Closures as Function Parameters

Introduction

In this lab, you will learn how functions can take closures as parameters, allowing any function that satisfies the trait bound of the closure to be used as an argument. The Fn, FnMut, and FnOnce traits dictate how a closure captures variables from the enclosing scope.

Note: If the lab does not specify a file name, you can use any file name you want. For example, you can use main.rs, compile and run it with rustc main.rs && ./main.

Input functions

Since closures may be used as arguments, you might wonder if the same can be said about functions. And indeed they can! If you declare a function that takes a closure as parameter, then any function that satisfies the trait bound of that closure can be passed as a parameter.

// Define a function which takes a generic `F` argument
// bounded by `Fn`, and calls it
fn call_me<F: Fn()>(f: F) {
    f();
}

// Define a wrapper function satisfying the `Fn` bound
fn function() {
    println!("I'm a function!");
}

fn main() {
    // Define a closure satisfying the `Fn` bound
    let closure = || println!("I'm a closure!");

    call_me(closure);
    call_me(function);
}

As an additional note, the Fn, FnMut, and FnOnce traits dictate how a closure captures variables from the enclosing scope.

Summary

Congratulations! You have completed the Input Functions lab. You can practice more labs in LabEx to improve your skills.