Closures as struct property: using generics and using trait object.

closure_as_struct_property.rs

struct Data<F>
where
    F: FnMut(&i32) -> i32,
{
    value: i32,
    dyn_value: i32,
    // parser, once assigned, cannot change because each closure is unique
    parser: F,
    dyn_parser: Box<dyn FnMut(&i32) -> i32>, // dynamic, parser can change any time
}

impl<F> Data<F>
where
    F: FnMut(&i32) -> i32,
{
    fn exec(&mut self) {
        self.value = (self.parser)(&self.value);
    }
    
    fn dyn_exec(&mut self) {
        self.dyn_value = (self.dyn_parser)(&self.dyn_value);
    }
}


fn main() {
    let mut d = Data {
        value: 2,
        dyn_value: 2,
        parser: |&x| -> i32 {
            x * x
        },
        dyn_parser: Box::new(|&x| -> i32 {
            x * x
        }),
    };
    d.exec();
    // parser cannot change
    //d.parser = |&x| -> i32 { x + 1 };
    d.dyn_exec();
    
    d.dyn_parser = Box::new(|&x| -> i32 { x + 1 }); // can be changed
    d.dyn_exec();
    
    println!("- value: {}", d.value);
    println!("- dyn_value: {}", d.dyn_value);
}