Lesson 11 of 59

5 mins read

Pattern Matching & Destructuring in Dart 3

In Plain English

Pattern Matching in Dart 3 enables expressive data destructuring and validation. Patterns match values against shapes, extract subcomponents into local variables, and assert conditions simultaneously. You can destructure Lists, Records, Maps, Object properties, and apply Relational Patterns (`>= 100`, `< 0`).

The Mental Model

"Pattern matching is like a coin sorter: coins roll down a slot where each coin fits into a matching hole and drops into a categorized sorting bin automatically."

Deep Dive: How It Works

List Patterns: `final [first, second, ...rest] = items;` extracts elements and gathers remaining items.

Record Destructuring: `final (name, age: userAge) = ("Alice", age: 30);`.

Relational Patterns: `case >= 18 && <= 65 => "Working Age"` matches numeric ranges directly in switch blocks.

Object Patterns: `case User(role: "admin", :email) => print("Admin: $email");`.

Syntax Blueprint

// List destructuring
final [a, b, ...tail] = [1, 2, 3, 4, 5];

// Relational switch pattern
final desc = switch (score) {
  >= 90 => "A",
  >= 80 => "B",
  _ => "F",
};

Destructuring sequences and matching relational boundary patterns.

Core Rules to Remember

Simultaneous Match & Extract: Validates structure and binds sub-variables in a single operation.

Rest Elements (...): Gathers remaining unassigned elements into a new sub-list.

Relational Patterns: Use comparison operators (`>`, `<`, `>=`, `<=`) directly inside switch cases.

Common Beginner Traps & How to Fix Them

Mistake:Trying to match a fixed-length list pattern against a list with different item counts without `...`.

Why it happens: `[a, b]` expects exactly 2 elements; if the list has 3, the match fails.

How to fix: Use `[a, b, ...]` or `if-case` matching.

Pro Tip: Use `if (json case {'id': int id, 'name': String name})` for bulletproof JSON API response validation and destructuring.

Live Interactive Example

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List Destructuring and Relational Pattern Matching

Python 3.12

Output Console

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How it works: Destructured list elements into x and y and evaluated speed using relational ranges.

Your Turn: Micro Challenge

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

Micro Exercise

Destructure First and Last with Rest Pattern

Given list `final scores = [100, 85, 90, 70, 95];`.
Use list pattern destructuring `final [first, ..., last] = scores;`.
Print `First: $first, Last: $last`.

Sandbox Output

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