Support some non-structural (nominal) type matching

[iislucas]

Proposal: support non-structural typing (e.g. new user-defined base-types, or some form of basic nominal typing). This allows programmer to have more refined types supporting frequently used idioms such as:

1) Indexes that come from different tables. Because all indexes are strings (or numbers), it's easy to use the an index variable (intended for one table) with another index variable intended for a different table. Because indexes are the same type, no error is given. If we have abstract index classes this would be fixed.

2) Certain classes of functions (e.g. callbacks) can be important to be distinguished even though they have the same type. e.g. "() => void" often captures a side-effect producing function. Sometimes you want to control which ones are put into an event handler. Currently there's no way to type-check them.

3) Consider having 2 different interfaces that have different optional parameters but the same required one. In typescript you will not get a compiler error when you provide one but need the other. Sometimes this is ok, but very often this is very not ok and you would love to have a compiler error rather than be confused at run-time.

Proposal (with all type-Error-lines removed!):

// Define FooTable and FooIndex
nominal FooIndex = string;  // Proposed new kind of nominal declaration.
interface FooTable {
  [i: FooIndex]: { foo: number };
}
let s1: FooIndex;
let t1: FooTable;

// Define BarTable and BarIndex
nominal BarIndex = string; // Proposed new kind of nominal declaration.
interface BarTable {
  [i: BarIndex]: { bar: string };
}
let s2: BarIndex;
let t2: BarTable;

// For assignment from base-types and basic structures: no type-overloading is needed.
s1 = 'foo1';
t1 = {};
t1[s1] = { foo: 1 };

s2 = 'bar1';
t2 = { 'bar1': { bar: 'barbar' }};

console.log(s2 = s1); // Proposed to be type error.
console.log(s2 == s1); // Proposed to be type error.
console.log(s2 === s1); // Proposed to be type error.

t1[s2].foo = 100; // Gives a runtime error. Proposed to be type error.
t1[s1].foo = 100;

function BadFooTest(t: FooTable) {
  if (s2 in t) {  // Proposed to be type error.
    console.log('cool');
    console.log(t[s2].foo); // Proposed to be type error.
  }
}

function GoodBarTest(t: BarTable) {
  if (s2 in t) {
    console.log('cool');
    console.log(t[s2].bar);
  }
}

BadFooTest(t1); // Gives runtime error;
BadFooTest(t2); // No runtime error, Proposed to be type error.
GoodBarTest(t1); // Gives runtime error; Proposed to be type error.
GoodBarTest(t2);

[nahuel]

@aleksey-bykov

your workaround seems to fail for the number type (but string, boolean, etc, works ok):

const enum AsOrderId {}
type OrderId = number & AsOrderId;
let orderId = <OrderId> 123;
orderId = 124; // <-- passes ok (but is wrong)

[zpdDG4gta8XKpMCd]

that's right, because numbers coerce to enums and enums coerce to numbers, in order to overcome this you might want consider the following:

module AsOrderId { export const enum Brand {} }
interface AsOrderId { '': AsOrderId.Brand; }
type OrderId = number & AsOrderId;
let orderId = <OrderId> 123;
orderId = 124; // <-- now complaints as expected

[dead-claudia]

Here's a use case where I know I'll need nominal types for: number subtyping. In a performance-critical section of code for a project I'm intermittently working on, I need several different pointer types used with a reference-counting cache. Because of certain API hooks, I already have to do significant manual management, but mostly using numbers will help reduce memory dramatically and drastically increase speed for this.

Something like this, but imagine several files full of this madness:

// ./types.ts
export nominal Pointer extends number {}
export nominal TypePointer extends Pointer {}
export nominal AttrPointer extends Pointer {}
export nominal ChildrenPointer extends Pointer {}
export nominal ChildPointer extends Pointer {}
export interface Node {}

// ./caches.ts
import {AttrPointer, ChildrenPointer} from "./types"

class ReferenceCache<T, U extends number> {
  // properties
  function get(entry: U): T { /* whatever */ }
  function add(entry: T): U { /* whatever */ }
  function delete(entry: U): void { /* whatever */ }
}

export const attrCache = new ReferenceCache<any, AttrPointer>()
export const childCache = new ReferenceCache<Child[], ChildrenPointer>()

// ./patch.ts
import {attrCache, childCache} from "./caches"
import {Pointer, Node, AttrPointer, ChildrenPointer} from "./types"

function patch(nodes: Pointer[], start: i, node: Node) {
  const attrs = attrCache.get(<AttrPointer> nodes[i + 1])
  checkAttrs(attrs, node.attrs)

  const children = childCache.get(<ChildrenPointer> nodes[i + 2])
  const normalized = normalize(node.children)
  checkChildren(attrs, node.attrs)
}

[patrickjuchli]

I disagree with the chicken example by @AJamesPhillips, the distinction between classes and interfaces @plalx made is important.

If two interface declarations are structurally the same then they offer the same behaviour and should be treated the same. Isn't that the main point of interfaces? To implement a Toaster and a Mixer and have them satisfy the same IPowerSwitch interface? From the perspective of such an interface, a Toaster and a Mixer are the same thing. And if someone creates a second interface having the same behaviour then that means that it doesn't matter which one you use. Maybe I have this opinion because to me interfaces are first about behaviour and less about what something is.

[dead-claudia]

@patrickjuchli BTW, some libraries return instances of classes and match with instanceof, but don't provide the constructor.

Although IMHO after #7642 gets addressed, the only desire I have for nominal typing is for opaque numeric subtypes (used as pointers) and types whose structure is purely an implementation detail.

I'd love to see if I can crack the nut of asm.js in TypeScript, but numeric subtypes are the only realistic way I could do that and keep types straight.

[AJamesPhillips]

@patrickjuchli I think I've come round to that point of view. For "nominal" interfaces I'm using the following:

interface IJetPlane {
  IJetPlane: string;
  id: number;
  name: string;
  optional?: string;
}

It's slightly ungainly. It is a leaky implementation of nominal interfaces but serves it's purpose for my project using POJOs & Redux (I don't want to continuously go from class instances and back). Alternatively aleksey-bykov's comment is useful.

[dead-claudia]

@patrickjuchli @AJamesPhillips

What I'm currently using is this (it's in a .d.ts file):

interface AddUpdate<T> extends Update {
  "update add": this;
  mask: nodes;
  index: number;
  child: Child<T>;
}

Granted, this would fare better with const enums also being permitted as types, but that or nominal types would be okay.

interface AddUpdate<T> extends Update {
  mask: nodes.ADD;
  index: number;
  child: Child<T>;
}

[spion]

Note that nominal types will lead to surprising results with npm (node_modules) style commonjs modules. Nominal types that come from different versions of the same module will be different and incompatible (unless they are explicitly told what their unique name is).

Should be less of an issue with npm version 3.

[dead-claudia]

By the way, I would expect nominal types to still match equivalent structural interfaces. But I would mainly find nominal types useful for keeping number subtypes straight. This varies from units in scientific calculations to numeric pointers to types that their type is computed from the result of a value (such as a combination of bitwise flags).

The last one requires dependent types (types dependent on their value), which are probably not making it into TypeScript anytime soon, since it's only just now getting past the purely academic and experimental world into Haskell. It also makes a type system Turing complete, which a lot of language writers don't usually like, by allowing type-level functions and lambdas.

On Tue, Apr 12, 2016, 20:02 Gorgi Kosev notifications@github.com wrote:

Note that nominal types will lead to surprising results with npm (node_modules) style commonjs modules (unless they are explicitly told what their unique name is). Nominal types that come from different versions of the same module will likely be different.

— You are receiving this because you commented. Reply to this email directly or view it on GitHub https://github.com/Microsoft/TypeScript/issues/202#issuecomment-209156716

[dead-claudia]

I need a nominal type for an opaque object whose structure is an implementation detail. I need this badly.

[simonbuchan]

Intersecting with a branded interface seems pretty close to what we want to be possible:

> enum YearsBrand {}
undefined
> type Years = number & { _brand: YearsBrand }
undefined
> enum DaysBrand {}
undefined
> type Days = number & { _brand: DaysBrand }
undefined
> function yearsToDays(years: Years): Days { return <Days> years * 365; }
⨯ Unable to compile TypeScript
[eval].ts (6,51): Neither type 'number & { _brand: YearsBrand; }' nor type 'number & { _brand: DaysBrand; }' is assignable to the other.
  Type 'number & { _brand: YearsBrand; }' is not assignable to type '{ _brand: DaysBrand; }'.
    Type '{ _brand: YearsBrand; }' is not assignable to type '{ _brand: DaysBrand; }'.
      Types of property '_brand' are incompatible.
        Type 'YearsBrand' is not assignable to type 'DaysBrand'. (2352)
[eval].ts (6,51): The left-hand side of an arithmetic operation must be of type 'any', 'number' or an enum type. (2362)
[eval].ts (6,51): Type 'number' is not assignable to type 'number & { _brand: DaysBrand; }'.
  Type 'number' is not assignable to type '{ _brand: DaysBrand; }'. (2322)
> function yearsToDays(years: Years): Days { return <Days> (<number> years * 365); }
undefined
> yearsToDays(30)
⨯ Unable to compile TypeScript
[eval].ts (7,13): Argument of type 'number' is not assignable to parameter of type 'number & { _brand: YearsBrand; }'.
  Type 'number' is not assignable to type '{ _brand: YearsBrand; }'. (2345)
> let age = <Years> 30
undefined
> yearsToDays(age)
10950

It would be nice if Typescript gave better errors and supported lifting operators, which implies a declaration syntax (I favor new type Foo = Bar to avoid more keywords, and it looks like the same feature in Haskell)

Being able to use them as keys would also be handy:

> enum UserIdBrand {}
undefined
> type UserId = string & { _brand: UserIdBrand }
undefined
> interface SessionCache { [userId: UserId]: any }
⨯ Unable to compile TypeScript
[eval].ts (10,27): An index signature parameter type must be 'string' or 'number'. (1023)

[dead-claudia]

@simonbuchan

That's the essence of nominal subtypes: sometimes you need numbers with formally correct unit handling, sometimes you need just an ordered, statically known set of numbers (that's enums of today), and sometimes you need to handle certain string keys as if they were their own type (where there are n possibilities, but they are more than just strings).

I've had a case where I need nominal interfaces for objects, because it would be an acceptable compromise to dependent types for a definition file. In my case, the object's type is dependent on the first few bits of a mask on the object itself, but since dependent types (types that depend on the value) won't likely exist in TypeScript for quite a while if ever, nominal interfaces with casting would work.

[simonbuchan]

@isiahmeadows Yes? My comment was that Typescript already allows the creation of distinct (that is, nominal) types without changing the runtime shape (e.g. { ObjectId_value: string }), so the (real!) concern of "How do we avoid breaking the entire typesystem" can be approached as "How do we make this terrible hack on the type system less unusable", or in other words: if it would be broken with a new feature, it may already be broken! (In a bizzare edge-case, not a hopefully heavily used new feature)

[simonbuchan]

Of course, you can get many of the same "benefits" with just class Foo {}; let x = <Foo><any> 123 - the intersect hack is only slightly safer without the extra <any> (and of course it doesn't generate any extra JS)

[dead-claudia]

@simonbuchan I'd rather not even expose members that don't exist in the first place. That's why I try to avoid those hacks in the first place.

[patrickjuchli]

The V8 team is doing some experiments with SaneScript and SoundScript (in this order). Short description here where they also refer to TypeScript to be used as a basis.

There is also a presentation with detailed information. On pages 25 and following they talk about introducing nominal classes. Maybe these experiments could be helpful in this discussion.

[svieira]

A slightly generified version of @simonbuchan's state of the art for nominal types in TypeScript is:

/**
 * Nominal typing for any TypeScript interface or class.
 *
 * If T is an enum type, any type which includes this interface
 * will only match other types that are tagged with the same
 * enum type.
 */
interface Nominal<T> { 'nominal structural brand': T }

Usage is identical to his version (and the error messages are slightly better, in my opinion):

declare module Dates {
  export const enum Years {}
  export const enum Days {}
}

type Years = Nominal<Dates.Years> & number;
type Days = Nominal<Dates.Days> & number;

var day: Days;
var years: Years;
day = years;
// Type 'Nominal<Year> & number' is not assignable to type 'Nominal<Day> & number'.
//   Type 'Nominal<Year> & number' is not assignable to type 'Nominal<Day>'.
//     Type 'number' is not assignable to type 'Nominal<Day>'.

If it were possible to restrict the type of T in the Nominal interface to enum types this could be completely type-safe from the consumer's perspective.

A syntax proposal

Assuming that there was some way of restricting T to enum subtypes, the Nominal interface could be added as (for example) lang.Nominal and some syntactical sugar could be added for creating an interface or class that uses Nominal. I suggest the keyword distinct to tag a type which should be treated as a nominal type:

distinct interface SomeInterface { x: number, y: number, z: number }
distinct class SomeClass { a: string; b: string; c: string; }
distinct type SomeId = number

would be translated by the compiler (for ES5 and lower targets) as:

interface __SomeInterface  { x: number, y: number, z: number }
const enum __SomeInterfaceTag {}
type SomeInterface = __SomeInterface & lang.Nominal<__SomeInterfaceTag>

class SomeClass implements lang.Nominal<__SomeClassTag>  {
  'nominal structural brand': __SomeClassTag;
  a: string; b: string; c: string;
}
const enum __SomeClassTag {}

type SomeId = number & lang.Nominal<__SomeIdTag>;
const enum __SomeIdTag {};

the __-prefixed enums and interfaces being compiler-generated and inaccessible[1] to user code.

Usage

Casts would be necessary for any usage of a distinct type or distinct interface (but they are necessary for other languages too - such as the Scala shapeless library's Tagged utility). At it's simplest[2]:

// Types and interfaces just need to be "tagged"
var interfaceTagging = {x: 1, y: 1, z: 1} as SomeInterface;
var typeTagging = 123 as SomeId;

// Classes should be constructed specifically by the nominal constructor
var classCreation = new SomeClass('a', 'b', 'c');
// A cast is also possible, but at that point all type-safety is out the window.

Notes

1. The compiler-generated enum is available to consuming code if it accesses the 'nominal structural brand' field and uses a type query. This could be mitigated by using a Symbol for the key.

2. A specific keyword could be added for a "tagging" cast:

   var x = new Point(1, 2, 3) taggedAs SomeInterface;
   // Errors if SomeInterface is not a lang.Nominal

   I would argue for it since it documents the purpose of the cast and prevents using non-nominative types in nominative (i. e. "tagging") casts.

3. @SimonMeskens points out an excellent ES2015+ approach that could be very useful as well - instead of using the string-keyed 'nominal structural brand' use Symbol.species for the brand key and the enum (or the type itself) for the type.

[dead-claudia]

@svieira :+1:, but I'd like to point out that classes are already nominally typed, so the distinct keyword redundant for those cases.

[svieira]

@isiahmeadows - thanks!

Regarding classes being nominally typed, I may be reading section 8.1 wrong but I am pretty sure that they are still structurally typed:

The following example introduces both a named type called 'Point' (the class type) and a named value called 'Point' (the constructor function) in the containing declaration space.

class Point {  
    constructor(public x: number, public y: number) { }  
    public length() { return Math.sqrt(this.x * this.x + this.y * this.y); }  
    static origin = new Point(0, 0);  
}

The named type 'Point' is exactly equivalent to

interface Point {  
    x: number;  
    y: number;  
    length(): number;  
}

The named value 'Point' is a constructor function whose type corresponds to the declaration

var Point: {  
    new(x: number, y: number): Point;  
    origin: Point;  
};

[dead-claudia]

@svieira Okay...that's news to me. I didn't know that. I went and checked in the playground, and you are correct. Still, :+1: from me.

[dead-claudia]

I just had an idea: if you modeled primitive types as nominal types extending interfaces (mod typeof narrowing, and assuming this isn't already the case), that could make this easier to do.

nominal boolean extends Boolean {}
nominal number extends Number {}
nominal string extends String {}
nominal symbol extends Symbol {}
// `void` is basically the same as `undefined | null`
// functions and objects use syntax instead.

(Note that primitives will need compiler help, though.)

[shelby3]

@danquirk wrote:

Your latest example seems emblematic of some of my concerns. I take it you just want an error on the last line. But then in the same small snippet of code you've taken advantage of structural typing (a number is an Age since Age has no members) while also doing a nominal comparison a few lines later (to get an error using Height where Age is expected).

He could just as well have expressed the type as any, number, or string | number as desired. He wasn't forced to use structural typing, but probably did so as convenience. What is the concern specifically? Would you prefer that structural coercions are not allowed to be assigned to nominal types thus he would have been forced to declare the type?

Essentially every type comparison operation would require you to do the additional work of deducing whether the comparison was a structural or a nominal one, rather than having an easily predictable set of behavior.

When we compare any two instances of anything, we need to know their types to understand implications of implicit coercions (conversions) which don't even emit a compile-time error. How is it more difficult knowing whether only one of the types is nominal which will emit a compiler error if it is not?

For that scenario my preferred solution would be something like F#'s Units of Measure ( #364).

That seems to me to be the "kitchen sink" syntax pollution school-of-design. Nominal types can accomplish that and are the more general, generative essence construct.

[shelby3]

@plalx wrote:

In my opinion, the default behavior should have been that structural type checking is used only where contracts are defined in terms of interfaces and nominal typing is used when contracts are defined in terms of concrete classes

Agreed. See below...

Changing the implementation and have a private _value: string as well as a get value(): string getter would achieve a form of nominal typing naturally because different types with private member properties cannot be made equal, but supporting a nominal keyword which decorates interfaces & classes would be much better.

Agreed. See below...

Note if we must decorate class, I would prefer nominal class instead of named class, because class can also be anonymous (nameless) so named is confusing. Also I would prefer a breaking change that is only turned on by flag where by default undecorated class is nominal and use interface for all structural typing.

@sampsyo wrote:

I'm using a void field as a workaround. It works nicely and doesn't get in the way.

If we make the unused field private, it doesn't appear in the JS output and it doesn't get in the way of structural casts if there is a matching member property:

class Bar {
    private _nominal:void
}

class Baz {
    value: number
    private _nominal:void
}
let x: Bar = <Bar>42,          // Error
    y: Bar = <Bar>{},          // Ok
    z: Baz = <Baz>{value: 42}  // Ok
y = z                          // Error

Since classes can also be used as interfaces, then that is also the way to create a nominal "interface", so we can leave interface for structural (and since interface can't have private member properties).

Tangentially, I'd prefer the abstract keyword is removed. When you don't provide an implementation for a member property, then it is abstract. Why the need to decorate it? Verbosity is code smell. Is it to be sure I didn't intend to provide an implementation? As if I can't see that I didn't, lol.

To nominally subtype the built-in nominal types, which I think is more self-documenting and less likely to produce an accidental collision that the recommendation currently in the FAQ:

class Nominal<T> {
    private _nominal:T
}
type Bar = number & Nominal<'Bar'> // employing string literal type
type Baz = number & Nominal<'Baz'>
let y: Bar = <Bar>42,         // Ok
    z: Baz = <Baz>53          // Ok
y = z                         // Error

So that now that we see TypeScript already effectively has nominal typing, I suppose the need to justify it's implementation is vacated.

Edit: afaics instanceof is always nominal and a sealed keyword would be required for exhaustive checking with a class subtyping nominal sum type if SPOT and DNRY are to be respected. Typeclasses would be an alternative to formalizing class nominal subtyping.

Edit#2: I wanted to change from private _nominal:void to private _nominal = undefined to make it explicit that no other class (not even a subclass given it's private not protected) is able to set the member property to any value, but this creates a property in the emitted output. In version 2.0, I anticipate this could be instead private _nominal: undefined, but I haven't tested it.

[dead-claudia]

@shelby3

1. Classes aren't nominal. Not in the slightest.

2. The nominal subtyping mechanism you mention isn't really nominal. It's still structural; it's just using a string literal type to namespace it. With TypeScript 2.0, I believe you may be able to create nominal types with private opaque enum values like this:

   const enum Marker {Value}
   class Private { private _marker: Marker.Value }
   export type MyType = number & Private

   I'd recommend not trying to force nominal types into TypeScript in a way that requires frequent explicit casting in the meantime, though. If you focus on the data instead of the interface, you'd be surprised how far duck typing actually gets you.

[shelby3]

@isiahmeadows wrote:

1​. Classes aren't nominal. Not in the slightest.

I never wrote that classes are nominal or providing any effectively nominal typing simulation by default. I hope you were not implying that I made such an incorrect assertion.

2​. The nominal subtyping mechanism you mention isn't really nominal. It's still structural

I didn't write that the nominal typing simulation isn't achieved with the structural type checker. The ability to enforce that every private or protected property is unique between nominal classes, is in effect a nominally associated typing feature. Also apparently instanceof remains nominal for instantiated classes.

I'd recommend not trying to force nominal types into TypeScript in a way that requires frequent explicit casting in the meantime, though. If you focus on the data instead of the interface, you'd be surprised how far duck typing actually gets you.

I intend to convince[1] that subclassing is a non-extensible anti-pattern and typeclasses is the superior paradigm for nominal typing.

[1] See the THE COVARIANCE OF GENERICS section and the link back to a TypeScript issue.

[shelby3]

There is some evidence that TypeScript is conflating it design patterns around the separation-of-concepts for structural and nominal typing.

I hope we can get more organized.

[shelby3]

Why is it nearly everyone wants to deny that the prototype chain is a feature of JavaScript that enables nominal typing (i.e. deny that JavaScript has nominal typing)?

And that nominal typing is not only subclassing?

[MartinJohns]

@shelby3 Can you please elaborate how prototype would be a suitable candidate to solve nominal typing?

The example I have in mind is that my API returns the following JSON:

{
    "id": "ProductId",
    "name": "name"
}

Which then is deserialized to the following data structure:

interface Product {
    id: ProductId,
    name: string
}

The ProductId should be nominal typed. It's a string behind the scenes, but you can't just assign random strings to it. Same as you could not assign a UserId to it, even when that nominal type is also a string.

Since the data structure is deserialized I don't see how the prototype chain would work.

[dead-claudia]

@shelby3

There is some evidence that TypeScript is conflating it design patterns around the separation-of-concepts for structural and nominal typing.

I hope we can get more organized.

---

Why is it nearly everyone wants to deny that the prototype chain is a feature of JavaScript that enables nominal typing (i.e. deny that JavaScript has nominal typing)?

And that nominal typing is not only subclassing?

1. Please keep this civil. Accusing people of some supposed wrongdoing or speaking down to a group of people, isn't going to get you anywhere, even if you're right. This includes the above two comments, as well as both of the links to your comments from these two above ones.
2. @spion in his comment did not in fact claim, much less imply, that nominal typing is not just subclassing. He was stating benefits of having nominally different structural types that still matched, using Thenables in the promise world as an example. This has nothing to do with subclassing, and is merely the fundamental difference between structural and nominal types.
3. Yes, the prototype chain can be used for nominal typing. But is that useful in practice? It depends on the programmer. If you focus on the interface, then yes, nominal typing is a must. If you focus on the data, then nominal typing can actually get in the way at times. Call me crazy, but I'd rather have one way to define a simple point, not three different classes to define the same thing in different precisions (float, double, and int), extending a superclass which only matches the precision of one of them. Structural typing lets me instead return {x: 1, y: 2}, while descriptively naming the instance interface Point { x: number; y: number }. I tend to focus on the data, not the interface, so structural typing works fairly well. YMMV, though.

[zpdDG4gta8XKpMCd]

Classes are much more nominal than anything else in TS.

[shelby3]

@isiahmeadows wrote:

Accusing people of some supposed wrongdoing or speaking down to a group of people

What group of people did I doxx? Please be specific and show proof.

The way I see it, I referred to a common mindset in the JS community-at-large, including myself in the recent past. Douglas Crockford even points out that we don't give prototype chain inheritance enough attention.

All the worrying about little nuances of intent is getting tiresome. It isn't a big problem. Our egos are not that fragile.

@spion in his comment did not in fact claim, much less imply, that nominal typing is not just subclassing.

Incorrect. Typeclasses can use nominal typing and they don't have the problem he was asserting is a problem with nominal typing. The problem he was referring to is caused by subclassing. Apparently you didn't understand this. So now you do.

Yes, the prototype chain can be used for nominal typing. But is that useful in practice?

You will know soon.

[dead-claudia]

@shelby3 Both of these rhetorical questions come across as somewhat personal:

Why is it nearly everyone wants to deny that the prototype chain is a feature of JavaScript that enables nominal typing (i.e. deny that JavaScript has nominal typing)?

And that nominal typing is not only subclassing?

[shelby3]

Both of these rhetorical questions come across as somewhat personal:

I repeat:

All the worrying about little nuances of intent is getting tiresome. Our egos are not that fragile.

I will not reply further on that topic. Thanks. (No disrespect intended, just as I say it is tiresome)

[mhegazy]

@shelby3, over the past few days you have had multiple posts on this forum that are antagonizing and disrespectful to other participants. This is not a comment to this specific issue, but rather a general one. This is a technical forum for discussing the TypeScript design and bugs, and not a political or social discussion group; and we do expect all participants to interact in a civil, respectful and productive manner.

This is not a request to explain your comments, or tone; this is rather a statement of how many others perceive them. If you are interested in participating in this forum, and have vested interest in future of the TypeScript community and JavaScript tooling in general, I would suggest you keep the discussion in technical terms, and keep the tone professional.

[SimonMeskens]

I'm opposed to solutions that create a construction that works purely compile-time. Typescript should attempt to type existing javascript as precisely and concisely as possible. The problem with nominal types is that they are not nominal at run-time, unless they are tagged objects. Typescript can already type tagged objects, though obviously, we are waiting for symbol support in computed properties to really leverage this feature.

Can anyone explain to me something that nominal types can do, that tagged objects don't do?

[antanas-arvasevicius]

Looks like two worlds colliding here.. but in the reality there is no such thing as "nominal" in javascript, but it would be nice to have a compile time check for type mismatch in nominal way. Best example which would work for is if we had a lot of services with methods like update(id:SomethingId), update(id:OrderId), update(id:UserId) an such, and how to model API in a way that developers won't mix up these ids which all are "string", but meaning is different, and they should be able to pass any "string" in these methods if they explicitly cast it to appropriate type. Is that hard to implement in TS?

[RyanCavanaugh]

@antanas-arvasevicius this is already quite possible

// Use whatever magic string you like
type UserId = string & { "is user id": void }

at this point you can use any string as a UserId with a type assertion and everything basically works the way you'd expect. Technically this is still structural, though if everyone comes up with a unique magic string (not hard) it is effectively nominal.

[SimonMeskens]

@antanas-arvasevicius, as @RyanCavanaugh points out, Javascript has always had nominal types. Tagged objects are nominal if the tag is unique. By using a UUID as a property name, you ensure nominality. Even better is using the new ES2015 symbols, though Typescript can't properly type those yet (symbols in computed properties).

In fact, Javascript, as a prototypal language, provides a superset of classic OO, it is strictly MORE powerful. There isn't a single Java feature that Javascript can't do. The issue is: can Typescript express and typecheck the feature, which in case of nominal types, it can for UUIDs, but not for Symbols (yet).

interface Tagged {
    "d3a8e9b5-07c2-4a2d-a245-8a5908826b0f": void;
}

A handy semantic way is to use Symbol.species:

interface TaggedSpecies {
    [Symbol.species]: "d3a8e9b5-07c2-4a2d-a245-8a5908826b0f"
}

I haven't figured out a proper way to tag an object with multiple IDs yet when using the Symbol.species way, but I assume there's an elegant way to express it. Meanwhile, I suggest you just use the first sample I provided, which lets you tag as many IDs in an object as you want. Do note that for a robust code-base, you want to make sure that these tags are not enumerable, if you are actually adding them onto the object for runtime typechecking.

[antanas-arvasevicius]

But that way it looks like some language hack and is strange to read ;) would be nice to have such supportiness in a language itself instead of making constructs like these every time. I don't know maybe that behavior is significant to minorities and will be rarely used then whatever.

[RyanCavanaugh]

You can fancy it up a little with string literal types

// Once
type Branded<T, U> = T & { '__ kind': U };

// Elsewhere
type OrderID = Branded<string, 'Order ID'>;

[SimonMeskens]

@antanas-arvasevicius It's not a hack, and it's widely used today. It may look strange, but what something like Java does, is add that same tag behind the scenes. I agree it might not look super elegant, but I tried suggesting a more elegant syntax and unfortunately, something like that is not within the current scope of Typescript.

[SimonMeskens]

@RyanCavanaugh You should probably use Symbol.species instead of __kind, which is spec compliant and more semantic:

// Once
type Branded<T, U> = T & { [Symbol.species]: U };

// Elsewhere
type OrderID = Branded<string, 'Order ID'>;

And remove the magic string maybe:

type OrderUUID = "d3a8e9b5-07c2-4a2d-a245-8a5908826b0f";

// Once
type Branded<T, U> = T & { [Symbol.species]: U };

// Elsewhere
type OrderID = Branded<string, OrderUUID>;

[spion]

@shelby3 I was not referring to subclassing. That was only the mechanism via which I illustrated the problem with nominal types.

Typeclasses only solve the initial step where you have to convince the owner to implement your nominal interface; you no longer need to do that - you can write your own instance. However, you still have to convince the first author as well as existing library writers to adopt your typeclass i.e. write their code as follows:

function f(x:TheNominalInterface) { ... }

rather then write their functions against the original nominal type written by the first author:

function f(x: TheNominalOriginalPromise) { ... }

In short, I believe that nominal types are not what you want by default, and it was brilliant of TypeScript to bring structural types to the mainstream. I also quite like the idea (its fresh, really) of a type system that doesn't put absolute type safety and soundness on a pedestal above all else, but gives it rational practical consideration with all other concerns.

[MartinJohns]

@SimonMeskens Unfortunately these hack widely used workaround won't allow you to define a nominal alias for string, so that it can be used as an argument in an indexer.

[SimonMeskens]

@MartinJohns Not hacks or workarounds at all, as evidenced by the inclusion of Symbol into the standard. Strings are object like anything else, so it does work for strings too. You are correct they are not usable in indexers at present, which is a feature I could get behind (allow types that are either a subclass of string or a union with string in indexers). This is however a shortcoming of Typescript, not a shortcoming of tagged objects. It's also an issue easily fixed without adding new syntax to the language that creates a compile-time only construct. I was wrong, see comment further down

I get a bit tired when people call tagged objects hacks/workarounds. The literature disagrees with you and so does the standard.

[MartinJohns]

@SimonMeskens By no means I want to be offensive or disrespectful by calling it a hack. It's merely my current understanding of the proposed solution, unless it gets further language support.

Am I understand this correctly that it would not only be a compile time thing, but also a runtime thing by adding a field to the JavaScript object? If yes, how would this play together with data structures parsed from, e.g. using JSON.parse()? The API backend surely would not include this brand information in the JSON. Then it also wouldn't be nominal typing, but still structural typing, wouldn't it?

[SimonMeskens]

@MartinJohns The last project I worked on, which was purely Javascript, already used this concept. The API backend did indeed send brand information, facilitated by the fact that it was running on .NET and thus had the type information needed to brand JSON sent to client. I started adding TypeScript annotations, precisely to leverage this existing behavior. If you are in the unfortunate situation of having to work with a backend that sends back unbranded objects, you would simply brand them yourself, as they come in. Separation of concerns dictates that you would already put your JSON.parse() call in the layer where you fetch the data, which also wraps the ajax calls, etc. In that same layer, you do structural integrity checks, to make sure the data received is valid, at that point, you can easily brand the objects if they prove to be valid.

Apologies for being a bit snippy about the use of the word hack, I just think that lots of developers shy away from using this pattern, because of a bias. This is probably not the place to discuss that though.

[RyanCavanaugh]

I think there are two different things being discussed here -- you can have branded types in TypeScript (using whatever property key you want since the property is not actually manifest at runtime) using intersection types or brand properties of an arbitrary property type. This is useful because you can brand things which can't actually have extra properties, such as strings and numbers.

A different problem is discriminating objects that are manifest, via some key. Here a discriminator property, usually with a string value, is well-supported.

I would call the first thing about a 4 on a 0-10 "hack" scale (anything 6 or above I would not even mention in this forum without being forced to; people who were trying to fake non-nullability with T | void scored an 8 or 9), the second thing scores a solid 0.

[SimonMeskens]

Fair enough, yeah, that's why I mentioned earlier I don't like the idea of forcing anything into the language that is not present at runtime (TypeScript should be descriptive of Javascript at all times imo). In case of tagging objects, I don't see why you wouldn't just tag the object for real, unless you are indeed forced to use a pre-existing framework that doesn't let you tag objects, something that does come up in real code.

[SimonMeskens]

@MartinJohns I made a mistake earlier, you are indeed correct that if you want to use these nominal strings for property names, that is not possible and it's not a feature TypeScript could ever support in the way described. You are mistaken in thinking that you tag the property names though, all suggestions here for branding through a tag are by making the value a unique literal and using TypeScript's excellent support for literal string types:

// this works!
interface Example {
    someTag: "unique string goes here"
}

By forcing any objects that satisfy this interface to have a tag with a unique value, you don't need the property name to be unique, and thus, indexers do not matter. The suggested semantic way I described uses [Symbol.species] as a property name, and a UUID as a value, to get both decent semantics and proper uniqueness.

My apologies for the confusion, I should've gone to bed quite a while ago.