The correlation between methods in programming languages

Question

The correlation between methods in programming languages

Can a class or object be created with type constraints between methods?

abstract class Example<T>{
  abstract methodOne(): T
  abstract methodTwo (arg: T):any
}

I am looking to ensure that the argument of methodTwo is the same type as the return type of methodOne - any differences should result in a type error. The specific type itself is not the main focus.

While I was able to achieve this using a generic class, I found it cumbersome to have to specify the type in advance, which feels unnecessary to me.

Is there a way to accomplish this without needing to define the type beforehand, or without relying on a generic class?
As an additional question - is it feasible to reference another method's class or return type? For example, something like
```
methodTwo(arg: ReturnType<this.methodOne>):any
```
or
```
methodTwo(arg: typeof this.methodOne):any
```
?

typescript generics

Answer 1

Answer №1

Resolution

It is recommended to utilize

ReturnType<Test['method1']>

in place of ReturnType<this.method1>:

class NewClass{
  newMethod(): number {
    return 1;
  }
}

let newFunction = function() : boolean {
  return true;
}

class AnotherClass{
  methodA(): string {
    return '';
  };
  methodB (arg: ReturnType<AnotherClass['methodA']>): any {
    return null;
  }
  methodC (arg: ReturnType<NewClass['newMethod']>): any {
    return null;
  }
  methodD (arg: ReturnType<typeof newFunction>): any {
    return null;
  }
}

let anotherTest = new AnotherClass();

anotherTest.methodB('abc'); // Valid
anotherTest.methodB(3); // TypeScript compilation error. Expects a string

anotherTest.methodC(6); // Valid
anotherTest.methodC('xyz'); // TypeScript compilation error. Expects a number

anotherTest.methodD(false); // Valid
anotherTest.methodD('def'); // TypeScript compilation error. Expects a boolean

View and test this code with the TypeScript errors on stackblitz.

Note: The usage of ReturnType<> is supported from TypeScript version 2.8 onwards.

Answer 2

Resolution

It is recommended to utilize

ReturnType<Test['method1']>

in place of ReturnType<this.method1>:

class NewClass{
  newMethod(): number {
    return 1;
  }
}

let newFunction = function() : boolean {
  return true;
}

class AnotherClass{
  methodA(): string {
    return '';
  };
  methodB (arg: ReturnType<AnotherClass['methodA']>): any {
    return null;
  }
  methodC (arg: ReturnType<NewClass['newMethod']>): any {
    return null;
  }
  methodD (arg: ReturnType<typeof newFunction>): any {
    return null;
  }
}

let anotherTest = new AnotherClass();

anotherTest.methodB('abc'); // Valid
anotherTest.methodB(3); // TypeScript compilation error. Expects a string

anotherTest.methodC(6); // Valid
anotherTest.methodC('xyz'); // TypeScript compilation error. Expects a number

anotherTest.methodD(false); // Valid
anotherTest.methodD('def'); // TypeScript compilation error. Expects a boolean

View and test this code with the TypeScript errors on stackblitz.

Note: The usage of ReturnType<> is supported from TypeScript version 2.8 onwards.

Answer 3

Answer №2

If you're looking to create linked pairs of functions for any type T, rather than using functions with specific types, you'll need a generic type parameter in order to connect the return type of one function to the parameter of another.

One approach could be:

interface Linked<T> {
    (): T;
    method2: (arg: T) => any;
}

const method1String: Linked<string> = () => "foo";
method1String.method2 = arg => arg.length;

When trying this out, it offers two advantages:

You'll receive a compiler error if the method2 property is missing.
The argument type for method2 is correctly inferred, and you'll get an error if you try to define it differently.

However, understanding the specific context is important here. If these functions are meant to work together in some way, why not simply constrain them like this:

function doSomethingWithLinkedFunctions<T>(function1: () => T, function2: (arg: T) => any) {
}

This approach avoids the need for awkward data types and allows the inference of T from function1.

Alternatively, if the pair of functions must always be used together:

type Linked<T> = [() => T, (arg: T) => any];

function doSomething([func1, func2]: Linked<string>) {
}

Answer 4

If you're looking to create linked pairs of functions for any type T, rather than using functions with specific types, you'll need a generic type parameter in order to connect the return type of one function to the parameter of another.

One approach could be:

interface Linked<T> {
    (): T;
    method2: (arg: T) => any;
}

const method1String: Linked<string> = () => "foo";
method1String.method2 = arg => arg.length;

When trying this out, it offers two advantages:

You'll receive a compiler error if the method2 property is missing.
The argument type for method2 is correctly inferred, and you'll get an error if you try to define it differently.

However, understanding the specific context is important here. If these functions are meant to work together in some way, why not simply constrain them like this:

function doSomethingWithLinkedFunctions<T>(function1: () => T, function2: (arg: T) => any) {
}

This approach avoids the need for awkward data types and allows the inference of T from function1.

Alternatively, if the pair of functions must always be used together:

type Linked<T> = [() => T, (arg: T) => any];

function doSomething([func1, func2]: Linked<string>) {
}

The correlation between methods in programming languages

Answer №1

Resolution

Answer №2

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