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Support some non-structural (nominal) type matching #202

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iislucas opened this issue Jul 22, 2014 · 387 comments · May be fixed by #33038
Open

Support some non-structural (nominal) type matching #202

iislucas opened this issue Jul 22, 2014 · 387 comments · May be fixed by #33038

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@iislucas
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@iislucas iislucas commented Jul 22, 2014

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
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@RyanCavanaugh RyanCavanaugh commented Jul 23, 2014

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

+Needs Proposal

@iislucas
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@iislucas iislucas commented Jul 23, 2014

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
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@samwgoldman samwgoldman commented Jul 23, 2014

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

@RyanCavanaugh
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@RyanCavanaugh RyanCavanaugh commented Jul 23, 2014

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
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@iislucas iislucas commented Jul 23, 2014

@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
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@danquirk danquirk commented Jul 23, 2014

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

@iislucas iislucas changed the title Support non-structural (abstract) types Support some non-structural (nominal) type matching Jul 23, 2014
@zpdDG4gta8XKpMCd
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@zpdDG4gta8XKpMCd zpdDG4gta8XKpMCd commented Jul 25, 2014

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
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@iislucas iislucas commented Jul 26, 2014

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
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@basarat basarat commented Jul 26, 2014

@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
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@Steve-Fenton Steve-Fenton commented Jul 31, 2014

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
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@basarat basarat commented Jul 31, 2014

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

👍

@Steve-Fenton
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@Steve-Fenton Steve-Fenton commented Jul 31, 2014

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
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@RyanCavanaugh RyanCavanaugh commented Jul 31, 2014

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

What would that mean?

@Steve-Fenton
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@Steve-Fenton Steve-Fenton commented Jul 31, 2014

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
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@RyanCavanaugh RyanCavanaugh commented Jul 31, 2014

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
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@Steve-Fenton Steve-Fenton commented Jul 31, 2014

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
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@Steve-Fenton Steve-Fenton commented Jul 31, 2014

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
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@basarat basarat commented Aug 1, 2014

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
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@ComFreek ComFreek commented Aug 1, 2014

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.

@jonathandturner
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@jonathandturner jonathandturner commented Aug 1, 2014

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
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@danquirk danquirk commented Aug 1, 2014

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
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@Steve-Fenton Steve-Fenton commented Aug 1, 2014

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
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@danquirk danquirk commented Aug 1, 2014

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
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@iislucas iislucas commented Aug 1, 2014

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
#202 (comment)
.

Lucas Dixon | Google Ideas

@jonathandturner
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@jonathandturner jonathandturner commented Aug 1, 2014

@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
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@iislucas iislucas commented Aug 1, 2014

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
#202 (comment)
.

Lucas Dixon | Google Ideas

@artalar
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@artalar artalar commented Sep 3, 2019

Awesome! It will so helpful for refinement types in https://github.com/rtcad/rtcad/blob/master/src/__tests__/index.ts

@lu4
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@lu4 lu4 commented Sep 29, 2019

Ok, I've came to another way to declare a nominal type, it allows somewhat little more comfortable declaration, though two declarations per type are necessary:

export type Unique<T> = T & { readonly '': unique symbol };

export enum DbProto { }
export type DbContext = Unique<DbProto>;

export enum BatchProto { }
export type BatchContext = Unique<BatchProto>;

Use cases:

enum EnumType { }

var x: EnumType = 1; // Not fails => not typesafe
var y: Unique<EnumType> = 1; // Fails => typesafe
var z: Unique<EnumType> = 1 as Unique<EnumType>; // More or less ok since the user has provided the proof that he knows what he is doing
y = z; // Ok, typesafe
y = y + z; // Fails, typesafe
y += 1; // Fails, typesafe
y += z; // Fails, typesafe

However:
y++; // Not fails => not typesafe!

Also it is possible to declare only the EnumType and use Unique<EnumType> everywhere, like so:

export type Unique<T> = T & { readonly '': unique symbol };

export enum DbProto { }
export enum BatchProto { }

var x: Unique<DbProto> = 1 as Unique<DbProto>

@BobNobrain
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@BobNobrain BobNobrain commented Oct 30, 2019

For all the cases above, it seems like we can use something like "narrowing" mechanism for primitive types. The case is that we need to have types like Guid that are represented with a string but indeed are subtypes of it. But since such a type includes too many values to enumerate in union type (possibly even an infinite amount of values), we just need another way to tell the compiler that our Guid type is a subtype of string, for example, like this:

type Guid extends string; // AFAIK, this will not break any existing syntax

So the compiler knows that every type that extends string is just its another subtype, and all of these subtypes are not assignable to each other by default. Consider this example:

type A extends string;
type B extends string;

declare let a: A;
declare let b: B;
declare let s: string;

s = a; // ok
s = b; // ok
a = s; // error; need to use typecast
a = s as a; // ok
a = b; // error;
a = b as a; // error, '... neither type sufficiently overlaps with the other ...'

This also can be enhanced with more advanced subtyping, like type A extends string; type B extends A;, and type C extends B, A (meaning that C is a subset of intersection of A and B).

Indeed, this is still a kind of nominal typing, since the compiler knows nothing about Guid internal structure and therefore it has to resolve subtypes using their names. But it seems to create an isolated subset of types with no interference with any existing ones that are structurally compared, so I consider this to be easier to implement.

@unional
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@unional unional commented Jan 4, 2020

In case it is useful in this discussion, I learn that there are at least two flavor of nominal types.
I expose those types in https://github.com/unional/type-plus
They are described by Drew Colthorp in this article: https://spin.atomicobject.com/2018/01/15/typescript-flexible-nominal-typing/#comment-604580

@MaxGraey
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@MaxGraey MaxGraey commented Jan 5, 2020

How about newtype similar to Haskell?

newtype f32 = number;
newtype f64 = f32;

or (with avoiding extra keyword reservation)

type f32 = new number;
type f64 = new f32;
@omidkrad
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@omidkrad omidkrad commented Feb 8, 2020

My workaround for this currently is to use a marker interface like this.

interface TickerSymbol extends String {}

The only problem is that when I want to use it as a index key, I have to cast it to string.

interface TickerSymbol extends String {}
var symbol: TickerSymbol = 'MSFT';
// declare var tickers: {[symbol: TickerSymbol]: any}; // Error: index key must be string or number
declare var tickers: {[symbol: string]: any};
// tickers[symbol]; // Type 'TickerSymbol' cannot be used as an index type
tickers[symbol as string]; // OK

However, JavaScript seems to be fine with index type of String (with capital S).

var obj = { one: 1 }
var key = new String('one');
obj[key]; // TypeScript Error: Type 'String' cannot be used as an index type.
// but JS gives expected output:
// 1
@whzx5byb
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@whzx5byb whzx5byb commented Feb 23, 2020

Using typeof X where X is a symbol constant is better than unique symbol.

Consider this code:

export type Nominal<T, U extends string> = T & { [K in U]: unique symbol };

type Foo = Nominal<Object, 'Foo'>;
let foo: Foo = { Foo: Symbol() };  // should have an error here, but succeed.

A better solution:

const UniqueSymbol = Symbol();
export type Nominal<T, U extends string> = T & { [K in U]: typeof UniqueSymbol };

type Bar = Nominal<Object, 'Bar'>;
let bar: Bar = { Bar: Symbol() }; 

The constant symbol UniqueSymbol could be hidden in an external module which only exposes Nominal as a helper type.

@ljharb
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@ljharb ljharb commented Feb 23, 2020

That doesn’t work for types that aren’t symbols, like if i want a “FooID” string type to be unique (yet be recognized as a string)

@pauldraper
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@pauldraper pauldraper commented Feb 26, 2020

@lu4 points out that TypeScript already has nominal typing in symbols and enums.

Perhaps someone familiar with those could describe if the same approach could be used for other nominal types.

Language-wise newtype-like proposal seems straightforward.

type meters = new number;
type width = new meters;

type thing = { a: string };
type otherthing = new thing;
@kristiandupont
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@kristiandupont kristiandupont commented Feb 26, 2020

For what it's worth, this used to be very important to me but I found once I let my OCD calm down, the hacks that exist work really well. Especially because I ended up using "flavoring" instead of "branding" (https://spin.atomicobject.com/2018/01/15/typescript-flexible-nominal-typing/). I imagine an official solution would give me semantics closer to branding.

@ExE-Boss
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@ExE-Boss ExE-Boss commented Feb 26, 2020

There’s also somewhat nominal typing for classes with private fields (both hard and soft private):

declare class FooHardPrivate {
	#private;
}

// https://github.com/sandersn/downlevel-dts result:
declare class FooSoftPrivate {
	private "#private";
}

See TypeScript FAQ § When and why are classes nominal? for details.

@ryansmith94
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@ryansmith94 ryansmith94 commented Jun 18, 2020

I've been working on a collection of validation rules and branded types which have been making this easier for us. Sharing it here in case it's useful to anyone.
https://github.com/ryansmith94/rulr

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