Inferencing partial types in Typescript

Question

Inferencing partial types in Typescript

I'm completely stuck on this and can't seem to figure it out without using a second function:

interface Fixed { a: number }
const fn = <A, B extends {} = {}>(b: B) => {
  return b
}

fn({ a: 1 }) // { a: number }
fn<Fixed>({ a: 1 }) // {}

const fn2 = <A>() => <B extends {} = {}>(b: B) => {
  return b
}

const result = fn2<Fixed>()({ a: 1 }) // { a: number }

Why is TypeScript not able to infer the type of B when I specify type A? Everything works again if I return a function that attempts to infer the type of B.

typescript type-inference

Answer 1

Answer №1

Type inference operates on the principle of all or nothing. In the initial scenario:

fn({ a: 1 })

where no generic type parameters are provided, it will infer:

B as { a: number } based on the function argument;
and A as unknown since it is not utilized within the function.

In the subsequent case:

fn<Fixed>({ a: 1 })

when one generic type is specified, type inference will not apply to the remaining type parameters – hence:

A is set as Fixed;

B</bode> remains unspecified and defaults to <code>{}

.

This behavior may be frustrating, but it reflects the workings of TypeScript. Using two function calls, like in your second example, is a common workaround for this limitation.

Find more information about this topic in this GitHub issue.

Answer 2

Type inference operates on the principle of all or nothing. In the initial scenario:

fn({ a: 1 })

where no generic type parameters are provided, it will infer:

B as { a: number } based on the function argument;
and A as unknown since it is not utilized within the function.

In the subsequent case:

fn<Fixed>({ a: 1 })

when one generic type is specified, type inference will not apply to the remaining type parameters – hence:

A is set as Fixed;

B</bode> remains unspecified and defaults to <code>{}

.

This behavior may be frustrating, but it reflects the workings of TypeScript. Using two function calls, like in your second example, is a common workaround for this limitation.

Find more information about this topic in this GitHub issue.

Answer 3

Answer №2

In my opinion, the concept of currying provides a comprehensive solution. Here is a brief example to illustrate:

const x = <I>(v: I) => <O>((v: I) => O) => O

// fully typed
x<number>(10)<string>((n) => `${n * 2}`);

// partial inference NO.1
x(10)<string>((n) => `${n * 2}`);

// partial inference NO.2
x<number>(10)((n) => `${n * 2}`);

// full inference
x(10)((n) => `${n * 2}`);

Answer 4

In my opinion, the concept of currying provides a comprehensive solution. Here is a brief example to illustrate:

const x = <I>(v: I) => <O>((v: I) => O) => O

// fully typed
x<number>(10)<string>((n) => `${n * 2}`);

// partial inference NO.1
x(10)<string>((n) => `${n * 2}`);

// partial inference NO.2
x<number>(10)((n) => `${n * 2}`);

// full inference
x(10)((n) => `${n * 2}`);

Answer 5

Answer №3

To work around this issue, one possible solution is to introduce a second parameter solely for the purpose of type inference. This can be achieved by explicitly casting an empty object {} to the desired type:

interface Specified { x: string }
const func = <X, Y extends {} = {}>(y: Y, _placeholder_for_type_inference: X) => {
  return y
}

func({ x: 'example' }, {} as Specified)
/*
const func: <Specified, {
    x: string;
}>(y: {
    x: string;
}, _placeholder_for_type_inference: Specified) => {
    x: string;
}
*/

Answer 6

To work around this issue, one possible solution is to introduce a second parameter solely for the purpose of type inference. This can be achieved by explicitly casting an empty object {} to the desired type:

interface Specified { x: string }
const func = <X, Y extends {} = {}>(y: Y, _placeholder_for_type_inference: X) => {
  return y
}

func({ x: 'example' }, {} as Specified)
/*
const func: <Specified, {
    x: string;
}>(y: {
    x: string;
}, _placeholder_for_type_inference: Specified) => {
    x: string;
}
*/

Inferencing partial types in Typescript

Answer №1

Answer №2

Answer №3

Similar questions

NPM Package: Accessing resources from within the package

Error message TS2339 in Typescript: The property 'super' is not found on the type '($element: any, options: any) => any'

Unexpected Null Object Error in TypeScript Function

Exploring the power of Next.js, Styled-components, and leveraging Yandex Metrica Session Replay

You cannot employ typed arguments in combination with Typescript within the VueJS framework

Reasons why a functional component may not trigger a rerender after a state change using useReducer()

I am struggling to understand the significance of the $ symbol in this particular context

Encountering error codes TS1005 and TS1109 while trying to run an Angular 6 project

Utilizing TypeScript in an AngularJS (1.x) project alongside Webpack: A Step-By-Step Guide

What is the reason for Object.keys not returning a keyof type in TypeScript?

The issue I'm facing with my webpack-build is the exclusive appearance of the "error" that

Leveraging Json data in Angular components through parsing

Tips for selecting the best className type for material-ui components

Unable to determine the data type of the JSON object during the

Transform leaflet marker plugin into Typescript format

io-ts: Defining mandatory and optional keys within an object using a literal union

What methods are available to expedite webpack compilation (or decouple it from server restart)?

Issue arises when isomorphic-dompurify is used alongside dompurify in Next.js 13 causing compatibility problems

Why does Material-UI's "withStyles()" not function properly with a specified constructor in Typescript?

Tips for simulating behavior in express using Typescript and the Mocha library

Inferencing partial types in Typescript

Answer №1

Answer №2

Answer №3

Similar questions

NPM Package: Accessing resources from within the package

Error message TS2339 in Typescript: The property '__super__' is not found on the type '($element: any, options: any) => any'

Unexpected Null Object Error in TypeScript Function

Exploring the power of Next.js, Styled-components, and leveraging Yandex Metrica Session Replay

You cannot employ typed arguments in combination with Typescript within the VueJS framework

Reasons why a functional component may not trigger a rerender after a state change using useReducer()

I am struggling to understand the significance of the $ symbol in this particular context

Encountering error codes TS1005 and TS1109 while trying to run an Angular 6 project

Utilizing TypeScript in an AngularJS (1.x) project alongside Webpack: A Step-By-Step Guide

What is the reason for Object.keys not returning a keyof type in TypeScript?

The issue I'm facing with my webpack-build is the exclusive appearance of the "error" that

Leveraging Json data in Angular components through parsing

Tips for selecting the best className type for material-ui components

Unable to determine the data type of the JSON object during the

Transform leaflet marker plugin into Typescript format

io-ts: Defining mandatory and optional keys within an object using a literal union

What methods are available to expedite webpack compilation (or decouple it from server restart)?

Issue arises when isomorphic-dompurify is used alongside dompurify in Next.js 13 causing compatibility problems

Why does Material-UI's "withStyles()" not function properly with a specified constructor in Typescript?

Tips for simulating behavior in express using Typescript and the Mocha library

Error message TS2339 in Typescript: The property 'super' is not found on the type '($element: any, options: any) => any'