pub trait Fn<Args>: FnMut<Args>{
// Required method
extern "rust-call" fn call(&self, args: Args) -> Self::Output;
}The version of the call operator that takes an immutable receiver.
Instances of Fn can be called repeatedly without mutating state.
This trait (Fn) is not to be confused with function pointers (fn).
Fn is implemented automatically by closures which only take immutable references to captured variables or donât capture anything at all, as well as (safe) function pointers (with some caveats, see their documentation for more details). Additionally, for any type F that implements Fn, &F implements Fn, too.
Since both FnMut and FnOnce are supertraits of Fn, any instance of Fn can be used as a parameter where a FnMut or FnOnce is expected.
Use Fn as a bound when you want to accept a parameter of function-like type and need to call it repeatedly and without mutating state (e.g., when calling it concurrently). If you do not need such strict requirements, use FnMut or FnOnce as bounds.
See the chapter on closures in The Rust Programming Language for some more information on this topic.
Also of note is the special syntax for Fn traits (e.g. Fn(usize, bool) -> usize). Those interested in the technical details of this can refer to the relevant section in the Rustonomicon.
let square = |x| x * x;
assert_eq!(square(5), 25);Fn parameter
fn call_with_one<F>(func: F) -> usize
where F: Fn(usize) -> usize {
func(1)
}
let double = |x| x * 2;
assert_eq!(call_with_one(double), 2);fn_traits #29625)
Performs the call operation.
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