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);

[RyanCavanaugh]

Is there a better keyword here than "abstract" ? People are going to confuse it with "abstract class".

+Needs Proposal

[iislucas]

w.r.t. Needs Proposal: do you mean how to implement it? For compilation to JS, nothing needs to be changed. But would need internal identifiers for new types being introduced and an extra check at assignment.

[samwgoldman]

Regarding a name, what about "nominal" types? Seems pretty common in literature.

[RyanCavanaugh]

We're still writing up the exact guidelines on suggestions, but basically "Needs Proposal" means that we're looking for someone to write up a detailed formal explanation of what the suggestion means so that it can be more accurately evaluated.

In this case, that would mean a description of how these types would fit in to all the various type algorithms in the spec, defining in precise language any "special case" things, listing motivating examples, and writing out error and non-error cases for each new or modified rule.

[iislucas]

@RyanCavanaugh Thanks! Not sure I have time for that this evening :) but if the idea would be seriously considered I can either do it, to get someone on my team to do so. Would you want an implementation also? Or would a clear design proposal suffice?

[danquirk]

@iislucas no implementation is necessary for "Needs Proposal" issues, just something on the more formal side like Ryan described. No rush ;)

[zpdDG4gta8XKpMCd]

There is a workaround that I use a lot in my code to get nominal typing, consider:

interface NominalA {
   'I am a nominal type A, nobody can match me to anything I am not': NominalA;
    value: number;
}

interface NominalB {
   'I am a nominal type B, mostly like A but yet quite different': NominalB;
   value: number;
}

// using <any> on constructing instances of such nominal types is the price you have to pay
// I use special constructor functions that do casting internally producing a nominal object to avoid doing it everywhere
var a : NominalA = <any>  { value: 1 };
var b : NominalB = <any>  { value: 2 };

a = b; // <-- problema

[iislucas]

Neat trick! Slight optimization, you can use:

var a = <NominalA>  { value: 1 };
var b = <NominalB>  { value: 2 };

(Slightly nicer/safer looking syntax) [Shame it doesn't work for creating distinct types for string that you want to be indexable]

[basarat]

@aleksey-bykov nice trick. We have nominal Id types on the server (c#) that are serialized as strings (and we like this serialization). We've wondered of a good way to do that without it all being string on the client. We haven't seen bugs around this on the client but we still would have liked that safety. Based on your code the following looks promising (all interfaces will be codegened):

// FOO 
interface FooId{
    'FooId':string; // To prevent type errors
}
interface String{   // To ease client side assignment from string
    'FooId':string;
}
// BAR
interface BarId{
    'BarId':string; // To prevent type errors
}
interface String{   // To ease client side assignment from string
    'BarId':string;
}

var fooId: FooId;
var barId: BarId;

// Safety!
fooId = barId; // error 
barId = fooId; // error 
fooId = <FooId>barId; // error 
barId = <BarId>fooId; // error

// client side assignment. Think of it as "new Id"
fooId = <FooId>'foo';
barId = <BarId>'bar';

// If you need the base string 
// (for generic code that might operate on base identity)
var str:string;
str = <string>fooId;
str = <string>barId;  

[Steve-Fenton]

We could look at an implementation that largely left the syntax untouched: perhaps we could add a single new keyword that switches on "nominality" for a given interface. That would leave the TypeScript syntax largely unchanged and familiar.

class Customer {
    lovesUs: boolean;
}

named class Client {
    lovesUs: boolean;
}

function exampleA(customer: Customer) {

}

function exampleB(customer: Client) {

}

var customer = new Customer();
var client = new Client();

exampleA(customer);
exampleA(client);

exampleB(customer); // <- Not allowed
exampleB(client);

So you can use a Client where a Customer is needed, but not vice versa.

You could fix the error in this example by having Customer extend Client, or by using the correct named type - at which point the error goes away.

You could use the "named" switch on classes and interfaces.

[basarat]

You could use the "named" switch on classes and interfaces.

:+1:

[Steve-Fenton]

You could also use it to make a type nominal in a specific context, even if the type was not marked as nominal:

function getById(id: named CustomerId) {
    //...

[RyanCavanaugh]

You could also use it to make a type nominal in a specific context

What would that mean?

[Steve-Fenton]

When used as part of a type annotation, it would tell the compiler to compare types nominally, rather than structurally - so you could decide when it is important for the exact type, and when it isn't.

It would be equivalent to specifying it on the class or interface, but would allow you to create a "structural" interface that in your specific case is treated as "nominal".

Or, I have jumped the shark :) !

[RyanCavanaugh]

An example of an error (or non-error) would be nice. I can't figure out how you'd even use this thing

interface CustomerId { name: string }
interface OrderId { name: string }
function getById(id: named CustomerId) {
    //...
}
var x = {name: 'bob'};
getById(x); // Error, x is not the nominal 'named CustomerId' ?

function doubleNamed1(a: named CustomerId, b: named OrderId) {
    a = b; // Legal? Not legal?
}
function doubleNamed2(a: named CustomerId, b: named CustomerId) {
    a = b; // Legal? Not legal?
}
function namedAnon(x: named { name: string }) {
     // What does this even mean? How would I make a value compatible with 'x' ?
}

[Steve-Fenton]

This is why I'm not a language designer :)

I've shown in the example below that the keyword applies for the scope of the variable. If you make a parameter nominal, it is nominal for the whole function.

interface CustomerId { name: string }
interface OrderId { name: string }
function getById(id: named CustomerId) {
    //...
}
var x = {name: 'bob'};
getById(x); // Error, x is not the nominal 'named CustomerId'

function doubleNamed1(a: named CustomerId, b: named OrderId) {
    a = b; // Not legal, a is considered to be a nominal type
}
function doubleNamed2(a: named CustomerId, b: named CustomerId) {
    a = b; // Legal, a is compared nominally to b and they are the same type
}
function singleNamed1(a: named CustomerId, b: CustomerId) {
    a = b; // Legal, a is compared nominally to b and they are the same type
}
function namedAnon(x: named { name: string }) {
     // Compiler error - the "named" keyword can only be applied to interfaces and classes
}

[Steve-Fenton]

I admit that the notion of marking an item as nominal temporarily as per these recent examples may have been a little flippant - in the process of thinking through the implications of the feature I'm happy to accept it may be a terrible idea.

I'd hate for that to affect the much more straightforward idea marking a class or interface as nominal at the point it is defined.

[basarat]

Will need an inline creation syntax. Suggestion, a named assertion:

var x = <named CustomerId>{name: 'bob'};  // x is now named `CustomerId`
getById(x); // okay

Perhaps there can be a better one.

[ComFreek]

I wonder what the use cases for library developers are to not request nominal type checking via name.

Wouldn't you always be on the safe side if you use name by default? If the caller does have the right type, all is fine. If he doesn't, he must convert it (e.g. using the syntax @basarat suggested). If the conversion works, but doesn't work as expected, it's the user's fault and not the library developer's fault.

Maybe the whole problem is the duck typing system itself. But that's one problem TypeScript shouldn't solve, I suppose.

[sophiajt]

Not to sound like a sour puss, but being a structural type system is a fork in the road early on in how the type system works. We intentionally went structural to fit in better with JavaScript and then added layer upon layer of type system machinery on top of it that assumes things are structural. To pull up the floor boards and rethink that is a ton of work, and I'm not clear on how it adds enough value to pay for itself.

It's worth noting, too, the complexity it adds in terms of usability. Now people would always need to think about "is this type going to be used nominally or structurally?" Like Ryan shows, once you mix in patterns that are common in TypeScript the story gets murky.

It may have been mentioned already, but a good article for rules of thumb on new features is this one: http://blogs.msdn.com/b/ericgu/archive/2004/01/12/57985.aspx

The gist is that assume every new feature starts at -100 points and has to pay for itself in terms of added benefit. Something that causes a deep rethink of the type system is probably an order of magnitude worse. Not to say it's impossible. Rather, it's highly unlikely a feature could be worth so much.

[danquirk]

Agree with Jonathan here. I would have to see an extremely thorough proposal with some large code examples that prove this doesn't quickly become unmanageable. I have a hard time imagining how you could effectively use this modifier in a restricted set of circumstances without it leaking everywhere and ending up with you needing to use it on every type in your program (or giving up entirely on things like object literals). At that point you're talking about a different language that is basically incompatible with JavaScript.

Remember that nominal systems come with pain too and have patterns they don't represent as well. The trade off to enable those patterns with a structural system is the occasional overlap of structurally equal but conceptually different types.

[Steve-Fenton]

So the most common use case for this (that I can think of) is type-safe ids. Currently, you can create these in TypeScript by adding a private member to a class (or a crazy identifier on an interface, although that only reduces the chance, whereas the private member trick works as expected).

You have already made the decision that you want a nominal type when you create a type safe id class, because that is the purpose of such a class (and is the reason you aren't simply using number).

So my question is as follows, this code does what a lot of people want:

    class ExampleId {
        constructor(public value: number){}
        private notused: string;
    }

i.e. you cannot create another type that will satisfy this structure, because of the private member...

1. Would it be possible to formalise this behaviour with a keyword so the private member isn't needed?
2. Would it be possible to get this behaviour for interfaces?

The first of these two questions would probably cover 80% of the use cases. The second would allow similar cases and would be very useful from a .d.ts perspective.

This limits the feature to the creation of types that cannot be matched, which is already possible as described and for classes simply moves a "magic fix" into a more deliberate keyword.

I would be happy to write up something for this.

[danquirk]

Certainly feel free to try to write up something more complete that can be evaluated, although I will be honest and say the chances of us taking a change like seem quite slim to me.

Another data point to consider is that TypeScript classes had this behavior by default for some time (ie always behaved as a nominal type) and it was just very incongruous with the rest of the type system and ways in which object types were used. Obviously the ability to turn nominal on/off is quite different from always on but something to consider nonetheless. Also, as you note this pattern does allow some amount of nominal typing today, so it would be interesting to see if there are any codebases that have used this intermixing to a non-trivial degree (in a way that isn't just all nominal all the time).

[iislucas]

Note: lets not mix up the baby and bathwater here: the proposal in this issue is not a nominal keyword for any type, but to support a specific interface declaration of a nominal type. Nominal types are easy get right, and pretty well understood to provide value; while a 'sticky' nominal type annotation is tricky to do right. I'd suggest moving discussion of a anywhere nominal type-tag to a different issue so as not to confuse the two.

On Fri, Aug 1, 2014 at 4:37 PM, Dan Quirk notifications@github.com wrote:

Certainly feel free to try to write up something more complete that can be evaluated, although I will be honest and say the chances of us taking a change like seem quite slim to me.

Another data point to consider is that TypeScript classes had this behavior by default for some time (ie always behaved as a nominal type) and it was just very incongruous with the rest of the type system and ways in which object types were used. Obviously the ability to turn nominal on/off is quite different from always on but something to consider nonetheless. Also, as you note this pattern does allow some amount of nominal typing today, so it would be interesting to see if there are any codebases that have used this intermixing to a non-trivial degree (in a way that isn't just all nominal all the time).

— Reply to this email directly or view it on GitHub https://github.com/Microsoft/TypeScript/issues/202#issuecomment-50931990 .

Lucas Dixon | Google Ideas

[sophiajt]

@iislucas - as mentioned earlier, structural and nominal are fundamental choices in the type system. Any time you rethink part of the fundamental design choices, you need to understand the full impact. Even if it seems to be isolated to a small set of scenarios.

The best way to full understand the impact is to have a more complete suggestion. I wouldn't confuse @danquirk's response as throwing the baby with the bathwater, but instead as the minimal amount of work any proposal would need that touches a fundamental design choice.

[iislucas]

I agree that a fully proposal is a good idea, and I'll do that. I worked a long time in type-systems, so I'm pretty confident in my understanding of whats involved here. But there are wildly different things being suggested. So probably good to put each one into it's own discussion :)

On Fri, Aug 1, 2014 at 5:17 PM, Jonathan Turner notifications@github.com wrote:

@iislucas https://github.com/iislucas - as mentioned earlier, structural and nominal are fundamental choices in the type system. Any time you rethink part of the fundamental design choices, you need to understand the full impact. Even if it seems to be isolated to a small set of scenarios.

The best way to full understand the impact is to have a more complete suggestion. I wouldn't confuse @danquirk https://github.com/danquirk's response as throwing the baby with the bathwater, but instead as the minimal amount of work any proposal would need that touches a fundamental design choice.

— Reply to this email directly or view it on GitHub https://github.com/Microsoft/TypeScript/issues/202#issuecomment-50935866 .

Lucas Dixon | Google Ideas

[iislucas]

@jonathandturner just wanted to check what you think of the example use-cases proposed in the description. Do they make sense? Happy to clarify/expand on them if that helps (and also of https://github.com/Microsoft/TypeScript/issues/202#issuecomment-50930292 which is a case of entry (2) in the issue description)

And also w.r.t. the suggestion of @aleksey-bykov : https://github.com/Microsoft/TypeScript/issues/202#issuecomment-50199713 it seems that it could be implemented as a simple abbreviation to existing type-script without introducing any new concepts. The only optimization that would make sense to add would be for the compiler to optimize away self-referential nominal field.

I realize that I've offered to do something but not actually 100% sure what you'd like me to write - I could look through code and start pointing to how I would implement it? but maybe there's something more to do before that?

Nominal types (and new kinds of indexable types) that don't get confused between each other would be a big improvement in my opinion. They let you separate layers of abstraction and implementation details. And they are widely used in other typed functional languages too (F#, Haskell, SML/Ocaml, etc).

Thanks,

[sophiajt]

Hi @iislucas,

For a proposal for nominal types, you'll need to show strong use cases. As listed in the design goals for TypeScript: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals, being structural is something that TypeScript is committed to. The assumption that checking is structural is carried throughout the type checker, the spec, and how it's used in practice. In short, augmenting or going away from structural is going to be uphill all the way.

Adding some additional nominal checking would need to be weighed against the benefit. For this to have a chance of success, the types of new patterns that the new nominal capabilities provide would need to enable fundamental improvements in how JavaScript is typed.

Looking at the examples in the original post, I'm not sure which JavaScript patterns this enables. Starting with that might the best place. Also, you may want to look at other new functionality, like type guards, and how this might impact that.

[iislucas]

@jonathandturner I don't see why you make it seem like an exclusive choice of nominal vs structural: sometimes structural checking is what you want, sometimes nominal is. There are already a set of base nominals (string, number, etc). What I see as a good insight in typescript, and in the design goals, is that structural checking is important and will be supported. Many typed languages don't have this. It sounded to me that support for structural checking in the design goals was saying it's something TypeScript wanted to include rather than a claim about never adding an more base-types or never supporting nominal definitions. I don't see this as being a philosophical question that undermines TypeScript.

w.r.t. JavaScript patterns: it is not a JavaScript pattern that it enables. It is enables stronger levels of abstraction and more type-checking.

w.r.t. difficulty: support for nominal types is the easiest kind of extension there is to a type-system. We're not here proposing dependent types, or any fancy new computation in typing (although coming from that universe, I'm tempted... ;-) ). Just the ability to define new nominal types/interfaces.

But if I've mis-understood TypeScirpt's goals, and it is a goal to not support nominal types, then of course much stronger arguments and examples are needed.

[sophiajt]

@iislucas - TypeScript's main goal is to enable large-scale JavaScript applications. It does this by staying very close to JavaScript. Most of what the type system was built for is working with JavaScript and enabling JavaScript patterns in a type-safe way.

When we were setting ourselves up for how to go about that, we went back and forth a bit on supporting both nominal and structural. At one point, we had a structural system that would tag classes with "brands" that would allow users to use them more nominally.

The more we worked with the system, though, the more it felt like going completely structural seemed closer to the spirit of JavaScript and how flexible it is in practice. Once we realized that, we went completely in the direction of structural and aren't looking to backtrack.

That's why I said it would take a lot of strong benefits to make us backtrack and re-think going fully structural.

[iislucas]

@jonathandturner - thanks for the history and background. JS has generally been a difficult platform for creating large scale JS applications, and supporting this is why I and my team use it. Type-abstraction is specifically thought to help with this, and in my experience in other languages, it helps a lot. There's a long history of it in many languages, but primarily in the typed functional programming world (e.g. SML and Haskell). So if the problem is that it may be difficult to (re)implement, the thing to do is look into the details of what it would take to support it. If it is easy, then it's less of a issue. If it's hard and requires significant refactoring, then it makes sense to discuss examples and try and persuade more people - probably to do some research and point to it's role in other languages/frameworks.

The proposal of @aleksey-bykov : https://github.com/Microsoft/TypeScript/issues/202#issuecomment-50199713 seems easy to me. What would you say the next step in exploring this is for those of us who think it would make the language better? Is the next step a patch for typescript, or more examples using the proposed TypeScript syntax, along with errors/compilations?

[danquirk]

The implementation is not particularly difficult (nominal classes existed before, and you can hack the same effect in today via private members). What's difficult is reconciling how nominal code would interact with structural parts of a program and all the JS libraries that are built on structural typing. As mentioned, we essentially had a system like this before (classes were nominal, interfaces and object literals were structural) and it did not mesh very well with what people already do with JS.

[iislucas]

Good to know the implementation is not difficult.

w.r.t. existing libraries: how about leaving all the existing JS libraries as is (using structural typing)? Nominal types can be included incrementally in new projects to catch more errors at compile time and support abstraction in newer larger software projects - it gives a nice way to move towards increased reliability in a project, e.g. to start you might introduce some nominal types for different kinds of strings to avoid mixing them up.

I would start using new nominal types in my projects pretty quickly, but I don't mind that libraries I use don't. I can always introduce an abstraction layer (e.g. a new interface) by writing a new .d.ts file for an existing library when I feel it would help me (e.g. to start with, I might do so for things like socket identifiers for chrome-app environments, see: https://developer.chrome.com/apps/sockets_tcp ).

[zpdDG4gta8XKpMCd]

@danquirk, your major argument is that all existing libs are structural, people tend to think structural, etc

Well fine, having nominal types in addition to everything else being structural by default isn't going to hurt, is it?

The difficulties of matching structural types to nominal and shortcomings of not being able to do so 100% nicely is going to be a problem of nominal types where compromises are possible since these are the problems of minority of nominal types and doesn't affect how structural types work which are in the heart of the language.

[RyanCavanaugh]

Well fine, having nominal types in addition to everything else being structural by default isn't going to hurt, is it?

An increase in complexity needs to more than pay for itself in terms of value added. Every language feature we add is another row and column in the matrix of interactions between different aspects of the language. Especially for something as core as the rules of type compat, we're talking about permanently complicating the language in a major way and potentially preventing future features.

The thing I'm not seeing in this thread is a bunch of real-world examples where people would have gotten desired behavior with nominal types, but instead got poor behavior because of structural types. When we have more urgent gaps in the language, those examples are very common and easy to find.

[iislucas]

I don't see where the potential bad interactions would be - maybe you can say a bit more about that? Happy to think about that more and document what the expected interactions might be.

If you remove nominal types, strictly more types match, so removing it isn't difficult in the future. So, I don't see it as long-term challenge to maintain. Also, sound like it's not complicated to implement either.

w.r.t. more examples of where abstract types naturally occur, here's 3 groups that come off the top of my head:

• Socket ids in pretty much all platforms (node, Chrome Apps, Firefox Add-ons; UDP and TCP).
• User-names as keys in a table where you don't want to mix up usernames with other strings.
• Crypto-keys or passwords; again to avoid mixup with other string values.

[zpdDG4gta8XKpMCd]

@RyanCavanaugh, Tiny types is where nominals would be so very handy. Consider:

interface Age {}
interface Height {}
interface Person {
   height: Height;
   age: Age;
}

function ageFrom(value: number): Age {
   if (value > 0) return <any> value;
   throw new Error('Invalid age');
}

function heightFrom(value: number): Height {
  if (value > 0) return <any> value;
  throw new Error('Invalid height');
}

function formatAge(age: Age): string {
   return (<any> age).toFixed(0);
}

function formatHeightheight: Height) : string {
   return (<any> height).toFixed(0);
}

var person: Person = { age: ageFrom(35), height: heightFrom(180) };
console.log(formatAge(person.height)); // <= problem!

This gives another type safety measure that prevents you from mistaking a height for an age. Which otherwise would be interchangeable if plain numbers were used.

In Haskell same thing is done by newtype and it's a crazy popular feature.

Your point that there is little demand for nominal types is flawed, there is no demand because no one ever seen them in JavaScript and don't know they could have existed let alone possible at all. People including myself coming from other languages where there is a feature like this would certainly appreciate it.

[danquirk]

For that scenario my preferred solution would be something like F#'s Units of Measure ( https://github.com/Microsoft/TypeScript/issues/364). There's essentially a huge class of types inside number that cannot be specified as incompatible with one another. What you want are basically additional (user specified) primitive types.

There are scenarios where nominal types give you safety over structural types. There are also scenarios where nominal types create unnecessary friction trying to represent patterns that structural typing easily models. There are trade offs with each. Most languages I can think of have had to choose one or the other as trying to mix them is confusing or painful or both. 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). 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.

[zpdDG4gta8XKpMCd]

@danquirk, I don't mind trading nominals for units of measure if... units of measure can be applicable to any type, not necessarily just numbers. Strings are the next after numbers problematic type that just too broad to be used safely. So doing something like this for string would be just great: "aleksey.bykov@gmail.com"<email>. Now if we go further down this road why not to make Dates capable of units? I remember lots of bugs when I did heavy calculations on dates cause by mistaking one date for another. Do you think units of measure have a better chance for making it as a feature?

[danquirk]

Yeah I can see the applicability to other primitive types given JavaScript's penchant for string based APIs. We haven't had any serious discussions about UoM (ES6 features are keeping us fairly busy as far as new language stuff goes) but yes I would say it's more likely we do that at some point than add nominal semantics.

[zpdDG4gta8XKpMCd]

Put together an article on faking nominal types in TypeScript: https://gist.github.com/aleksey-bykov/0ab85f0b5e83fc848f85

[iislucas]

Thank you! This is very helpful. Obviously, I've love to see native support this in TS, but this is the nicest workaround I've seen so far.

[sampsyo]

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

class Foo {};
let x : Foo = 42;  // OK :(

class Bar {
    _nominal_Foo: void;
};
let y : Bar = 42;  // Error :)

Playground.

[basarat]

I'm using a void field as a workaround

making it any is just as good. And I prefer the term Brand as that is what is used by the TypeScript team e.g. : https://github.com/Microsoft/TypeScript/blob/7b48a182c05ea4dea81bab73ecbbe9e013a79e99/src/compiler/types.ts#L693-L698

    export interface Expression extends Node {
        _expressionBrand: any;
        contextualType?: Type;  // Used to temporarily assign a contextual type during overload resolution
    }

[zpdDG4gta8XKpMCd]

The real type or a brand propeety is an empty enum, since enums are unique in ts. On Sep 21, 2015 8:19 PM, "Basarat Ali Syed" notifications@github.com wrote:

I'm using a void field as a workaround

making it any is just as good. And I prefer the term brand as that is what is used by the TypeScript team e.g. : https://github.com/Microsoft/TypeScript/blob/7b48a182c05ea4dea81bab73ecbbe9e013a79e99/src/compiler/types.ts#L693-L698

export interface Expression extends Node {
    _expressionBrand: any;
    contextualType?: Type;  // Used to temporarily assign a contextual type during overload resolution
}

— Reply to this email directly or view it on GitHub https://github.com/Microsoft/TypeScript/issues/202#issuecomment-142144357 .

[plalx]

I agree that structural typing almost always creates abstractions that are far too wide to be practical in real-world scenarios. A concept is usually bound to a context and there's a lot of cases where two objects will accidentally share the same structure but shouldn't be interchangeable.

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: concrete classes also makes the promise of an implementation which is a form of contract.

Consider the code below:

class Username {
    public value: string;

    constructor(value: string) {
        if (!(this.value = value.trim())) throw new Error('value must not be empty');
        Object.freeze(this);
    }

    toString() { return this.value; }
}

let username: Username = { value: '' };

When Username is used as a contract it should not only carry the promise of having a value: string member, but also to be immutable and to wrap a non-empty string (supporting a final keyword would also be necessary to avoid breaking encapsulation through inheritance). The fact that you can accidentally break encapsulation could be problematic.

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 members cannot be made equal, but supporting a nominal keyword which decorates interfaces & classes would be much better.

[AJamesPhillips]

Can I confirm that this is still awaiting a proposal and that with a proposal this might move forwards? @aleksey-bykov 's contribution here looks interesting or does it need more refinement / improvement? To me though it's as "simple" as what @plalx said above:

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: concrete classes also makes the promise of an implementation which is a form of contract.

However if it was as "simple" as this it would have been implemented so I know I must be missing a lot of thinking around this simpler use case which would avoid very easy to make runtime bugs like #251.

Thanks :)

* edit * I retract my previous statement and disagree with @plalx . Interfaces should be treated nominally as well. For example if you serialised an a JetPlane, you want not want to be able to use it to construct a chicken:

interface IChicken {
  id: number;
  name: string;
}
class Chicken implements IChicken {
  id: number;
  name: string;
  constructor(kwargs: IChicken) {
    this.id = kwargs.id;
    this.name = kwargs.name;
  }
}

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

var serialisedJetPlane: IJetPlane = {id: 1, name: 'Vulcan'};
var chick = new Chicken(serialisedJetPlane);  // should error

If you actually wanted to do that then perhaps you could cast: var chick = new Chicken(<IChicken> serialisedJetPlane);

[antanas-arvasevicius]

Hi, I really miss a nominal types in TypeScript as it would allow code DSLs and other stuff more easily. Now we cannot constrain API to allow only certain type, for example

type OrderId = string;

function processOrder(id:OrderId)

now any id/string can be passed to any API and we just get very little by doing that - no compile type checking.

How could be useful nominal typing can be seen in this excellent example of usage in Scala's Squant library: http://www.squants.com/

[zpdDG4gta8XKpMCd]

@antanas-arvasevicius

nominals are scheduled some time past 2.0: https://github.com/Microsoft/TypeScript/wiki/Roadmap

for now the only type that is somewhat nominal is enum, in your example it would be

const enum AsOrderId {}
type OrderId = string & AsOrderId;
let orderId = <OrderId> '123';
orderId = '124'; // <-- problem

I am saying somewhat because enum's coerce to numbers and back.

some languages have units of measure as a first class feature: https://msdn.microsoft.com/en-us/library/dd233243.aspx

we here hope to get them too long time: https://github.com/Microsoft/TypeScript/issues/364

[antanas-arvasevicius]

Thank you Alexey! This should be enough for enforcing compiler to type check arguments.

Oh, didn't know that this kind of stuff already exists :)