feat: add Tagged for composable tagged types

index.d.ts

@@ -26,7 +26,7 @@ export type {PartialOnUndefinedDeep, PartialOnUndefinedDeepOptions} from './sour
 export type {ReadonlyDeep} from './source/readonly-deep';
 export type {LiteralUnion} from './source/literal-union';
 export type {Promisable} from './source/promisable';
-export type {Opaque, UnwrapOpaque} from './source/opaque';
+export type {Opaque, UnwrapOpaque, Tagged, UnwrapTagged} from './source/opaque';
 export type {InvariantOf} from './source/invariant-of';
 export type {SetOptional} from './source/set-optional';
 export type {SetReadonly} from './source/set-readonly';

source/exact.d.ts

@@ -1,5 +1,5 @@
 import type {KeysOfUnion, ArrayElement, ObjectValue} from './internal';
-import type {Opaque} from './opaque';
+import type {Opaque, TagContainer} from './opaque';
 import type {IsEqual} from './is-equal';
 
 /**
@@ -56,7 +56,7 @@ export type Exact<ParameterType, InputType> =
 		: ParameterType extends unknown[] ? Array<Exact<ArrayElement<ParameterType>, ArrayElement<InputType>>>
 			// In TypeScript, Array is a subtype of ReadonlyArray, so always test Array before ReadonlyArray.
 			: ParameterType extends readonly unknown[] ? ReadonlyArray<Exact<ArrayElement<ParameterType>, ArrayElement<InputType>>>
-				// For Opaque types, internal details are hidden from public, so let's leave it as is.
-				: ParameterType extends Opaque<infer OpaqueType, infer OpaqueToken> ? ParameterType
+				// Leave tagged types as-is. We could try to make the untagged part Exact, and just leave the tag as-is, but that seems to create instanitation excessively deep errors.
+				: ParameterType extends TagContainer<unknown> ? ParameterType
 					: ParameterType extends object ? ExactObject<ParameterType, InputType>
 						: ParameterType;

source/opaque.d.ts

@@ -1,17 +1,25 @@
 declare const tag: unique symbol;
 
-declare type Tagged<Token> = {
+declare type TagContainer<Token> = {
 	readonly [tag]: Token;
 };
 
+type MultiTagContainer<Token extends PropertyKey> = {
+	readonly [tag]: {[K in Token]: void};
+};
+
 /**
-Create an opaque type, which hides its internal details from the public, and can only be created by being used explicitly.
+Attach a "tag" to an arbitrary type. This allows you to create distinct types, that aren't assignable to one another, for runtime values that would otherwise have the same type. (See examples.)
+
+The generic type parameters can be anything.
+
+Note that `Opaque` is somewhat of a misnomer here, in that, unlike [some alternative implementations](https://github.com/microsoft/TypeScript/issues/4895#issuecomment-425132582), the original, untagged type is not actually hidden. (E.g., functions that accept the untagged type can still be called with the "opaque" version -- but not vice-versa.)
 
-The generic type parameter can be anything. It doesn't have to be an object.
+Also note that this implementation is limited to a single tag. If you want to allow multiple tags, use `Tagged` instead.
 
-[Read more about opaque types.](https://codemix.com/opaque-types-in-javascript/)
+[Read more about tagged types.](https://medium.com/@KevinBGreene/surviving-the-typescript-ecosystem-branding-and-type-tagging-6cf6e516523d)
 
-There have been several discussions about adding this feature to TypeScript via the `opaque type` operator, similar to how Flow does it. Unfortunately, nothing has (yet) moved forward:
+There have been several discussions about adding similar features to TypeScript. Unfortunately, nothing has (yet) moved forward:
 	- [Microsoft/TypeScript#202](https://github.com/microsoft/TypeScript/issues/202)
 	- [Microsoft/TypeScript#15408](https://github.com/Microsoft/TypeScript/issues/15408)
 	- [Microsoft/TypeScript#15807](https://github.com/Microsoft/TypeScript/issues/15807)
@@ -59,7 +67,7 @@ getMoneyForAccount(2);
 // You can use opaque values like they aren't opaque too.
 const accountNumber = createAccountNumber();
 
-// This will not compile successfully.
+// This will compile successfully.
 const newAccountNumber = accountNumber + 2;
 
 // As a side note, you can (and should) use recursive types for your opaque types to make them stronger and hopefully easier to type.
@@ -71,10 +79,10 @@ type Person = {
 
 @category Type
 */
-export type Opaque<Type, Token = unknown> = Type & Tagged<Token>;
+export type Opaque<Type, Token = unknown> = Type & TagContainer<Token>;
 
 /**
-Revert an opaque type back to its original type by removing the readonly `[tag]`.
+Revert an opaque or tagged type back to its original type by removing the readonly `[tag]`.
 
 Why is this necessary?
 
@@ -97,11 +105,101 @@ const money = moneyByAccountType.SAVINGS; // TS error: Property 'SAVINGS' does n
 
 // Attempting to pass an non-Opaque type to UnwrapOpaque will raise a type error.
 type WontWork = UnwrapOpaque<string>;
+
+// Using a Tagged type will work too.
+type WillWork = UnwrapOpaque<Tagged<number, 'AccountNumber'>>; // number
+```
+
+@category Type
+*/
+export type UnwrapOpaque<OpaqueType extends TagContainer<unknown>> =
+	OpaqueType extends MultiTagContainer<string | number | symbol>
+		? RemoveAllTags<OpaqueType>
+		: OpaqueType extends Opaque<infer Type, OpaqueType[typeof tag]>
+			? Type
+			: OpaqueType;
+
+/**
+Attach a "tag" to an arbitrary type. This allows you to create distinct types, that aren't assignable to one another, for runtime values that would otherwise have the same type. (See examples.)
+
+A type returned by `Tagged` can be passed to `Tagged` again, to create a type with multiple tags.
+
+[Read more about tagged types.](https://medium.com/@KevinBGreene/surviving-the-typescript-ecosystem-branding-and-type-tagging-6cf6e516523d)
+
+There have been several discussions about adding similar features to TypeScript. Unfortunately, nothing has (yet) moved forward:
+  - [Microsoft/TypeScript#202](https://github.com/microsoft/TypeScript/issues/202)
+  - [Microsoft/TypeScript#4895](https://github.com/microsoft/TypeScript/issues/4895)
+  - [Microsoft/TypeScript#33290](https://github.com/microsoft/TypeScript/pull/33290)
+
+@example
+```
+import type {Tagged} from 'type-fest';
+
+type AccountNumber = Tagged<number, 'AccountNumber'>;
+type AccountBalance = Tagged<number, 'AccountBalance'>;
+
+function createAccountNumber(): AccountNumber {
+	// As you can see, casting from a `number` (the underlying type being tagged) is allowed.
+  return 2 as AccountNumber;
+}
+
+function getMoneyForAccount(accountNumber: AccountNumber): AccountBalance {
+  return 4 as AccountBalance;
+}
+
+// This will compile successfully.
+getMoneyForAccount(createAccountNumber());
+
+// But this won't, because it has to be explicitly passed as an `AccountNumber` type!
+getMoneyForAccount(2);
+
+// You can use opaque values like they aren't opaque too.
+const accountNumber = createAccountNumber();
+
+// This will compile successfully.
+const newAccountNumber = accountNumber + 2;
+```
+
+@category Type
+*/
+export type Tagged<Type, Tag extends PropertyKey> = Type & MultiTagContainer<Tag>;
+
+/**
+Revert a tagged type back to its original type by removing the readonly `[tag]`.
+
+Why is this necessary?
+
+1. Use a `Tagged` type as object keys
+2. Prevent TS4058 error: "Return type of exported function has or is using name X from external module Y but cannot be named"
+
+@example
+```
+import type {Tagged, UnwrapTagged} from 'type-fest';
+
+type AccountType = Tagged<'SAVINGS' | 'CHECKING', 'AccountType'>;
+
+const moneyByAccountType: Record<UnwrapTagged<AccountType>, number> = {
+	SAVINGS: 99,
+	CHECKING: 0.1
+};
+
+// Without UnwrapTagged, the following expression would throw a type error.
+const money = moneyByAccountType.SAVINGS; // TS error: Property 'SAVINGS' does not exist
+
+// Attempting to pass an non-Tagged type to UnwrapTagged will raise a type error.
+type WontWork = UnwrapTagged<string>;
 ```
 
 @category Type
 */
-export type UnwrapOpaque<OpaqueType extends Tagged<unknown>> =
-	OpaqueType extends Opaque<infer Type, OpaqueType[typeof tag]>
-		? Type
-		: OpaqueType;
+export type UnwrapTagged<TaggedType extends MultiTagContainer<PropertyKey>> =
+RemoveAllTags<TaggedType>;
+
+type RemoveAllTags<T> = T extends MultiTagContainer<infer ExistingTags>
+	? {
+		[ThisTag in ExistingTags]:
+		T extends Tagged<infer Type, ThisTag>
+			? RemoveAllTags<Type>
+			: never
+	}[ExistingTags]
+	: T;

test-d/exact.ts

@@ -1,3 +1,4 @@
+import {expectError} from 'tsd';
 import type {Exact, Opaque} from '../index';
 
 { // Spec - string type
@@ -353,7 +354,7 @@ import type {Exact, Opaque} from '../index';
 	}
 }
 
-// Spec - special test case for Opaque type
+// Spec - special test case for Opaque types
 // @see https://github.com/sindresorhus/type-fest/issues/508
 {
 	type SpecialName = Opaque<string, 'special name'>;
@@ -390,6 +391,16 @@ import type {Exact, Opaque} from '../index';
 	});
 }
 
+// Spec - test the above for tagged types too.
+{
+	type TaggedNumber = Opaque<number, 'tag'>;
+
+	const fn = <T extends Exact<{a: TaggedNumber}, T>>(arguments_: T) => arguments_;
+
+	fn({a: 1 as TaggedNumber});
+	expectError(fn({a: 1 as TaggedNumber, b: true}));
+}
+
 // Spec - special test case for deep optional union
 // https://github.com/sindresorhus/type-fest/issues/545
 {

test-d/opaque.ts

@@ -1,5 +1,5 @@
 import {expectAssignable, expectNotAssignable, expectNotType, expectType} from 'tsd';
-import type {Opaque, UnwrapOpaque} from '../index';
+import type {Opaque, UnwrapOpaque, Tagged, UnwrapTagged} from '../index';
 
 type Value = Opaque<number, 'Value'>;
 
@@ -9,9 +9,12 @@ const value: Value = 2 as Value;
 // The underlying type of the value is still a number.
 expectAssignable<number>(value);
 
-// You cannot modify an opaque value.
+// You cannot modify an opaque value (and still get back an opaque value).
 expectNotAssignable<Value>(value + 2);
 
+// But you can modify one if you're just treating it as its underlying type.
+expectAssignable<number>(value + 2);
+
 type WithoutToken = Opaque<number>;
 expectAssignable<WithoutToken>(2 as WithoutToken);
 
@@ -49,3 +52,63 @@ expectAssignable<PlainValue>(123);
 
 const plainValue: PlainValue = 123 as PlainValue;
 expectNotType<Value>(plainValue);
+
+// UnwrapOpque should work even when the token _happens_ to make the Opaque type
+// have the same underlying structure as a Tagged type.
+expectType<number>(4 as UnwrapOpaque<Opaque<number, {x: void}>>);
+
+// All the basic tests that apply to Opaque types should pass for Tagged types too.
+// See rationale for each test in the Opaque tests above.
+//
+// Tests around not providing a token, which Tagged requires, or using non-
+// `string | number | symbol` tags, which Tagged doesn't support, are excluded.
+type TaggedValue = Tagged<number, 'Value'>;
+type TaggedUUID = Tagged<string, 'UUID'>;
+
+const taggedValue: TaggedValue = 2 as TaggedValue;
+expectAssignable<number>(taggedValue);
+expectNotAssignable<TaggedValue>(value + 2);
+expectAssignable<number>(value + 2);
+
+const userEntities2: Record<TaggedUUID, Foo> = {
+	['7dd4a16e-d5ee-454c-b1d0-71e23d9fa70b' as UUID]: {bar: 'John'},
+	['6ce31270-31eb-4a72-a9bf-43192d4ab436' as UUID]: {bar: 'Doe'},
+};
+
+const johnsId2 = '7dd4a16e-d5ee-454c-b1d0-71e23d9fa70b' as TaggedUUID;
+
+const userJohn2 = userEntities2[johnsId2];
+expectType<Foo>(userJohn2);
+
+// Tagged types should support multiple tags,
+// by intersection or repeated application of Tagged.
+type AbsolutePath = Tagged<string, 'AbsolutePath'>;
+type NormalizedPath = Tagged<string, 'NormalizedPath'>;
+type NormalizedAbsolutePath = AbsolutePath & NormalizedPath;
+
+type UrlString = Tagged<string, 'URL'>;
+type SpecialCacheKey = Tagged<UrlString, 'SpecialCacheKey'>;
+
+expectNotAssignable<NormalizedPath>('' as AbsolutePath);
+expectNotAssignable<NormalizedAbsolutePath>('' as AbsolutePath);
+expectAssignable<AbsolutePath>('' as NormalizedAbsolutePath);
+expectAssignable<NormalizedPath>('' as NormalizedAbsolutePath);
+
+expectNotAssignable<SpecialCacheKey>('' as UrlString);
+expectAssignable<UrlString>('' as SpecialCacheKey);
+
+// A tag that is a union type should be treated as multiple tags.
+// This is the only practical-to-implement behavior, given how we're storing the tags.
+// However, it's also arguably the desirable behavior, and it's what the TS team planned to implement:
+// https://github.com/microsoft/TypeScript/pull/33290#issuecomment-529710519
+expectAssignable<Tagged<number, 'Y'>>(4 as Tagged<number, 'X' | 'Y'>);
+
+// UnwrapOpaque and UnwrapTagged both work on Tagged types.
+type PlainValueUnwrapOpaque = UnwrapOpaque<TaggedValue>;
+type PlainValueUnwrapTagged = UnwrapTagged<TaggedValue>;
+
+const unwrapped1 = 123 as PlainValueUnwrapOpaque;
+const unwrapped2 = 123 as PlainValueUnwrapTagged;
+
+expectType<number>(unwrapped1);
+expectType<number>(unwrapped2);