feat(value): add validator for value

validators/value.ts

@@ -0,0 +1,23 @@
+// Copyright 2023-latest Tomoki Miyauchi. All rights reserved. MIT license.
+// This module is browser compatible.
+
+import { format, ScalarValidator } from "../utils.ts";
+import type { Assert } from "../types.ts";
+import error from "./error.json" assert { type: "json" };
+
+export class ValueValidator<Out = unknown> extends ScalarValidator
+  implements Assert<unknown, Out> {
+  declare [Assert.symbol]: Out;
+  constructor(public value: Out) {
+    super();
+    super.expect(({ input }) => format(error.should_be_but, this, input));
+  }
+
+  is(input: unknown): boolean {
+    return input === this.value;
+  }
+
+  toString(): string {
+    return `${this.value}`;
+  }
+}