Proposal: New Dart Lint For Constructor Body Order

This article discusses a proposal for a new lint in Dart, sort_declaring_constructor_body_below_header. This lint aims to enforce a specific style for placing the body of a primary constructor within a class, enum, or extension type.

Background and Motivation

Currently, the Dart language doesn't mandate a specific location for the body of a primary constructor within a class definition. As noted by @eernstg in this issue, the placement is a matter of style. However, a consistent style can improve code readability and maintainability.

The proposal suggests that placing the constructor body near the top of the class body might be a preferred style, potentially enforced by a lint rule. This would ensure that the core logic of the constructor is easily visible and accessible to developers.

The sort_declaring_constructor_body_below_header Lint

The proposed lint, sort_declaring_constructor_body_below_header, aims to address this stylistic concern. It would encourage developers to place the body of the primary constructor below the header, ensuring a consistent code structure. This lint would operate independently of other lint rules like sort_constructors_first or sort_unnamed_constructors_first, providing more granular control over constructor ordering.

Why Not Use "First"?

An alternative name considered was "first," but this was rejected to avoid conflicts with sort_unnamed_constructors_first. The sort_unnamed_constructors_first rule can be beneficial for developers who want to quickly identify the default way of instantiating a class, even if it differs from other constructors.

Impact on Code

This lint would primarily affect the ordering of elements within a class definition. Consider the following example:

class C._(int v) {
 @metadata
 this;

 new(int v) = C._;

 factory other(int v) => C(v);
}

In this example, the lint would not change the order of the elements. The primary constructor body (this;) is already placed below the constructor header (C._(int v)).

Benefits of the Proposed Lint

Implementing the sort_declaring_constructor_body_below_header lint offers several potential benefits:

• Improved Code Readability: By enforcing a consistent style for constructor body placement, the lint can make code easier to read and understand.
• Enhanced Maintainability: Consistent code structure simplifies maintenance and reduces the cognitive load on developers.
• Granular Control: The lint provides fine-grained control over constructor ordering, independent of other lint rules.
• Reduced Cognitive Load: By consistently placing the constructor body in a predictable location, developers can quickly find and understand the initialization logic of a class. This reduces the cognitive load required to navigate and comprehend code, especially in large projects with complex class structures. Consistent placement of the constructor body becomes a visual cue, allowing developers to immediately identify the core initialization logic without having to scan the entire class definition.
• Streamlined Code Reviews: With a standardized approach to constructor body placement, code reviews become more efficient. Reviewers can quickly verify that the code adheres to the established style, focusing on the logic and functionality rather than stylistic inconsistencies. This uniformity also simplifies the process of onboarding new team members, as they can quickly adapt to the project's coding conventions. The lint acts as an automated style guide, ensuring that all developers follow the same rules for constructor body placement.
• Facilitates Code Navigation: When constructor bodies are consistently placed below the header, it becomes easier to navigate through the code. IDEs and code editors can leverage this consistency to provide better navigation tools, such as jump-to-definition and code folding. Developers can quickly locate the constructor body, inspect its logic, and understand how the class is initialized. This improved navigation enhances productivity and reduces the time spent searching for specific code elements.
• Promotes Code Consistency Across Projects: By adopting the sort_declaring_constructor_body_below_header lint, developers can promote code consistency across different projects. This consistency reduces the learning curve when switching between projects and makes it easier to share code and libraries. A standardized style for constructor body placement contributes to a more unified and cohesive Dart ecosystem. Consistency across projects fosters collaboration and knowledge sharing among developers.
• Supports a More Predictable Code Structure: The lint helps establish a more predictable code structure, which is essential for large and complex projects. Predictability reduces the risk of errors and makes it easier to reason about the code's behavior. Developers can rely on the consistent placement of constructor bodies to quickly understand the initialization process and identify potential issues. A predictable code structure improves the overall quality and reliability of the software.

Example Scenarios

To further illustrate the impact of the proposed lint, let's consider a few example scenarios.

Scenario 1: Simple Class with Primary Constructor

class MyClass {
 int _value;

 MyClass(this._value) {
 print('Initializing MyClass with value: $_value');
 }

 int getValue() => _value;
}

In this scenario, the sort_declaring_constructor_body_below_header lint would ensure that the constructor body (the block containing print('Initializing MyClass with value: $_value');) is placed immediately after the constructor header (MyClass(this._value)). This placement makes the initialization logic immediately visible.

Scenario 2: Class with Multiple Constructors

class MyClass {
 int _value;

 MyClass(this._value);

 MyClass.named(int value) : _value = value {
 print('Initializing MyClass with named constructor');
 }

 int getValue() => _value;
}

In this case, the lint would apply to the named constructor (MyClass.named) as well. The body of the named constructor (the block containing print('Initializing MyClass with named constructor');) would be placed directly below the constructor header (MyClass.named(int value) : _value = value). This consistency helps in quickly identifying the initialization logic for different types of constructors.

Scenario 3: Class with Factory Constructor

class MyClass {
 int _value;

 MyClass._internal(this._value);

 factory MyClass(int value) {
 if (value < 0) {
 throw ArgumentError('Value cannot be negative');
 }
 return MyClass._internal(value);
 }

 int getValue() => _value;
}

Here, the lint would not directly affect the factory constructor (MyClass(int value)), as factory constructors don't have a body in the same way as primary and named constructors. However, it would still apply to the internal constructor (MyClass._internal(this._value)), ensuring that its (potentially empty) body is placed below the header. This maintains a consistent style even for internal constructors.

Potential Drawbacks

While the proposed lint offers numerous advantages, it's essential to acknowledge potential drawbacks:

• Initial Adjustment: Developers accustomed to different styles might need time to adjust to the new lint rule. However, the benefits of improved consistency and readability should outweigh the initial inconvenience. Adopting a new coding style always requires some adjustment, but the long-term benefits are significant.
• Enforcement Complexity: Implementing the lint might require careful consideration of edge cases and complex scenarios. The Dart analyzer needs to accurately identify constructor bodies and enforce the placement rule without introducing false positives or negatives. Thorough testing and validation are crucial to ensure the lint's effectiveness.
• Subjectivity of Style: Some developers might argue that constructor body placement is a matter of personal preference and shouldn't be enforced by a lint rule. However, the goal of the lint is to promote a consistent style that benefits the entire development team, even if it means sacrificing some individual preferences. Balancing individual preferences with team-wide consistency is a key aspect of establishing coding standards.

Conclusion

The sort_declaring_constructor_body_below_header lint represents a valuable addition to the Dart ecosystem. By promoting a consistent style for constructor body placement, it can significantly improve code readability, maintainability, and overall code quality. While there might be some initial adjustment and potential drawbacks, the long-term benefits of this lint are substantial. This proposal is a step towards creating more standardized and understandable Dart codebases.

For more information on Dart linting and style guidelines, visit the official Dart Style Guide.