Add enum support

The `enum` is a popular aspect of TypeScript, and can be embedded in
interfaces as a concise, but descriptive, shorthand for literal string
unions:

```typescript
enum Colour {
  White = '000000',
  Black = 'ffffff'
}
```

This change adds an exported member `enum` to `io-ts`, based on
[this suggestion][1] by @noe132

It means that `enum`s can be reused directly in `io-ts`:

```typescript
const T = t.enum(Colour)
```

[1]: #216 (comment)

CHANGELOG.md

@@ -14,6 +14,11 @@
 **Note**: Gaps between patch versions are faulty/broken releases. **Note**: A feature tagged as Experimental is in a
 high state of flux, you're at risk of it changing without notice.
 
+# 2.1.0
+
+- **New Feature**
+  - Add support for `enum`
+
 # 2.0.1
 
 - **Bug Fix**

README.md

@@ -312,6 +312,7 @@ You can also use the [`withMessage`](https://gcanti.github.io/io-ts-types/module
 | union                       | `A \| B`                    | `t.union([ A, B ])`                                                  |
 | intersection                | `A & B`                     | `t.intersection([ A, B ])`                                           |
 | keyof                       | `keyof M`                   | `t.keyof(M)` (**only supports string keys**)                         |
+| enum                        | `enum A {}`                 | `t.enum(A)`                                                          |
 | recursive types             |                             | `t.recursion(name, definition)`                                      |
 | branded types / refinements | ✘                           | `t.brand(A, predicate, brand)`                                       |
 | integer                     | ✘                           | `t.Int` (built-in branded codec)                                     |

package.json

@@ -1,6 +1,6 @@
 {
   "name": "io-ts",
-  "version": "2.0.1",
+  "version": "2.1.0",
   "description": "TypeScript compatible runtime type system for IO validation",
   "files": [
     "lib",

src/index.ts

@@ -689,6 +689,24 @@ export const array = <C extends Mixed>(codec: C, name: string = `Array<${codec.n
     codec
   )
 
+/**
+ * @since 2.1.0
+ */
+export class EnumType<A> extends Type<A> {
+  public readonly _tag: 'EnumType' = 'EnumType'
+  constructor(name: string, is: EnumType<A>['is'], validate: EnumType<A>['validate'], encode: EnumType<A>['encode']) {
+    super(name, is, validate, encode)
+  }
+}
+
+/**
+ * @since 2.1.0
+ */
+const enumType = <A>(e: any, name: string = 'Enum'): EnumType<A> => {
+  const is = (u: unknown): u is A => Object.keys(e).some(k => e[k] === u)
+  return new EnumType<A>(name, is, (u, c) => (is(u) ? success(u) : failure(u, c)), identity)
+}
+
 /**
  * @since 1.0.0
  */
@@ -1614,6 +1632,13 @@ export {
   undefinedType as undefined
 }
 
+export {
+  /**
+   * @since 2.1.0
+   */
+  enumType as enum
+}
+
 export {
   /**
    * Use `UnknownArray` instead

test/enum.ts

@@ -0,0 +1,53 @@
+import * as assert from 'assert'
+import * as t from '../src/index'
+import { assertSuccess, assertFailure } from './helpers'
+
+describe('enum', () => {
+  enum A {
+    Foo = 'foo',
+    Bar = 'bar'
+  }
+
+  describe('name', () => {
+    it('should assign a default name', () => {
+      const T = t.enum(A)
+      assert.strictEqual(T.name, 'Enum')
+    })
+
+    it('should accept a name', () => {
+      const T = t.enum(A, 'T')
+      assert.strictEqual(T.name, 'T')
+    })
+  })
+
+  describe('is', () => {
+    it('should check an enum value', () => {
+      const T = t.enum(A)
+      assert.strictEqual(T.is(A.Foo), true)
+      assert.strictEqual(T.is('bar'), true)
+      assert.strictEqual(T.is('invalid'), false)
+      assert.strictEqual(T.is(null), false)
+      assert.strictEqual(T.is(A), false)
+    })
+  })
+
+  describe('decode', () => {
+    it('should decode an enum value', () => {
+      const T = t.enum(A)
+      assertSuccess(T.decode(A.Foo), A.Foo)
+      assertSuccess(T.decode('bar'), 'bar')
+    })
+
+    it('should fail decoding an invalid value', () => {
+      const T = t.enum(A)
+      assertFailure(T, 'invalid', ['Invalid value "invalid" supplied to : Enum'])
+    })
+  })
+
+  describe('encode', () => {
+    it('should encode an enum value', () => {
+      const T = t.enum(A)
+      assert.deepStrictEqual(T.encode(A.Foo), A.Foo)
+    })
+  })
+})