add override utility

src/structs/utilities.ts

@@ -1,6 +1,12 @@
 import { Struct, Context, Validator } from '../struct'
 import { object, optional, type } from './types'
-import { ObjectSchema, Assign, ObjectType, PartialObjectSchema } from '../utils'
+import {
+  ObjectSchema,
+  Assign,
+  ObjectType,
+  PartialObjectSchema,
+  OverrideSourceType,
+} from '../utils'
 
 /**
  * Create a new struct that combines the properties properties from multiple
@@ -59,6 +65,20 @@ export function assign(...Structs: Struct<any>[]): any {
   return isType ? type(schema) : object(schema)
 }
 
+/**
+ * Create a new struct based on target struct and overridden by source struct.
+ */
+
+export function override<
+  A extends ObjectSchema,
+  B extends OverrideSourceType<A>
+>(
+  target: Struct<ObjectType<A>, A>,
+  source: B
+): Struct<ObjectType<Assign<A, B>>, Assign<A, B>> {
+  return assign(target, type(source))
+}
+
 /**
  * Define a new struct type with a custom validation function.
  */

src/utils.ts

@@ -291,6 +291,15 @@ export type ObjectType<S extends ObjectSchema> = Simplify<
   Optionalize<{ [K in keyof S]: Infer<S[K]> }>
 >
 
+/**
+ * A schema for source type of override utility.
+ */
+
+export type OverrideSourceType<T extends ObjectSchema> = Partial<
+  Record<keyof T, AnyStruct>
+> &
+  ObjectSchema
+
 /**
  * Omit properties from a type that extend from a specific type.
  */

test/validation/override/invalid-object.ts

@@ -0,0 +1,28 @@
+import { object, override, string, number } from '../../../src'
+
+const Target = object({ a: string() })
+
+export const Struct = override(Target, { a: number(), b: number() })
+
+export const data = {
+  a: 'invalid',
+  b: 2,
+  c: 5,
+}
+
+export const failures = [
+  {
+    value: 'invalid',
+    type: 'number',
+    refinement: undefined,
+    path: ['a'],
+    branch: [data, data.a],
+  },
+  {
+    branch: [data, data.c],
+    path: ['c'],
+    refinement: undefined,
+    type: 'never',
+    value: 5,
+  },
+]

test/validation/override/invalid-type.ts

@@ -0,0 +1,21 @@
+import { type, override, string, number } from '../../../src'
+
+const Target = type({ a: string() })
+
+export const Struct = override(Target, { a: number(), b: number() })
+
+export const data = {
+  a: 'invalid',
+  b: 2,
+  c: 5,
+}
+
+export const failures = [
+  {
+    value: 'invalid',
+    type: 'number',
+    refinement: undefined,
+    path: ['a'],
+    branch: [data, data.a],
+  },
+]

test/validation/override/valid-object.ts

@@ -0,0 +1,15 @@
+import { type, override, string, number } from '../../../src'
+
+const Target = type({ a: string() })
+
+export const Struct = override(Target, { a: number(), b: number() })
+
+export const data = {
+  a: 1,
+  b: 2,
+}
+
+export const output = {
+  a: 1,
+  b: 2,
+}

test/validation/override/valid-type.ts

@@ -0,0 +1,17 @@
+import { type, override, string, number } from '../../../src'
+
+const Target = type({ a: string() })
+
+export const Struct = override(Target, { b: number() })
+
+export const data = {
+  a: '1',
+  b: 2,
+  c: 3,
+}
+
+export const output = {
+  a: '1',
+  b: 2,
+  c: 3,
+}