Demo Option to handle None

Let's see another, probably a bit more realistic, example of this situation.

In this example when we declare the variable input we also initialize it to None.

Then we collect the values from the command line into a variable called args. Let's disregard the details of this for now. I wanted to create a working example for the None-handling and this is the simplest I managed to do.

If there are more than 1 values in that args variable, meaning if the user supplied at least one value after the name of the program then we assign this value to the input variable. However, we don't just assign the value as it arrived from the user, we wrap it in the Some() expression. It might look like a function, but it is not really one. It just wraps the value. We'll discuss the details later.

So now, after the if-statement we either have None in the input variable or we have Some(value) in there.

#![allow(clippy::needless_late_init)]

fn main() {
    let mut input = None;

    let args = std::env::args().collect::<Vec<String>>();
    if args.len() > 1 {
        input = Some(&args[1]);
    }

    match input {
        Some(thing) => {
            println!("The input is {thing}");
        }
        None => {
            println!("There was no input")
        }
    }
}

In order to use it this variable properly Rust makes us check if the content of the variable is None or Some(value). Rust has many nice ways to do this, probably the clearest is using he match pattern-matching statement.

We know that the input variable has two possible states hence we need two "arms" in the match statement. One will be triggered if the variable contains Some(value) the other when the variable contains None. In both cases we can provide a block of code to be executed.

In our example we provided a variable called thing, yes that is a variable we can use any name there. The value provided by the user that was later wrapped in the Some() construct will be assigned to this variable for the duration of the block after the fat-arrow.

Usage

We can provide the command-line parameters right after we typed in cargo run:

$ cargo run
There was no input

$ cargo run hello
The input is hello

Some extra notes

• In this example we had to use the mut keyword to make the variable mutable.
• In this case, because each arm of the match statement has only one statement (a println! in both cases) we could get-by without the curly braces.