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Enum and patterns

Learn enums with pattern matching.

Pattern matching

Dart 3's pattern matching is very powerful with enums. switch can now be written as an expression that returns a value; no break needed. Each case matches a pattern and produces a value. The result can be assigned directly to a variable.

Example: final label = switch (s) { Status.active => "running", Status.idle => "waiting", Status.maintenance => "under maintenance" };. The expression to the right of the arrow (=>) is the case's result. Cases are separated by commas, no break. When all constants are covered, default is not needed; this produces short, safe code.

Practice Your turn: switch expression

Define an enum Approval with pending, approve, reject. In main assign final a = Approval.approve; and use a switch expression to return "approved" for Approval.approve. Print the result. Cover all constants.

Starter code

main.dart
void main() {
  // Define Approval enum: pending, approve, reject
  // Use a switch expression to produce a label and print
}

Sample solution

main.dart
enum Approval { pending, approve, reject }

void main() {
  final a = Approval.approve;
  final label = switch (a) {
    Approval.pending => "pending",
    Approval.approve => "approved",
    Approval.reject => "rejected",
  };
  print(label);
}

Expected output

approved

Destructuring

Pattern matching does not just catch the enum constant; it can also destructure fields of an enhanced enum. Inside a case you write the constant name and the field names in parentheses to access them directly. Like case Heat high(: final degrees):. This is especially powerful with multi-field enhanced enums.

Moreover, the underscore _ catches the "all other cases" pattern; _ => "unknown". This is like default but shorter. Still, with _ the compiler will not warn when a new constant is added, so writing all constants out is usually safer. Pattern matching, switch expressions and enhanced enums form a strong Dart 3 trio.

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 The switch expression arrow

Question 
final label = switch (s) { Status.active ____ "running", Status.idle => "waiting" };
Answer 
final label = switch (s) { Status.active => "running", Status.idle => "waiting" };

Explanation: => binds the pattern on the left to the value on the right.

2 When is a switch expression more useful?

main.dart
final s = switch (x) { A => 1, B => 2 };
  • default is required
  • break is required
  • Each case returns a value Correct answer
  • it cannot replace if

Explanation: A switch expression returns a value; no break needed.

3 Match pattern

Term Meaning
pattern shape to match
=> produces a value
_ all other cases
destructure opens fields

All lessons in Beginner

1 Dart basics

  1. What is Dart?
  2. Your first program
  3. dart run
  4. Comments
  5. String interpolation
  6. Summary - Dart basics

2 Variables

  1. var and type
  2. final
  3. const
  4. late
  5. Null safety
  6. Summary - Variables

3 Data types

  1. Integer types
  2. Floating-point types
  3. bool and String
  4. Type inference
  5. dynamic
  6. Summary - Data types

4 Project: Calculator

  1. Project design
  2. Add and subtract
  3. Multiply and divide
  4. Error handling
  5. Putting it together

5 Operators

  1. Arithmetic operators
  2. Assignment operators
  3. Comparison operators
  4. Logical operators
  5. Null-aware operators
  6. Summary - Operators

6 Strings

  1. Creating strings
  2. String interpolation
  3. length and methods
  4. substring
  5. contains and indexOf
  6. Summary - Strings

7 Conditionals

  1. The if statement
  2. else and else if
  3. switch
  4. Pattern matching
  5. Ternary
  6. Summary - Conditionals

8 Loops

  1. The for loop
  2. The while loop
  3. The do-while loop
  4. for-in
  5. break and continue
  6. Summary - Loops

9 Project: Shopping cart

  1. Cart design
  2. Adding items
  3. Calculating totals
  4. Listing the cart
  5. Putting it together

10 Lists

  1. Creating lists
  2. List access
  3. add and remove
  4. length and contains
  5. List methods
  6. Summary - Lists

11 Sets and Maps

  1. Set
  2. Set operations
  3. Map
  4. Map operations
  5. Map iteration
  6. Summary - Sets and Maps

12 Functions

  1. Defining functions
  2. Return values
  3. Arrow functions
  4. Optional parameters
  5. First-class functions
  6. Summary - Functions

13 Null safety in depth

  1. Nullable types
  2. Null-aware operators
  3. The ! operator
  4. late
  5. Null check
  6. Summary - Null safety

14 Class basics

  1. class
  2. Constructor
  3. Instance variables
  4. Methods
  5. this
  6. Summary - Class basics

15 Project: To-do CLI

  1. Task model
  2. Adding tasks
  3. Listing tasks
  4. Completing tasks
  5. Putting it together

16 Advanced string operations

  1. split and join
  2. replace
  3. trim and pad
  4. RegExp
  5. StringBuffer
  6. Summary - Advanced strings

17 Enums

  1. Defining enums
  2. Enum values
  3. Enhanced enums
  4. switch with enum
  5. Enum and patterns
  6. Summary - Enums

18 Exceptions

  1. try/catch
  2. throw
  3. finally
  4. The Exception class
  5. on clause
  6. Summary - Exceptions

19 Async basics

  1. Future
  2. async/await
  3. then
  4. Future.error
  5. Future.wait
  6. Summary - Async basics

20 Beginner final

  1. Project design
  2. Classes
  3. Collections
  4. Error handling
  5. Putting it together