Lesson 12 of 20

6 mins read

Pattern Matching with match Expressions

In Plain English

The match expression compares a value against a series of patterns and executes code based on which pattern matches. match is exhaustive: the compiler forces you to handle every possible value, eliminating unhandled state bugs.

The Mental Model

"A match expression is like a biometric sorting machine: coins roll down chutes with precision-cut slots. Every single coin denomination has an exact matching slot, with a catch-all tray (_) at the bottom for foreign coins."

Deep Dive: How It Works

Exhaustiveness: You must cover all possible cases (or provide a catch-all _ arm).

Match Arms: pattern => expression, separated by commas.

Match Guards: pattern if condition => expr adds extra boolean constraints.

Syntax Blueprint

match code { 200 => "OK", 404 => "Not Found", _ => "Unknown" }

Evaluates top-to-bottom and returns the resulting expression of the first matching arm.

Core Rules to Remember

Exhaustive: Compiler guarantees zero missed enum variants or numbers.

Wildcard _: Acts as default fallback for all remaining unhandled patterns.

Multiple Patterns: Match multiple values in one arm: 1 | 2 | 3 => "Low".

Common Beginner Traps & How to Fix Them

Mistake:Non-exhaustive match on integers or enums.

Why it happens: Matching integers 1, 2 without a wildcard _ causes E0004: non-exhaustive patterns.

How to fix: Always include the wildcard pattern _ => ... for unbounded types.

Pro Tip: You can match ranges directly in match arms: 1..=10 => "Single digit or ten".

Live Interactive Example

Hit Run Code to see it live

HTTP Status Code Matcher

Rust 1.78 / rustc

Output Console

Click "Run Code" to view the rendered output.

How it works: The match expression exhaustively evaluates http_status and binds the descriptive text.

Your Turn: Micro Challenge

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

Micro Exercise

Match dice roll outcome

Given let roll = 6;
Use match roll { 6 => "Critical Hit", 1 => "Critical Miss", _ => "Normal Roll" };
Print "Outcome: Critical Hit".

Sandbox Output

Click "Run & Check" to test your solution.

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