feat: add t.opaque

README.md

@@ -42,6 +42,7 @@ The validation errors are detailed. Adapted from the brilliant work in `flow-run
     - [`t.readonlyArray(t.number())`](#treadonlyarraytnumber)
     - [`t.object(properties)`](#tobjectproperties)
     - [`t.object({ required?, optional?, exact? })`](#tobject-required-optional-exact-)
+    - [`t.opaque<DateString>(() => t.string())`](#topaquedatestring--tstring)
     - [`t.readonly(objectType)`](#treadonlyobjecttype)
     - [`t.merge(...objectTypes)`](#tmergeobjecttypes)
     - [`t.mergeInexact(...objectTypes)`](#tmergeinexactobjecttypes)
@@ -386,9 +387,13 @@ PersonType.assert({ name: 1 }) // error
 PersonType.assert({ name: 'dude', age: 'old' }) // error
 ```
 
+### `t.opaque<DateString>(() => t.string())`
+
+A validator that requires the value to be a string, but presents the type as `DateString` (for instance with `export opaque type DateString = string`)
+
 ### `t.readonly(objectType)`
 
-Use `t.readOnly(t.object(...))` or `t.readOnly(t.merge(...))` etc. Doesn't require the object to be frozen, just allows the extracted type to be readonly.
+Use `t.readonly(t.object(...))` or `t.readonly(t.merge(...))` etc. Doesn't require the object to be frozen, just allows the extracted type to be readonly.
 
 ### `t.merge(...objectTypes)`
 

src/index.js.flow

@@ -14,6 +14,7 @@ import NumberType from './types/NumberType'
 import NumericLiteralType from './types/NumericLiteralType'
 import ObjectType from './types/ObjectType'
 import ObjectTypeProperty from './types/ObjectTypeProperty'
+import OpaqueType from './types/OpaqueType'
 import oneOf from './oneOf'
 import PrimitiveLiteralType from './types/PrimitiveLiteralType'
 import RecordType from './types/RecordType'
@@ -47,6 +48,7 @@ export {
   NumericLiteralType,
   ObjectType,
   ObjectTypeProperty,
+  OpaqueType,
   oneOf,
   PrimitiveLiteralType,
   RecordType,
@@ -68,6 +70,7 @@ export {
 
 declare export function any(): Type<any>
 declare export function unknown(): Type<mixed>
+declare export function opaque<T>(type: () => Type<any>): OpaqueType<T>
 
 declare export function array<T>(elementType: Type<T>): Type<T[]>
 declare export function readonlyArray<T>(

src/index.ts

@@ -12,6 +12,7 @@ import NumberType from './types/NumberType'
 import NumericLiteralType from './types/NumericLiteralType'
 import ObjectType from './types/ObjectType'
 import ObjectTypeProperty from './types/ObjectTypeProperty'
+import OpaqueType from './types/OpaqueType'
 import oneOf from './oneOf'
 import PrimitiveLiteralType from './types/PrimitiveLiteralType'
 import RecordType from './types/RecordType'
@@ -45,6 +46,7 @@ export {
   NumericLiteralType,
   ObjectType,
   ObjectTypeProperty,
+  OpaqueType,
   oneOf,
   PrimitiveLiteralType,
   RecordType,
@@ -66,6 +68,8 @@ export {
 
 export const any = (): Type<any> => new AnyType()
 export const unknown = (): Type<unknown> => new UnknownType()
+export const opaque = <T>(type: () => Type<any>): OpaqueType<T> =>
+  new OpaqueType<T>(type)
 
 export const array = <T>(elementType: Type<T>): Type<T[]> =>
   new ArrayType(elementType)

src/types/OpaqueType.js.flow

@@ -0,0 +1,11 @@
+// @flow
+
+import Type from './Type'
+
+declare class OpaqueType<T> extends Type<T> {
+  type: () => Type<any>;
+
+  constructor(type: () => Type<any>): void;
+}
+
+export default OpaqueType

src/types/OpaqueType.ts

@@ -0,0 +1,37 @@
+import Type from './Type'
+import Validation, { IdentifierPath } from '../Validation'
+import RuntimeTypeErrorItem from '../errorReporting/RuntimeTypeErrorItem'
+
+export default class TypeReference<T> extends Type<T> {
+  typeName = 'TypeReference'
+  readonly type: () => Type<any>
+
+  constructor(type: () => Type<any>) {
+    super()
+    this.type = type
+  }
+
+  resolveType(): Type<any> {
+    return this.type().resolveType()
+  }
+
+  *errors(
+    validation: Validation,
+    path: IdentifierPath,
+    input: any
+  ): Iterable<RuntimeTypeErrorItem> {
+    yield* this.type().errors(validation, path, input)
+  }
+
+  accepts(input: any): input is T {
+    return this.type().accepts(input)
+  }
+
+  get acceptsSomeCompositeTypes(): boolean {
+    return this.type().acceptsSomeCompositeTypes
+  }
+
+  toString(): string {
+    return this.type().toString()
+  }
+}

test/opaque.ts

@@ -0,0 +1,11 @@
+import * as t from '../src'
+import { expect } from 'chai'
+
+describe(`t.opaque`, function() {
+  const DateString: t.OpaqueType<any> = t.opaque(() => t.string())
+
+  it(`works`, function() {
+    DateString.assert('foo')
+    expect(DateString.accepts('foo')).to.be.true
+  })
+})