How to pass anonymous functions as parameters in Rust? - function-pointers

Question

I've been playing around with Rust the past week. I can't seem to figure out how to pass a function that is defined as a parameter when calling the method, and haven't come across any documentation that shows them being used in that fashion.
Is it possible to define a function in the parameter list when calling a function in Rust?
This is what I've tried so far...
fn main() {
// This works
thing_to_do(able_to_pass);
// Does not work
thing_to_do(fn() {
println!("found fn in indent position");
});
// Not the same type
thing_to_do(|| {
println!("mismatched types: expected `fn()` but found `||`")
});
}
fn thing_to_do(execute: fn()) {
execute();
}
fn able_to_pass() {
println!("Hey, I worked!");
}

Answer 1

In Rust 1.0, the syntax for closure parameters is as follows:
fn main() {
thing_to_do(able_to_pass);
thing_to_do(|| {
println!("works!");
});
}
fn thing_to_do<F: FnOnce()>(func: F) {
func();
}
fn able_to_pass() {
println!("works!");
}
We define a generic type constrained to one of the closure traits: FnOnce, FnMut, or Fn.
Like elsewhere in Rust, you can use a where clause instead:
fn thing_to_do<F>(func: F)
where F: FnOnce(),
{
func();
}
You may also want to take a trait object instead:
fn main() {
thing_to_do(&able_to_pass);
thing_to_do(&|| {
println!("works!");
});
}
fn thing_to_do(func: &Fn()) {
func();
}
fn able_to_pass() {
println!("works!");
}