Question

How to use higher-order functions in Rust

Answer 1

// A higher‑order function is a function that does at least one of the following:
// 1. Takes another function as an argument
// 2. Returns a function as its result
// If it does either one, it qualifies.

// Higher-order function: takes a function as an argument
fn apply_twice<F>(fnc: F, x: i32) -> i32
where
    F: Fn(i32) -> i32,
{
    fnc(fnc(x))
}

// A simple function to pass in
fn add3(n: i32) -> i32 {
    n + 3
}

fn main() {
    // Use the higher-order function
    let result = apply_twice(add3, 5); // 5 + 3 = 8 + 3 = 11

    println!("{}", result);
}


/*
run:

11

*/

 

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Answer 2

// A higher‑order function is a function that does at least one of the following:
// 1. Takes another function as an argument
// 2. Returns a function as its result
// If it does either one, it qualifies.

// makeMultiplier is a higher‑order function because it RETURNS another function
fn make_multiplier(n: i32) -> impl Fn(i32) -> i32 {

    // This inner function forms a closure and remembers the value of n
    move |x| x * n // Uses the captured value n
}

fn main() {
    // double_val is now a function created by makeMultiplier(2)
    // It remembers n = 2 through closure
    let double_val = make_multiplier(2);

    // Calling double_val(10) multiplies 10 by the captured n (which is 2)
    println!("{}", double_val(10));
}


/*
run:

20

*/

 

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