Rust cannot infer Fn type where Option<Fn> is None

Question

So I have this situation, where Rust cannot infer the generic type arguments for a function, that takes and optional Fn as a parameter. When passing None into it, it seems like the generic type arguments are obsolete, but Rust needs to know their values to compile and I don't know what to tell him. I tried foo::<(), ()>(calculation_result, None), but that doesn't work.

Does anyone know how I can call this foo function when passing None instead of a closure into it? Is there a dummy type I can specify or something?

Answer 1

The problem is to call a function with an unspecified parameter.

fn foo<F, R>(arg: bool, f: Option<F>) -> MyResult<R>
    where F: Fn() -> R
{...}

foo(true, None); // <- R cannot be inferred

You have to manually specify the type. Keep in mind that Option gets optimised to the size of Box (not sizeof Box + 1) and does not allocate in the None case.

let empty: Option<Box<dyn Fn() -> ()>> = None;
foo(true, empty); // <- R is (), Option<Box<>>::None does not allocate