feat: add omittable type

package.json

@@ -6,7 +6,7 @@
     "url": "https://github.com/MonoidDev/reform"
   },
   "private": false,
-  "version": "1.0.3",
+  "version": "1.1.0",
   "license": "MIT",
   "publishConfig": {
     "access": "public"

src/index.ts

@@ -17,3 +17,4 @@ export * from './react/factories';
 export * from './fields/stringField';
 export * from './fields/numberField';
 export * from './fields/booleanField';
+export * from './types/UtilityTypes';

src/types/LiteralType.ts

@@ -2,7 +2,7 @@ import { makeLeft, makeRight } from './Either';
 import { ErrorMessage } from './ErrorMessage';
 import { makeResolver, Resolver } from './Resolver';
 
-export type LiteralValue = string | number | boolean;
+export type LiteralValue = string | number | boolean | null | undefined;
 
 export interface TLiteral<T extends LiteralValue>
   extends Resolver<unknown, T, ErrorMessage, 'literal'> {

src/types/UtilityTypes.ts

@@ -0,0 +1,5 @@
+import { AnyResolver, literal } from '..';
+import { union } from './CombinationTypes';
+
+export const omittable = <R extends AnyResolver>(r: R) =>
+  union([r, literal(undefined)]);

test/utilityTypes.test.ts

@@ -0,0 +1,26 @@
+import { omittable } from '../src';
+import {
+  any,
+  array,
+  boolean,
+  nullType,
+  number,
+  object,
+  string,
+  undefinedType,
+} from '../src/types/BasicTypes';
+import { InputOf, OutputOf } from '../src/types/infer';
+import { literal } from '../src/types/LiteralType';
+import { struct } from '../src/types/StructType';
+import { expectLeft, expectRight } from './utils';
+
+describe('utility types', () => {
+  it('omittable', () => {
+    expectRight(omittable(string()).resolve('x'));
+    expectRight(omittable(string()).resolve(undefined));
+    expectLeft(omittable(string()).resolve(1));
+
+    expectRight(omittable(number()).resolve(1));
+    expectRight(omittable(number()).resolve(undefined));
+  });
+});