a
+    const arg2 = (a: string, b: number) => `${a}${b}`
+    const arg3 = (a: number) => a
+
+    const _a1: number = apply1(countArgs)
+    const _a2: string = apply1(arg1)
+    // @ts-expect-error: Target signature provides too few arguments. Expected 2 or more, but got 1.
+    apply1(arg2)
+    // @ts-expect-error: Type 'string' is not assignable to type 'number'.
+    apply1(arg3)
+
+    const _b1: number = apply2(countArgs)
+    const _b2: string = apply2(arg1)
+    const _b3: string = apply2(arg2)
+    // @ts-expect-error: Type 'string' is not assignable to type 'number'.
+    apply1(arg3)
+  })
 })

packages/effect/src/Function.ts

@@ -171,7 +171,7 @@ export const dual: {
   }
 }
 /**
- * Apply a function to a given value.
+ * Apply a function to given values.
  *
  * @example
  * ```ts
@@ -184,7 +184,7 @@ export const dual: {
  *
  * @since 2.0.0
  */
-export const apply = <A>(a: A) => <B>(self: (a: A) => B): B => self(a)
+export const apply = <A extends ReadonlyArray<unknown>>(...a: A) => <B>(self: (...a: A) => B): B => self(...a)
 
 /**
  * A lazy argument.

packages/effect/test/Function.test.ts

@@ -7,7 +7,10 @@ const g = (n: number) => n * 2
 
 describe("Function", () => {
   it("apply", () => {
-    deepStrictEqual(Function.pipe(String.length, Function.apply("a")), 1)
+    const countArgs = (...args: Array<unknown>) => args.length
+
+    deepStrictEqual(Function.pipe(countArgs, Function.apply("a")), 1)
+    deepStrictEqual(Function.pipe(countArgs, Function.apply("a", "b", "c")), 3)
   })
 
   it("compose", () => {