Syntax for hinting literal type inference

Now that we have so many literal types we more than ever need new syntax that would make their use natural. Please consider the following:

const value = (true); // true
const value = true; // boolean

const value = ('a'); // 'a'
const value = 'a'; // string

const value = (1); // 1
const value = 1; // number

const value = ['a', 1]; // (string | number)[]
const value = (['a', 1]) // [string, number]
const value = ([('a'), (1)]) // ['a', 1];

Problem:

There is no way to get a literal type value without explicit type annotation.

Solution:

Add new syntax (or shall we say some new semantics to the old mostly unused syntax)

Highlights:

Composable.
Works fine with the existing syntax, doesn't need a new one.
Highly unlikely to be a breaking change.
Doesn't affect JavaScript semantics at all, can be executed as written and will work as expected.

Shortcomings:

Undiscoverable syntax.
Semantic conflicts (breaking changes) although rare:
- generated code
- conditional expressions
- unintended excessive syntax

Prior work:

Inspired by consider syntax for tuple literals: (['a', 2]) // [string, number] #9217 which is closed in favor of this one.

Can you explain what the problem with this syntax is? Too verbose? This works in Typescript today.

interface Tuple {
    [0]: string;
    [1]: number;
}

var value: Tuple = ['a', 1]; // type checks correctly

In fact, just tested and this works too:

interface Tuple<U, T> {
    [0]: U;
    [1]: T;
}

var value: Tuple<string, number> = ['a', 1];

I assume the problem here is that you need to manually annotate the value? You want sane implicit typing of literals?

Problem:
There is no way to get a literal type value without explicit type annotation.
yes
yes
yes

Ah, gotcha. Yeah, I hardly ever use implicit types, I manually type everything explicitly. I get the need for this proposal, but I personally think the proposed syntax is unexpected behavior. I don't think it will break anything, but there seem to be a lot of edge cases and as a user, I don't expect (true) to type differently than true. I don't like surprises in my type checking.

Personally, I would just write this and make the intent explicit:

const value = ['a', 1] as [string, number];

Of note, you can't use literal types in "as" notation, which I would propose to fix this issue personally:

const value = true as true;

could you give me an example when you casually type (true), please?

point is it's the waste of syntax, no one types (true) in their everyday work, why not to give it a better use?

A quick search of my code base of the previous big project I did in Typescript gives me this:

var borderWidth = (4);

...

borderWidth = 12;

I assume this breaks with your proposal? In which case, yes, your proposal would break the last big codebase I worked on. Why are those parenthesis there? I assume at one point it said something like borderWidth = (4 * someOtherValue);. Real life code bases are messy, someone forgot to take those parenthesis out. I wouldn't mind something like that breaking with an update of TypeScript btw, just saying that people do casually type stuff like that.

you are lucky to find one place that, well, was left unattended, rather than crafted the way it is on purpose, and it's just one scalepan... - your unintentionally overlooked code, the other scalepan is a new feature that enables the whole new world of exciting opportunities and universal happiness, now what exactly are we arguing about?

We're not arguing, I said from the beginning I like the idea of the feature 😉

I'm just not sure if the proposed syntax is to my liking, seems a bit unexpected. Then again, as I said, I don't use implicit typing, so it really doesn't mean much to me.

then give me some thumb's up's!

unexpectedness of the syntax is already spotted (in the parent proposal), admitted and listed here under "Shortcomings"

i confess i lived a sinful life, the proposal is not 100% perfect

Just one more observation: if I have to manually type those parenthesis, then I'm explicitly annotating that literal. I don't think your proposal is implicit annotation at all, it's just a shorthand explicit annotation. The shorthand is universal and saves you from explicitly mentioning the type, but it's explicit regardless. Anyway, I'm knee deep in physics integrators right now, time to get back.

hmmm how about a new operator? := This way, if the compiler, by any chance can infer the value it will explicitly assing the type.

const a := 1; // a is 1

// Can work with expression
const b := (Math.random() * 0); // b is 0
// As it won't be confused with explicit parentheses like:
const b = (Math.random() * 0) + 1; // b is number even though b is always 1

const c := Math.random(); // c is number

const d := 'hello'; // d is 'hello'

const e := !false; // e is true

:= kinda aliases explicit type annotation const a: 1 = 1 => const a := 1, the syntax is new and kinda expected 😁

well yeah, i never said i wanted it implicit, all i want it to get rid of

... explicit type annotations

while still being explicit about my intentions at defining a literal value

Yup, I get it now. I looked over your initial thread too. I'll give this a thumbs up, I do think it would be useful and it does have a parallel to the arrow syntax.

Ali Taheri Moghaddar (@alitaheri)

Ali Taheri Moghaddar (@alitaheri) good catch!

i agree the condition expression of the ternary operator is usually tend to be braced in parenthesis

here is the thing though:

we don't care about any expressions other than literal ones

a literal expression is an expression whose terms DO NOT contain variables

i hate the flatness of this statement but nevertheless it would enable what's required it if accepted

Ali Taheri Moghaddar (@alitaheri)

Ali Taheri Moghaddar (@alitaheri)

:= is limiting it to assignment cases only

how about binding arguments to parameters?

function id<a>(value: a): a { return value; }
const value = id('hey');

Igor Oleinikov (@Igorbek)

Igor Oleinikov (@Igorbek) Well that's true in that particular case, but let's take a look at another example that I just encountered:

function select<T, R extends {[P in ValueKey]: T}, ValueKey extends string = "code">(item: T, values: R[], options: {valueKey: ValueKey} = {valueKey: "code"} as any) {
    return values.find(value => value[options.valueKey] === item);
}

select(1, [{code: 1, label: "hello"}]); // Works, ValueKey = "code" which is the default.
select(1, [{id: 1, label: "hello"}], {valueKey: "id"}); // Error, ValueKey = string, understands that values should be {[key: string]: number} and label is a string
select(1, [{id: 1, label: "hello"}], {valueKey: "id" as "id"}); // Works, ValueKey = "id"

Of course, if the compiler could infer the type properly that would be great, but I'm not really sure how possible this is, especially if we care about backward compatibility.

I hope my PR #17785 would address this, by allowing people to reuse the const vs. let distinction to indicate whether they want [1,2,3] or number[]. There's obviously no silver bullet (what of [number]? (1|2|3)[]?), so there will always be cases where you may need casts. I think where this PR adds value though is by increasing user control.

I saw some comments proposing syntax that would enable literal inference only for assignation (:=), this is not enough. I type most of my declarations explicitly but I still had issues because of lack of literal type narrowing when I updated one of my libraries to use mapped types. Mapped types improved the "correctness" of the types by removing some anys but require the types of the various objects to be better inferred.

Here is a minimal example exposing the issue:

// Lib part: Provides classes to build schemas and test them at runtime

interface MetaType<T> {
  test(val: any): val is T;
}

// Represents a specific variant from an enum
class EnumLit<T> implements MetaType<T> {
  variant: T;
  constructor(enumVariant: T) {
    this.variant = enumVariant;
  }

  test(val: any): val is T {
    return val === this.variant;
  }
}

// Represents an object with multiple properties, each with their own type
class Doc<T extends {}> implements MetaType<T> {
  props: {[P in keyof T]: MetaType<T[P]>};
  constructor(props: {[P in keyof T]: MetaType<T[P]>}) {
    this.props = props;
  }

  test(val: any): val is T {
    for (const k in this.props) {
      if (!this.props[k].test(val[k])) { return false; }
    }
    return true;
  }
}

// User code

enum AnimalName {
  Duck,
  Cat,
}

interface Duck {
  name: AnimalName.Duck;
}

// This breaks because {name: MetaType<AnimalName>} is not assignable to {name: MetaType<AnimalName.Duck>}
// This worked previously because Doc.params was just `{[P in keyof T]: MetaType<any>}`
const $Duck = new Doc<Duck>({name: new EnumLit(AnimalName.Duck)});

// You have to explicitly state the generic parameter of EnumLit (really heavy due to repetition)
const $Duck2 = new Doc<Duck>({name: new EnumLit<AnimalName.Duck>(AnimalName.Duck)});

Complete error:

error TS2345: Argument of type '{ name: EnumLit<AnimalName>; }' is not assignable to parameter of type '{ name: MetaType<AnimalName.Duck>; }'.
  Types of property 'name' are incompatible.
    Type 'EnumLit<AnimalName>' is not assignable to type 'MetaType<AnimalName.Duck>'.
      Types of property 'test' are incompatible.
        Type '(val: any) => val is AnimalName' is not assignable to type '(val: any) => val is AnimalName.Duck'.
          Type predicate 'val is AnimalName' is not assignable to 'val is AnimalName.Duck'.
            Type 'AnimalName' is not assignable to type 'AnimalName.Duck'.

Regarding the syntax bikeshedding, the idea of parens is nice but I agree that it can be confusing and may break many code generation tools.
I'd propose an addition similar to the ! assertion operator: add a unary "literal type narrowing" operator. For example @ or # are unused currently.

Here is a comparison of what the various propositions may look like in my example:

// Parens (not very readable)
const $Duck = new Doc<Duck>({name: new EnumLit((AnimalName.Duck))});

// Diamond
const $Duck = new Doc<Duck>({name: new EnumLit(<> AnimalName.Duck)});

// Unit
const $Duck = new Doc<Duck>({name: new EnumLit(<unit> AnimalName.Duck)});

// Prefix @
const $Duck = new Doc<Duck>({name: new EnumLit(@AnimalName.Duck)});

// Postfix @
const $Duck = new Doc<Duck>({name: new EnumLit(AnimalName.Duck@)});

// Prefix #
const $Duck = new Doc<Duck>({name: new EnumLit(#AnimalName.Duck)});

// Postfix #
const $Duck = new Doc<Duck>({name: new EnumLit(AnimalName.Duck#)});

This operator could also be applied to expressions to ask the compiler to resolve the most specific type.
For example @(1 + 2) would be typed as 3.

Charles Samborski (@demurgos)

Charles Samborski (@demurgos) I'd argue specific types shouldn't require additional effort as type widening is mostly useful under specific circumstances (mutable variable, i.e. var/let assignment), meaning we already have a decent idea what default makes sense when.

For example @(1 + 2) would be typed as 3.

Seems they didn't like this, see #15645.

m93a

While it only works for a limited amount of cases, I would suggest overloading the ! postfix operator when used on literal value expressions; since we know that literally 1! would never be nullable, this now means that it's exactly one. I would also think that this has lower impact than the parens idea, since the postfix ! is already a typescript-only syntax, and the only people writing 1! would be doing it as a typo.

So, some examples using postfix !
(filtering out those that have been solved by #10676)

const value = ['a', 1]; // (string | number)[]
const value = ['a', 1]!; // [string, number]
const value = ['a'!, 1!]!; // ['a', 1]
const value = ['a'!, 1!]; // ('a' | 1)[]

const value = {a: 1} // {a: number}
const value = {a: 1!} // {a: 1}

cases that it doesn't solve

const foo = 'foo' // 'foo'
const bar = [foo!]! // would still be [string]
const value = {a: foo!} // still {a: string}

The other syntax solution I can think of (since I like keywords more than characters :P ) is to use as const as a postfix, for example

const value = {a: foo as const}

Or, maybe both? Allow postfix ! to narrow when it's a literal, and as const for more complex mappings?

Edit (2019-01-14): m93a as submitted this as a separate issue as #26979

good thing is that we can piggyback ride on the existing TypeScript only expression level syntax ! (which is so much at odds with its design goals but who cares right?)

bad part is that there is no way to see what is going on in const value = {a: foo!} without knowing what foo is, it's going to be a nightmare for code reviewers like myself

My interpretation was that ! would only have the literalizing effect on true literal expressions; even ("foo")! should be a no-op IMO. Otherwise you get into a ridiculous situation when expr: "foo" | null - do you then have to write expr!! to prevent it widening?

Yup, exactly what Ryan is saying. The ! would only apply on literals, and so const value = {a: foo!} would unambiguously be a non-null assertion.

Emily Marigold Klassen (@forivall)

Emily Marigold Klassen (@forivall) I coincidentally opened an issue about that syntax. Could you discuss about that in #22872 if you prefer?

This is great, why can't we focus on just string literal type for now?

let value = 'myType'; // string
let value = `myType`; // 'myType'
const value = ['myType']; // string[]
const value = [`myType`]; // 'myType'[]
const value = {a: 'myType'} // {a: string}
const value = {a: `myType`} // {a: 'myType'}

type myType = 'myType'; // OK
type myType = `myType`; // Error
let myType = `myType`; // OK 'myType'

Bastian (@aloifolia)

Bastian (@aloifolia)

Maybe the problem could be mitigated by telling the compiler more precisely which objects (i.e. standard objects and arrays) are actually constant and which should be changeable. Thus, the as const proposal by Emily Marigold Klassen (@forivall) seems to be a reasonable solution (although a bit cumbersome to use - which could be eased by having some sort of top-down cascading behaviour for, say as const!).

I proposed something like this too in #20195, which is to extend the readonly operator to be applicable to object (and array) literals, e.g.

const o = readonly { x: 3, y: 'hello' };
// o: { readonly x: 3; readonly y: 'hello' }

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