What is the method to define a function that strictly accepts a value of type T only if IsSomething<T> evaluates to true?

Question

What is the method to define a function that strictly accepts a value of type T only if IsSomething<T> evaluates to true?

In my system, there is a complex generic type called IsUnique<T> that evaluates to either true or false based on the input type T:

type IsUnique<T> = (/* ... */) ? true : false;

Now I am looking to develop a function that takes an unique value as the first argument, meaning a value with type U which would result in IsUnique<U> being true. How can I achieve this task?

const processUniqueValue: WhichFunctionTypeShouldIGoFor = value => {
  // ...
};

let x: T1; // Assuming `IsUnique<T1>` is evaluated as `true`
let y: T2; // Assuming `IsUnique<T2>` is evaluated as `false`

processUniqueValue(x); // Should work without any issues
processUniqueValue(y); // Must not work during compilation

typescript

Answer 1

Answer №1

If you make a slight modification to IsCool<T> in this manner:

type IsCool<T> = (/* ... */) ? T : never;

You can now define your function as follows:

const performOperationOnCoolValue = <T>(value: IsCool<T>) => {
  // ...
};

The compiler will give an error if you attempt to invoke performOperationOnCoolValue with a variable that would result in never for IsCool<type>.

Answer 2

If you make a slight modification to IsCool<T> in this manner:

type IsCool<T> = (/* ... */) ? T : never;

You can now define your function as follows:

const performOperationOnCoolValue = <T>(value: IsCool<T>) => {
  // ...
};

The compiler will give an error if you attempt to invoke performOperationOnCoolValue with a variable that would result in never for IsCool<type>.

What is the method to define a function that strictly accepts a value of type T only if IsSomething<T> evaluates to true?

Answer №1

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