Rust closures ans iterators

 

Rust provides closures and iterators to enable functional programming. In first look it looks great, like the same nice functional programming in other programming languages, but a deeper check exposes that rust suffers from non friendly requirements. See the following example which demonstrates the weird iterators and closure interactions.

fn main() {
    let mut updated_numbers: Vec<i32> = Vec::new();
    let numbers = vec![1, 5, 4, 7, 8, 9];

    // long and explicit definition of a closure
    let decrement_number = |x: &i32| -> i32{
        println!("decrement a number");
        // here we access a variable from outside the closure scope
        updated_numbers.push(x.clone());
        x - 1
    };

    // short hand implicit definition of a closure
    let increment_number = |x| x + 1;

    // we use iterators to create new vectors
    let incremented: Vec<i32> = numbers.iter().map(increment_number).collect();
    let back_to_origin: Vec<i32> = incremented.iter().map(decrement_number).collect();


    println!("numbers {:?}", numbers);
    println!("incremented {:?}", incremented);
    println!("and decremented back {:?}", back_to_origin);
    println!("we've updated the following {:?}", updated_numbers);


    /*
    It turns out functional programming in rust is no fun!
    iter() - creates a pointer to the items
    filter() - creates another pointer to the pointer to the items
    hence, we have a double pointer in the filter, very unexpected behavior
     */
    let filter_even = |x: &&i32| -> bool { return *x % 2 == 0 };
    let even: Vec<i32> = numbers.iter().filter(filter_even).map(|x: &i32| -> i32 { *x }).collect();

    /*
    This emphasise the complex rust internals.
    we must redefine the closures since the order of actions is different
     */
    let filter_even = |x: &i32| -> bool { return x % 2 == 0 };
    let decrement_number = |x| x -1;
    let odd: Vec<i32> = numbers.iter().map(increment_number).filter(filter_even).map(decrement_number).collect();

    println!("even numbers are {:?}", even);
    println!("odd numbers are {:?}", odd);
}