feat: improve method overload signature

_dev_deps.ts

@@ -3,6 +3,7 @@
 export {
   assert,
   assertEquals,
+  assertFalse,
 } from "https://deno.land/std@0.190.0/testing/asserts.ts";
 export { describe, it } from "https://deno.land/std@0.190.0/testing/bdd.ts";
 export {

emplace.ts

@@ -96,23 +96,23 @@ export function emplace<K, V, M>(
 export function emplace<K, V, M>(
   map: Readonly<MapLike<K, V>> & M,
   key: K,
-  handler: Readonly<{ insert: (key: K, map: M) => V }>,
+  handler: Readonly<Insertable<K, V, M>>,
 ): V;
 export function emplace<K, V, M>(
   map: Readonly<MapLike<K, V>> & M,
   key: K,
-  handler: Readonly<{ update: (existing: V, key: K, map: M) => V }>,
+  handler: Readonly<Updatable<K, V, M>>,
 ): V | undefined;
 export function emplace<K, V, M>(
   map: Readonly<MapLike<K, V>> & M,
   key: K,
-  { update, insert }: Partial<Readonly<EmplaceHandler<K, V, M>>>,
+  handler: Readonly<Insertable<K, V, M> | Updatable<K, V, M>>,
 ): V | undefined {
   if (map.has(key)) {
     const value = map.get(key)!;
 
-    if (update) {
-      const updated = update(value, key, map);
+    if ("update" in handler) {
+      const updated = handler.update(value, key, map);
 
       map.set(key, updated);
 
@@ -122,8 +122,8 @@ export function emplace<K, V, M>(
     }
   }
 
-  if (insert) {
-    const inserted = insert(key, map);
+  if ("insert" in handler) {
+    const inserted = handler.insert(key, map);
 
     map.set(key, inserted);
 

mixin.ts

@@ -13,8 +13,23 @@ export function emplace<K, V, M>(
   this: Readonly<MapLike<K, V>> & M,
   key: K,
   handler: Readonly<EmplaceHandler<K, V, M>>,
-): V {
-  return _emplace(this, key, handler);
+): V;
+export function emplace<K, V, M>(
+  this: Readonly<MapLike<K, V>> & M,
+  key: K,
+  handler: Readonly<Insertable<K, V, M>>,
+): V;
+export function emplace<K, V, M>(
+  this: Readonly<MapLike<K, V>> & M,
+  key: K,
+  handler: Readonly<Updatable<K, V, M>>,
+): V | undefined;
+export function emplace<K, V, M>(
+  this: Readonly<MapLike<K, V>> & M,
+  key: K,
+  handler: Readonly<Insertable<K, V, M> | Updatable<K, V, M>>,
+): V | undefined {
+  return _emplace(this, key, handler as EmplaceHandler<K, V, M>);
 }
 
 /** Mixin for {@link emplace}.

mixin_test.ts

@@ -1,18 +1,24 @@
 // Copyright 2023-latest Tomoki Miyauchi. All rights reserved. MIT license.
 
 import { emplace, Emplaceable } from "./mixin.ts";
-import { assert, assertEquals, describe, it } from "./_dev_deps.ts";
+import {
+  assert,
+  assertEquals,
+  assertFalse,
+  describe,
+  it,
+} from "./_dev_deps.ts";
 
 describe("emplace", () => {
   it("should bind map and do emplace", () => {
     const map = new Map();
 
     assertEquals(
-      emplace.bind(map)("a", { insert: () => "a", update: () => "b" }),
-      "a",
+      emplace.bind(map)("a", { update: () => "b" }),
+      undefined,
     );
 
-    assert(map.has("a"));
+    assertFalse(map.has("a"));
   });
 });