Dart Primary Constructors & New Constructor Syntax Skill

Use this skill when helping users write, refactor, or debug code using Dart's Primary Constructors feature.

Dart Version Requirements

• Dart 3.13 and above: Primary constructors are enabled by default.
• Dart 3.12: The feature is available but experimental. Users must explicitly enable the experiment flag primary-constructors via --enable-experiment=primary-constructors or in analysis_options.yaml:

analyzer:
  enable-experiment:
    - primary-constructors

• Dart 3.11 and earlier: Primary constructors are not supported.

1. Overview

Primary Constructors allow developers to declare a non-redirecting generative constructor as well as a set of instance variables directly in the class header. This significantly reduces boilerplate and improves code readability.

Key Benefits

• Combines field declaration, parameter declaration, and initialization into a single declaration known as a declaring parameter declaration.
• Enables safe reference to constructor parameters in non-late field initializers (Primary Initializer Scope).
• Allows empty declaration bodies to be represented concisely with a semicolon (;).
• Introduces abbreviated concise syntax for in-body constructors.

2. Syntax Reference

2.1 Basic Class Header Syntax

To declare a primary constructor, place a parameter list immediately after the type name (and optional type parameters):

// Declares fields x and y, and a generative constructor Point(this.x, this.y)
class Point(var int x, var int y);

// Declares final fields
class PointFinal(final int x, final int y);

2.2 Declaring, Initializing, and Plain Parameters

A primary constructor parameter list distinguishes between three types of parameters:

1. Declaring Parameters: Indicated by the var or final modifier (e.g., final int x). They implicitly create a corresponding instance field in the class.
2. Initializing Parameters: Indicated by the this. or super. prefix (e.g., this.x or super.x). They initialize an existing field or a super constructor parameter, respectively.
3. Regular Parameters: Declared without modifiers (e.g., int y). They do not become fields and are only available during initialization (e.g., in field initializers or the this : initializer list in the class body).

// `x` is a field and a parameter because it has the keyword `final`. In particular, we can use the name `x` in the initializer list in the in-body part of the primary constructor. 'y' is a only parameter because it has neither of the keywords `final` or `var`, but `y` is passed to the super constructor via the `this :` initializer list.
class C(final int x, int y) extends Base {
  this : super(y);
}

Declaring parameters and initializing parameters are two ways of achieving the same goal: declaring a class with instance fields which are set in the constructor. Regular parameters are different in that their values are not automatically routed to an instance field.

2.3 Constant Primary Constructors

To make a primary constructor const, place the const keyword before the class/type name in the declaration header:

class const Point(final int x, final int y);
extension type const Ext(int x);
enum const MyEnum(final int x) {
  entry(1);
}

2.4 Extension Types

Extension types must use primary constructors.

• The single parameter in the header is the representation field.
• The representation variable cannot use the var modifier (using var triggers the representation_field_modifier error).
• The representation variable can optionally use the final modifier. If final is not present then it is inferred; that is, the parameter is declaring whether or not it's explicitly final.

2.5 Empty Body Semicolon Shorthand (;)

When a class, mixin class, mixin, extension or extension type has an empty body, the {} braces can be replaced by a semicolon (;):

class C(int x);
mixin class MC;
extension type ET(int x);
mixin M;
extension Ext on C;

2.6 The In-Body Part of a Primary Constructor (this ...)

If a primary constructor requires assertions or custom field initializations, they can be declared in the body using the this : syntax:

class Point(var int x, var int y) {
  // Initializer list in class body
  this : assert(x >= 0), y = y * 2;
}

You can also write a constructor body with this syntax (this {...}).

2.7 Abbreviated Concise Constructor Syntax

For constructors declared within the class body, the class name can be omitted and replaced with the new or factory keywords:

3. Semantics & Scoping Rules

3.1 Primary Initializer Scope

When a primary constructor is declared, its formal parameters are introduced into the Primary Initializer Scope. This scope is the current scope for non-late field initializers in the class body and the primary constructor's initializer list (after this :). This allows non-late fields to reference constructor parameters directly during declaration:

class DeltaPoint(final int x, int delta) {
  // 'x' and 'delta' are in scope here
  final int y = x + delta;
}

3.2 Late Instance Variables Restriction

The primary initializer scope is not active for late instance variable initializers.

• Since late variables can be evaluated after construction has completed, their initializers cannot safely access constructor parameters.
• Attempting to access a primary constructor parameter in a late field initializer results in a compile-time error.

3.3 Shadowing

Primary constructor parameters shadow class members (fields) of the same name within the primary initializer scope:

• In a non-late initializer: int y = x refers to parameter x.
• In a late initializer: late int y = x refers to field x (if it exists) because the parameter x is out of scope.

3.4 Generative Constructor Restrictions

To guarantee that the primary constructor (and the associated initializer scope) always executes:

• A class, mixin class, or enum declaration with a primary constructor cannot declare any other non-redirecting generative constructors (except extension types).
• All other generative constructors declared in the body must redirect (directly or indirectly) to the primary constructor.

3.5 Parameter Mutation Errors

Primary constructor parameters are non-assignable inside the initialization phase.

• Any assignment to a parameter (e.g., p = value, p++) inside field initializers or the this : initializer list is a compile-time error.

3.6 Double Initialization Errors

Initializing a field twice (e.g., once in the field declaration/initializer and once in the this : initializer list or as an initializing formal) is a compile-time error.

4. Diagnostics & Troubleshooting

Most errors and lints have quick-fixes, run dart fix to fix those violations. For other common errors, fix them using the following table:

5. Step-by-Step Refactoring Workflows

Workflow 5.1: Migrating a Class to a Primary Constructor

Follow these steps to migrate a verbose class to the new primary constructor syntax:

1. Identify Candidate Fields and Constructor: Locate generative constructors and the fields they initialize. In this case, this would be the name and age fields.

   // Before
   class User {
     final String name;
     final int age;
     User(this.name, this.age);
   }

2. Move Fields to the Header: Place fields in the header with final or var modifiers and append a semicolon (;) if the body is empty. The name and age fields are now written the primary constructor as declaring parameters final String name and final int age, respectively.

   // After
   class User(final String name, final int age);

3. Handle Custom Initializers and Assertions: If there is an initializer list or assert block, move it to a this block inside the body:

   // Before
   class Point {
     final int x;
     final int y;
     Point(this.x, this.y) : assert(x >= 0);
   }
   
   // After
   class Point(final int x, final int y) {
     this : assert(x >= 0);
   }

4. Leverage Primary Initializer Scope for Calculations: If a field value is calculated from parameters, declare it inside the body and assign it directly using the parameters:

   // Before
   class Rect {
     final double width;
     final double height;
     final double area;
     Rect(this.width, this.height) : area = width * height;
   }
   
   // After
   class Rect(final double width, final double height) {
     // 'width' and 'height' are in scope here
     final double area = width * height;
   }

5. Convert In-Body Constructors to Redirecting: Ensure all in-body generative constructors redirect to the primary constructor:

   // Before
   class Point {
     final int x;
     final int y;
     Point(this.x, this.y);
     Point.zero() : x = 0, y = 0;
   }
   
   // After
   class Point(final int x, final int y) {
     new zero() : this(0, 0); // Redirects to primary
   }

Workflow 5.2: Applying Abbreviated (Concise) In-Body Constructors

When the user prefers to keep the constructor in the class body but wants to reduce verbosity, suggest the abbreviated constructor syntax:

// Before
class DatabaseService {
  final String url;
  DatabaseService(this.url);
  DatabaseService.local() : url = 'localhost';
  factory DatabaseService.create() => DatabaseService('default');
}

// After
class DatabaseService {
  final String url;
  new(this.url); // Omit class name, use 'new'
  new local() : url = 'localhost'; // Use 'new local' for named constructors
  factory create() => DatabaseService('default'); // Omit class name from factory
}