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.
 
+# 2.3.0
+
+- **New Feature**
+  - Add support for `enum`
+
 # 2.2.13
 
 - **Bug Fix**

src/index.ts

@@ -827,6 +827,36 @@ export const array = <C extends Mixed>(item: C, name: string = `Array<${item.nam
     item
   )
 
+enum Enum {}
+/**
+ * @since 2.3.0
+ */
+export class EnumType<E extends typeof Enum> extends Type<E[keyof E]> {
+  public readonly _tag: 'EnumType' = 'EnumType'
+  private _enum: E
+  private _enumValues: Set<string | number>
+  public constructor(e: E, name?: string) {
+    super(
+      name || 'enum',
+      (u): u is E[keyof E] => {
+        if (!this._enumValues.has(u as any)) return false
+        // Don't allow key names from number enum reverse mapping
+        if (typeof (this._enum as any)[u as string] === 'number') return false
+        return true
+      },
+      (u, c) => (this.is(u) ? success(u) : failure(u, c)),
+      identity
+    )
+    this._enum = e
+    this._enumValues = new Set(Object.values(e))
+  }
+}
+
+/**
+ * @since 2.3.0
+ */
+const enumType = <E extends typeof Enum>(e: E, name?: string) => new EnumType<E>(e, name)
+
 /**
  * @since 1.0.0
  */
@@ -1871,6 +1901,13 @@ export {
   undefinedType as undefined
 }
 
+export {
+  /**
+   * @since 2.1.0
+   */
+  enumType as enum
+}
+
 export {
   /**
    * Use `UnknownArray` instead

test/enum.ts

@@ -0,0 +1,85 @@
+import * as assert from 'assert'
+import * as t from '../src/index'
+import * as _ from '../src/Decoder'
+import { isLeft } from 'fp-ts/lib/Either'
+
+describe('enum', () => {
+  enum A {
+    Foo = 'foo',
+    Bar = 'bar'
+  }
+
+  enum B {
+    Foo,
+    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 string 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)
+    })
+
+    it('should check an enum integer value', () => {
+      const T = t.enum(B)
+      assert.strictEqual(T.is(B.Foo), true)
+      assert.strictEqual(T.is(1), true)
+      assert.strictEqual(T.is('Foo'), false)
+      assert.strictEqual(T.is('invalid'), false)
+      assert.strictEqual(T.is(null), false)
+      assert.strictEqual(T.is(B), false)
+    })
+  })
+
+  describe('decode', () => {
+    it('should decode an enum string value', () => {
+      const T = t.enum(A)
+      assert.deepStrictEqual(T.decode(A.Foo), _.success(A.Foo))
+      assert.deepStrictEqual(T.decode('bar'), _.success('bar'))
+    })
+
+    it('should decode an enum integer value', () => {
+      const T = t.enum(B)
+      assert.deepStrictEqual(T.decode(B.Foo), _.success(B.Foo))
+      assert.deepStrictEqual(T.decode(1), _.success(1))
+    })
+
+    it('should fail decoding an invalid string value', () => {
+      const T = t.enum(A)
+      assert.deepStrictEqual(isLeft(T.decode('invalid')), true)
+    })
+
+    it('should fail decoding an invalid integer value', () => {
+      const T = t.enum(B)
+      assert.deepStrictEqual(isLeft(T.decode(2)), true)
+    })
+  })
+
+  describe('encode', () => {
+    it('should encode an enum string value', () => {
+      const T = t.enum(A)
+      assert.deepStrictEqual(T.encode(A.Foo), A.Foo)
+    })
+
+    it('should encode an enum integer value', () => {
+      const T = t.enum(B)
+      assert.deepStrictEqual(T.encode(B.Foo), B.Foo)
+    })
+  })
+})