Statement and Expression in Rust 🦀

I'm learning Rust recently and I'm reading the Rust documentation. I found this page about statements and expressions. I think it's a good idea to write a blog about it.

In short

• Statements are instructions that perform some action and do not return a value.
• Expressions evaluate to a resultant value.

Let's look at a function in Rust

Function bodies are made up of a series of statements optionally ending in an expression.

In the above example, let x: i32 = 69; end with a semicolon, so it's a statement. It assigns the value 69 to the variable x.

Function definitions

Function definitions are also statements; the entire example above is a statement in itself.

Statements do not return values. Therefore, you can’t assign a let statement to another variable. For example, the following code will not compile:

However, in other language like Python you can do this. The assignment returns the value of the assignment. You can write x = y = 69 and have both x and y have the value 69.

How about expressions?

Expressions do not include ending semicolons. If you add a semicolon to the end of an expression, you turn it into a statement, and it will then not return a value. In the following example, j + 1 is an expression so it returns a value and assign it to i.

Function with return value

You can return early from a function by using the return keyword and specifying a value, but most functions return the last expression implicitly. In this case, x + 1 is the expression that we want to return.

If we place a semicolon at the end of the line containing x + 1, changing it from an expression to a statement. This code will not compile:

Instead you will get an mismatched type error message like this:

implicitly returns `()` as its body has no tail or `return` expression

We will not cover what unit type () is in this blog. If you are interested, you can read the Rust documentation about it here.

To summarize, statements do not return values, but expressions do. If you add a semicolon to the end of an expression, you turn it into a statement, which will then not return a value. Keep this in mind when writing Rust code. If you have any questions, feel free to leave a comment below. Thank you for reading!

• Rust documentation