proposal for data constructor defaults

packages/effect/src/Data.ts

@@ -451,3 +451,31 @@ export const TaggedError = <Tag extends string>(tag: Tag): new<A extends Record<
   ;(Base.prototype as any).name = tag
   return Base as any
 }
+
+/**
+ * @since 2.0.0
+ * @category constructors
+ */
+export const addDefaults: <Args, Defaults extends { [K in keyof Args]?: () => Args[K] }, Out extends object>(
+  newable: new(args: Args) => Out,
+  defaults: Defaults
+) => new(
+  args: Omit<Args, keyof Defaults> & { [K in keyof Args as K extends keyof Defaults ? K : never]?: Args[K] }
+) => Out = (
+  newable,
+  defaults
+) => {
+  return class extends (newable as any) {
+    constructor(args: any) {
+      super({
+        ...args,
+        ...Object.entries(defaults).reduce((acc, [cur, v]) => {
+          if (!(cur in args)) {
+            acc[cur] = (v as any)()
+          }
+          return acc
+        }, {} as Record<string, any>)
+      })
+    }
+  } as any
+}

packages/effect/test/Effect/error.test.ts

@@ -89,4 +89,14 @@ describe("Effect", () => {
     class MessageError extends Data.TaggedError("MessageError")<{}> {}
     assert.deepStrictEqual(new MessageError().toJSON(), { _tag: "MessageError" })
   })
+
+  it.it("withDefaults", () => {
+    class TestError extends Data.addDefaults(
+      Data.TaggedError("TestError")<{ a: string; b: number }>,
+      { b: () => 1 }
+    ) {}
+
+    assert.deepStrictEqual(new TestError({ a: "hello" }).toJSON(), { _tag: "TestError", a: "hello", b: 1 })
+    assert.deepStrictEqual(new TestError({ a: "hello", b: 2 }).toJSON(), { _tag: "TestError", a: "hello", b: 2 })
+  })
 })