How do you pass a Rust function as a parameter?

Question

Can I pass a function as a parameter? If not, what is a good alternative?

I tried some different syntaxes but I have not found the right one. I know I can do this:

fn example() {
    let fun: fn(value: i32) -> i32;
    fun = fun_test;
    fun(5i32);
}

fn fun_test(value: i32) -> i32 {
    println!("{}", value);
    value
}

but that's not passing the function as a parameter to another function:

fn fun_test(value: i32, (some_function_prototype)) -> i32 {
    println!("{}", value);
    value
}

Answer 1

Sure you can:

fn fun_test(value: i32, f: &dyn Fn(i32) -> i32) -> i32 {
    println!("{}", f(value));
    value
}

fn times2(value: i32) -> i32 {
    2 * value
}

fn main() {
    fun_test(5, &times2);
}

As this is Rust, you have to take into account the ownership and lifetime of the closure.

TL;DR; Basically there are 3 types of closures (callable objects):

1. Fn: It cannot modify the objects it captures.
2. FnMut: It can modify the objects it captures.
3. FnOnce: The most restricted. Can only be called once because when it is called it consumes itself and its captures.

See When does a closure implement Fn, FnMut and FnOnce? for more details

If you are using a simple pointer-to-function like closure, then the capture set is empty and you have the Fn flavor.

If you want to do more fancy stuff, then you will have to use lambda functions.

In Rust there are proper pointers to functions, that work just like those in C. Their type is for example fn(i32) -> i32. The Fn(i32) -> i32, FnMut(i32) -> i32 and FnOnce(i32) -> i32 are actually traits. A pointer to a function always implements all three of these, but Rust also has closures, that may or may not be converted to pointers (depending on whether the capture set is empty) to functions but they do implement some of these traits.

So for example, the example from above can be expanded:

fn fun_test_impl(value: i32, f: impl Fn(i32) -> i32) -> i32 {
    println!("{}", f(value));
    value
}
fn fun_test_dyn(value: i32, f: &dyn Fn(i32) -> i32) -> i32 {
    println!("{}", f(value));
    value
}
fn fun_test_ptr(value: i32, f: fn(i32) -> i32) -> i32 {
    println!("{}", f(value));
    value
}

fn times2(value: i32) -> i32 {
    2 * value
}

fn main() {
    let y = 2;
    //static dispatch
    fun_test_impl(5, times2);
    fun_test_impl(5, |x| 2*x);
    fun_test_impl(5, |x| y*x);
    //dynamic dispatch
    fun_test_dyn(5, &times2);
    fun_test_dyn(5, &|x| 2*x);
    fun_test_dyn(5, &|x| y*x);
    //C-like pointer to function
    fun_test_ptr(5, times2);
    fun_test_ptr(5, |x| 2*x); //ok: empty capture set
    fun_test_ptr(5, |x| y*x); //error: expected fn pointer, found closure
}

Answer 2

Fn, FnMut and FnOnce, outlined in the other answer, are closure types. The types of functions that close over their scope.

Apart from passing closures Rust also supports passing simple (non-closure) functions, like this:

fn times2(value: i32) -> i32 {
    2 * value
}

fn fun_test(value: i32, f: fn(i32) -> i32) -> i32 {
    println!("{}", f (value));
    value
}

fn main() {
    fun_test (2, times2);
}

fn(i32) -> i32 here is a function pointer type.

If you don't need a full-fledged closure than working with function types is often simpler as it doesn't have to deal with those closure lifetime nicities.