infer combine return value as a tuple

packages/resulto/src/result.ts

@@ -3,6 +3,7 @@ import {
   type ErrFn,
   type ErrPredicate,
   type Fn,
+  Mutable,
   type Predicate,
   ResultError,
   type UnwrapErrs,
@@ -805,10 +806,10 @@ export function fromThrowable<T, E>(
  * will contain an array of `Ok` values, otherwise the returned `Result` will
  * contain the first `Err` error.
  */
-export function combine<T extends Result<unknown, unknown>[]>(
-  results: T
+export function combine<T extends readonly Result<unknown, unknown>[]>(
+  results: [...T]
 ): Result<UnwrapOks<T>, UnwrapErrs<T>[number]> {
-  const unwrapped = [] as UnwrapOks<T>
+  const unwrapped = [] as Mutable<UnwrapOks<T>>
 
   for (const result of results) {
     if (result.isErr()) {
@@ -829,7 +830,10 @@ export function combine<T extends Result<unknown, unknown>[]>(
  * returned `AsyncResult` will contain the first `Err` error.
  */
 export function combineAsync<
-  T extends (Result<unknown, unknown> | AsyncResult<unknown, unknown>)[]
->(results: T): AsyncResult<UnwrapOks<T>, UnwrapErrs<T>[number]> {
+  T extends readonly (
+    | Result<unknown, unknown>
+    | AsyncResult<unknown, unknown>
+  )[]
+>(results: [...T]): AsyncResult<UnwrapOks<T>, UnwrapErrs<T>[number]> {
   return chain(Promise.all(results).then(combine))
 }

packages/resulto/src/utility.ts

@@ -9,7 +9,10 @@ export type Fn<T, U> = (value: T) => U
 export type ErrFn<E, F> = (error: E) => F
 
 export type UnwrapOks<
-  T extends (Result<unknown, unknown> | AsyncResult<unknown, unknown>)[]
+  T extends readonly (
+    | Result<unknown, unknown>
+    | AsyncResult<unknown, unknown>
+  )[]
 > = {
   [i in keyof T]: T[i] extends Result<infer U, unknown>
     ? U
@@ -19,7 +22,10 @@ export type UnwrapOks<
 }
 
 export type UnwrapErrs<
-  T extends (Result<unknown, unknown> | AsyncResult<unknown, unknown>)[]
+  T extends readonly (
+    | Result<unknown, unknown>
+    | AsyncResult<unknown, unknown>
+  )[]
 > = {
   [i in keyof T]: T[i] extends Result<unknown, infer U>
     ? U
@@ -28,6 +34,10 @@ export type UnwrapErrs<
     : never
 }
 
+export type Mutable<T> = {
+  -readonly [P in keyof T]: T[P]
+}
+
 export class ResultError extends Error {
   data: unknown
 

packages/resulto/test/result.test-d.ts

@@ -1,6 +1,7 @@
 import { assertType, describe, test } from "vitest"
 
-import { err, ok } from "../src"
+import { combine, err, ok } from "../src"
+import { combineAsync } from "../src/result"
 
 describe(".value, .error", () => {
   const value = 4
@@ -33,3 +34,19 @@ describe(".value, .error", () => {
     assertType<string>(errResult.error)
   })
 })
+
+describe("combine/combineAsync", () => {
+  test("infers Ok values", () => {
+    {
+      const res1 = ok([4])
+      const res2 = ok("")
+
+      assertType<[number[], string]>(combine([res1, res2]).unwrap())
+      assertType<Promise<[number[], string]>>(
+        combineAsync([res1, res2]).unwrap()
+      )
+    }
+
+    assertType<[number, string]>(combine([ok(4), ok("")]).unwrap())
+  })
+})