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 [email protected] 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 [email protected] 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.