main.rs

struct ApplyFnToParam<F> {
    f: F,
    param: u32
}

fn my_default_fn(param: u32) -> bool {
    param == 42
}

// This is the part which blocked me the whole day, and I finally figured it out myself.
// I don't know if I should cry or laugh.
impl ApplyFnToParam<fn(u32) -> bool> {
    fn new_with_default_fn(param: u32) -> ApplyFnToParam<fn(u32) -> bool> {
        ApplyFnToParam {
            f: my_default_fn as fn(u32) -> bool,
            param: param
        }
    }
}

impl<F> ApplyFnToParam<F> where F: Fn(u32) -> bool {
    fn new(f: F, param: u32) -> ApplyFnToParam<F> {
        ApplyFnToParam {
            f: f,
            param: param
        }
    }

    fn apply(self) -> bool {
        (self.f)(self.param)
    }
}

fn main() {
    let a = ApplyFnToParam::new(my_default_fn, 7).apply();
    let b = ApplyFnToParam::new(|x| x == 42, 42).apply();
    let c = ApplyFnToParam::new_with_default_fn(42).apply();
    println!("{} {} {}", a, b, c);
}