Lesson 05 of 20

6 mins read

Functions, Parameters & Return Expressions

In Plain English

Functions in Rust are declared with fn. Parameters must have explicit type annotations. Rust is an expression-oriented language: blocks evaluate to their final expression, meaning omitting the trailing semicolon acts as an implicit return.

The Mental Model

"In Rust, a function body is like an assembly line: steps ending in semicolons perform side effects, but the final un-punched part at the end of the belt is the finished product handed right back to the caller."

Deep Dive: How It Works

Statements vs Expressions: Statements perform an action and return () (e.g. let x = 5;). Expressions evaluate to a resulting value (e.g. 5 + 5).

Implicit Return: Omit the semicolon on the final line of a function to return that value.

Explicit Return: The return keyword is available for early exits from function bodies.

Syntax Blueprint

fn add(a: i32, b: i32) -> i32 { a + b }

Notice no semicolon after a + b—this marks it as the returned expression.

Core Rules to Remember

Explicit Signatures: Functions declare parameter types and return type (-> Type).

Expression Returns: The last expression without a semicolon is the return value.

Unit Type (): Functions that return nothing return the empty tuple () implicitly.

Common Beginner Traps & How to Fix Them

Mistake:Adding an accidental semicolon to the return expression.

Why it happens: fn add(a: i32, b: i32) -> i32 { a + b; } returns () instead of i32, causing mismatched types error.

How to fix: Remove the trailing semicolon from the return expression.

Pro Tip: Use block expressions to compute values inline: let total = { let sub = 100; sub + 15 };.

Live Interactive Example

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Expression-Based Functions

Rust 1.78 / rustc

Output Console

Click "Run Code" to view the rendered output.

How it works: Demonstrates typed parameters, implicit expression return, and formatted printing.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Write a multiply function

Write fn multiply(x: i32, y: i32) -> i32 that returns x * y without a semicolon.
In main(), call multiply(7, 8) and print "Product: 56".

Sandbox Output

Click "Run & Check" to test your solution.

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