Using Typescript to specify the parameter type of a function as a generic function

Question

Using Typescript to specify the parameter type of a function as a generic function

After creating a function called compose, it looks like this:

const composeTyped = <T, U, R>(f: (x: T) => U, g: (y: U) => R) => (x: T) => g(f(x));

It appears to me that both functions f and g fall under the type fGeneric, which is defined as follows:

type fGeneric = <T, R>(arg: T) => R;

The issue I'm facing is understanding whether and how I can utilize the fGeneric type to specify the types of f and g in composedType. Specifically, if I attempt the following:

const composeTyped_ = <T, U, R>(f: fGeneric, g: fGeneric) => (x: T) => g(f(x));

The composeTyped_ function ends up with the type (x: T) => unknown. However, my goal is to have it be of type (x: T) => R.

typescript typescript-generics

Answer 1

Answer №1

To make fGeneric accept generic type arguments, it needs to be defined as follows:

type fGeneric<T, R> = (arg: T) => R;

Next, the function composeTyped_ can be implemented like this:

const composeTyped_ = <T, U, R>(
  f: fGeneric<T, U>,
  g: fGeneric<U, R>
) => (x: T) => g(f(x));

After defining these functions, you can use them as shown below:

declare const f: (str: string) => number

declare const g: (num: number) => null

// This call should now work without any issues
composeTyped_(f, g)

// The following call will produce an error message due to incompatible types being passed
composeTyped_(g, f)

Answer 2

To make fGeneric accept generic type arguments, it needs to be defined as follows:

type fGeneric<T, R> = (arg: T) => R;

Next, the function composeTyped_ can be implemented like this:

const composeTyped_ = <T, U, R>(
  f: fGeneric<T, U>,
  g: fGeneric<U, R>
) => (x: T) => g(f(x));

After defining these functions, you can use them as shown below:

declare const f: (str: string) => number

declare const g: (num: number) => null

// This call should now work without any issues
composeTyped_(f, g)

// The following call will produce an error message due to incompatible types being passed
composeTyped_(g, f)

Using Typescript to specify the parameter type of a function as a generic function

Answer №1

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