Rust Style Expressions in Other Programming Languages

Published on 2021-05-29 by Nimai Patel

Lets say we have a variable animal and we want to initiate the value of a variable sound based on the value of animal. In JavaScript, we could do something like this:

let sound;
if (animal === 'dog') {
    sound = 'barks';
} else if (animal === 'cat') {
    sound = 'meows';

While this solution works as expected, if you're obsessed with functional programming like I am, the use of let in a codebase that otherwise adheres to FP concepts as much as possible might bother you.

The immutable solution in this case is actually pretty simple though. We can just use a ternary operator in the assignment for sound

const sound = animal === 'dog' ? 'barks' : 'meows'

Well, this kinda works. The problem is that sound is assigned 'meows' as long animal isn't 'dog'. So even if animal was 'pig', sound would be assigned 'meows'! Also, what do we do if we have more than two types of animals? We could start nesting ternary operators, but that would get ugly quickly!

Now the Rust programming language has a syntactically clean solution to this problem.

/* variables are immutable by default in Rust */
let sound = {
    if animal == "dog" {
    } else if animal == "cat" {
    } else if animal == "pig" {
    } else {
        "not valid animal"

Expressions inside braces in Rust can evaluate to values. This means you can use control flow key words like if-else statements and loops during the assignment of variables.

Now how do we go about doing something similar in other programming languages like JavaScript? Well, we can use anonymous functions and immediately call them on assignment.

const sound = (() => {
    if (animal === "dog") {
        return "barks"
    } else if (animal === "cat") {
        return "meows"
    } else if (animal === "pig") {
        return "oinks"

Great! And it doesn't look that bad either, no?