Typescript: generic that extends a type with a generic

Question

Say I have an interface

interface Applicative {}

Now I want to define a function f that:

• takes a funct

Answer 1

This is not exactly what you are looking for but it is as close as I think you can get:

interface Applicative {}

function f(fn: Function, a: U & Applicative): U & Applicative {

    return null;
}

a will have to be both U (whatever U is) and Applicative. U can not be defined to be a generic type explicitly I am afraid.

Better typing can be achieved by:

function f(fn: Function, a: U & Applicative): U & Applicative { }

I am not entirely sure that the return type in my example is exactly what you want. But you should be able to achieve your required result by adding/changing the required interface on the return type e.g.:

function f(fn: Function, a: U & A): U & A
function f(fn: Function, a: U & A): U & A & B

Or something similar.