Overview

Create custom linting rules for Ameba to enforce project-specific code quality standards and catch domain-specific code smells in Crystal projects.

Understanding Custom Rules

Custom Ameba rules allow you to:

• Enforce project-specific coding standards
• Catch domain-specific anti-patterns
• Validate business logic constraints
• Ensure consistency across large codebases
• Create reusable rule libraries for your organization
• Extend Ameba's built-in capabilities

Rule Anatomy

Basic Rule Structure

Every Ameba rule inherits from Ameba::Rule::Base and follows this structure:

module Ameba::Rule::Custom
  # Rule that enforces documentation on public classes
  class DocumentedClasses < Base
    properties do
      description "Enforces public classes to be documented"
    end

    MSG = "Class must be documented with a comment"

    def test(source)
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::ClassDef)
      return unless node.visibility.public?

      doc = node.doc
      issue_for(node, MSG) if doc.nil? || doc.empty?
    end
  end
end

Key Components

1. Module namespace - Custom rules typically use Ameba::Rule::Custom or Ameba::Rule::<Category>
2. Base class - All rules inherit from Ameba::Rule::Base
3. Properties block - Defines rule metadata and configuration
4. Message constant - The error message shown to users
5. Test method - Entry point that initializes the AST visitor
6. Overloaded test methods - Handle specific AST node types

Creating Your First Custom Rule

Step 1: Project Setup

Create a Crystal library for your custom rules:

# Initialize a new Crystal library
crystal init lib ameba-custom-rules
cd ameba-custom-rules

Update shard.yml:

name: ameba-custom-rules
version: 0.1.0
authors:
  - Your Name <your.email@example.com>

description: Custom Ameba rules for your project

crystal: ">= 1.0.0"

license: MIT

development_dependencies:
  ameba:
    github: crystal-ameba/ameba
    version: ~> 1.6.0

Important: Ameba should be a development dependency to avoid version conflicts.

Step 2: Implement a Simple Rule

Create src/ameba-custom-rules/no_sleep_in_production.cr:

require "ameba"

module Ameba::Rule::Custom
  # Prevents sleep() calls in production code
  class NoSleepInProduction < Base
    properties do
      description "Prevents sleep calls in production code"
      enabled true
    end

    MSG = "Avoid using sleep() in production code; use proper background jobs"

    def test(source)
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::Call)
      return unless node.name == "sleep"

      issue_for node, MSG
    end
  end
end

Step 3: Register and Use the Rule

Create main file src/ameba-custom-rules.cr:

require "ameba"
require "./ameba-custom-rules/*"

# Rules are automatically registered through inheritance

Update your project's Ameba configuration:

# .ameba.yml
Custom/NoSleepInProduction:
  Enabled: true
  Severity: Warning

Step 4: Test Your Rule

Create spec/ameba-custom-rules/no_sleep_in_production_spec.cr:

require "../spec_helper"

module Ameba::Rule::Custom
  describe NoSleepInProduction do
    it "reports sleep calls" do
      rule = NoSleepInProduction.new

      source = Source.new %(
        def process
          sleep 5.seconds
        end
      )

      rule.test(source)
      source.issues.size.should eq(1)
    end

    it "allows code without sleep" do
      rule = NoSleepInProduction.new

      source = Source.new %(
        def process
          puts "Processing"
        end
      )

      rule.test(source)
      source.issues.should be_empty
    end
  end
end

Advanced Rule Examples

Enforcing Naming Conventions

module Ameba::Rule::Custom
  # Enforces that service classes end with "Service"
  class ServiceClassNaming < Base
    properties do
      description "Service classes must end with 'Service'"
      enabled true
    end

    MSG = "Service class name should end with 'Service'"

    def test(source)
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::ClassDef)
      class_name = node.name.to_s

      # Check if class is in services directory
      return unless source.path.includes?("/services/")

      # Check if name ends with Service
      unless class_name.ends_with?("Service")
        issue_for node.name, MSG
      end
    end
  end
end

Detecting Dangerous Method Calls

module Ameba::Rule::Custom
  # Prevents dangerous ActiveRecord-like methods
  class NoDangerousDatabaseCalls < Base
    properties do
      description "Prevents dangerous database operations"
      dangerous_methods ["delete_all", "destroy_all", "update_all"]
    end

    MSG = "Dangerous method %s without conditions"

    def test(source)
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::Call)
      return unless dangerous_methods.includes?(node.name)

      # Check if call has arguments (conditions)
      if node.args.empty?
        message = MSG % node.name
        issue_for node, message
      end
    end
  end
end

Enforcing Error Handling

module Ameba::Rule::Custom
  # Ensures HTTP client calls have error handling
  class HttpErrorHandling < Base
    properties do
      description "HTTP client calls must handle errors"
      enabled true
    end

    MSG = "HTTP client calls should be wrapped in begin/rescue"

    def test(source)
      @in_rescue_block = false
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::ExceptionHandler)
      @in_rescue_block = true
      true  # Continue visiting children
    end

    def test(source, node : Crystal::Call)
      return if @in_rescue_block

      # Check for HTTP client calls
      if node.obj.try(&.to_s.includes?("HTTP"))
        issue_for node, MSG
      end
    end
  end
end

Validating Method Complexity

module Ameba::Rule::Custom
  # Limits method complexity
  class MethodComplexity < Base
    properties do
      description "Methods should not be too complex"
      max_complexity 10
    end

    MSG = "Method complexity (%d) exceeds maximum (%d)"

    def test(source)
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::Def)
      complexity = calculate_complexity(node)

      if complexity > max_complexity
        message = MSG % [complexity, max_complexity]
        issue_for node, message
      end
    end

    private def calculate_complexity(node)
      counter = ComplexityCounter.new
      node.accept(counter)
      counter.complexity
    end

    private class ComplexityCounter < Crystal::Visitor
      getter complexity : Int32 = 1

      def visit(node : Crystal::If)
        @complexity += 1
        true
      end

      def visit(node : Crystal::Case)
        @complexity += 1
        true
      end

      def visit(node : Crystal::While)
        @complexity += 1
        true
      end

      def visit(node : Crystal::Call)
        # Count logical operators
        if node.name.in?("&&", "||")
          @complexity += 1
        end
        true
      end

      def visit(node : Crystal::ASTNode)
        true
      end
    end
  end
end

Enforcing Documentation Standards

module Ameba::Rule::Custom
  # Requires documentation with specific format
  class DocumentationFormat < Base
    properties do
      description "Public methods must have documentation with examples"
      enabled true
      require_examples true
    end

    MSG_NO_DOC = "Public method must have documentation"
    MSG_NO_EXAMPLE = "Documentation must include usage examples"

    def test(source)
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::Def)
      return unless node.visibility.public?
      return if node.name.starts_with?("initialize")

      doc = node.doc

      if doc.nil? || doc.empty?
        issue_for node, MSG_NO_DOC
        return
      end

      if require_examples && !has_example?(doc)
        issue_for node, MSG_NO_EXAMPLE
      end
    end

    private def has_example?(doc : String)
      doc.includes?("```") || doc.includes?("Example:")
    end
  end
end

Working with AST Nodes

Common AST Node Types

# Class definitions
def test(source, node : Crystal::ClassDef)
  node.name            # Class name
  node.visibility      # public?, private?, protected?
  node.doc            # Documentation comment
  node.abstract?      # Is abstract class?
  node.superclass     # Parent class
end

# Method definitions
def test(source, node : Crystal::Def)
  node.name           # Method name
  node.args           # Arguments
  node.body           # Method body
  node.return_type    # Return type annotation
  node.visibility     # Visibility modifier
  node.doc           # Documentation
end

# Method calls
def test(source, node : Crystal::Call)
  node.name          # Method name
  node.obj           # Receiver object
  node.args          # Arguments
  node.named_args    # Named arguments
  node.block         # Block argument
end

# Variable assignments
def test(source, node : Crystal::Assign)
  node.target        # Left side (variable)
  node.value         # Right side (value)
end

# Conditionals
def test(source, node : Crystal::If)
  node.cond          # Condition
  node.then          # Then branch
  node.else          # Else branch
end

# Loops
def test(source, node : Crystal::While)
  node.cond          # Loop condition
  node.body          # Loop body
end

Traversal Patterns

# Pattern 1: Track state during traversal
class MyRule < Base
  def test(source)
    @inside_block = false
    @depth = 0
    AST::NodeVisitor.new self, source
  end

  def test(source, node : Crystal::Block)
    @inside_block = true
    @depth += 1
    true  # Continue visiting children
  end
end

# Pattern 2: Collect information then analyze
class MyRule < Base
  def test(source)
    @method_names = [] of String
    visitor = AST::NodeVisitor.new self, source
    analyze_collected_data(source)
  end

  def test(source, node : Crystal::Def)
    @method_names << node.name
  end

  private def analyze_collected_data(source)
    # Analyze @method_names
  end
end

# Pattern 3: Parent-child relationships
class MyRule < Base
  def test(source, node : Crystal::ClassDef)
    # Visit only methods in this class
    node.body.accept(MethodVisitor.new(self, source))
  end

  private class MethodVisitor < Crystal::Visitor
    def initialize(@rule : MyRule, @source : Source)
    end

    def visit(node : Crystal::Def)
      @rule.check_method(node, @source)
      true
    end

    def visit(node : Crystal::ASTNode)
      true
    end
  end
end

Rule Configuration

Configurable Properties

module Ameba::Rule::Custom
  class ConfigurableRule < Base
    properties do
      description "A rule with configurable properties"

      # Boolean properties
      enabled true
      strict_mode false

      # Numeric properties
      max_length 100
      min_length 3

      # String properties
      prefix "test_"
      suffix "_spec"

      # Array properties
      allowed_names ["foo", "bar", "baz"]
      excluded_paths ["spec/**/*", "lib/**/*"]
    end

    def test(source)
      # Use properties
      return unless enabled

      if strict_mode
        # Apply strict checks
      end

      AST::NodeVisitor.new self, source
    end
  end
end

Configuration in .ameba.yml

Custom/ConfigurableRule:
  Enabled: true
  Severity: Warning
  StrictMode: true
  MaxLength: 120
  MinLength: 5
  Prefix: "app_"
  AllowedNames:
    - "primary"
    - "secondary"
  ExcludedPaths:
    - "spec/fixtures/**"
    - "db/migrations/**"

Issue Reporting

Basic Issue Reporting

# Simple issue
issue_for node, "Error message"

# Issue with interpolation
issue_for node, "Found #{count} violations"

# Issue at specific location
issue_for node.name, "Method name violates convention"

# Issue with custom location
issue_for(
  {node.location.try(&.line_number) || 1, 1},
  node.end_location,
  "Custom message"
)

Issue Severity

# Controlled by configuration
Custom/MyRule:
  Severity: Error    # Blocks CI
  # or
  Severity: Warning  # Important but not blocking
  # or
  Severity: Convention  # Style preference

Rich Issue Messages

module Ameba::Rule::Custom
  class RichMessages < Base
    MSG_TEMPLATE = <<-MSG
      Method '%{method}' is too long (%{actual} lines, max %{max} allowed)
      Consider extracting to smaller methods or using composition.
      MSG

    def test(source, node : Crystal::Def)
      line_count = count_lines(node)

      if line_count > max_lines
        message = MSG_TEMPLATE % {
          method: node.name,
          actual: line_count,
          max: max_lines
        }
        issue_for node, message
      end
    end
  end
end

Testing Custom Rules

Comprehensive Test Suite

require "../spec_helper"

module Ameba::Rule::Custom
  describe DocumentedClasses do
    context "with documented class" do
      it "passes" do
        rule = DocumentedClasses.new

        source = Source.new %(
          # This is a documented class
          class MyClass
          end
        )

        rule.test(source)
        source.issues.should be_empty
      end
    end

    context "with undocumented public class" do
      it "reports an issue" do
        rule = DocumentedClasses.new

        source = Source.new %(
          class MyClass
          end
        )

        rule.test(source)
        source.issues.size.should eq(1)
        source.issues.first.message.should contain("documented")
      end
    end

    context "with private class" do
      it "allows undocumented private classes" do
        rule = DocumentedClasses.new

        source = Source.new %(
          private class InternalClass
          end
        )

        rule.test(source)
        source.issues.should be_empty
      end
    end

    context "with empty documentation" do
      it "reports an issue" do
        rule = DocumentedClasses.new

        source = Source.new %(
          #
          class MyClass
          end
        )

        rule.test(source)
        source.issues.size.should eq(1)
      end
    end

    context "configuration" do
      it "can be disabled" do
        rule = DocumentedClasses.new
        rule.enabled = false

        source = Source.new %(
          class MyClass
          end
        )

        rule.test(source)
        source.issues.should be_empty
      end
    end
  end
end

Test Helpers

# spec/spec_helper.cr
require "spec"
require "ameba"
require "../src/ameba-custom-rules"

module Ameba
  # Helper to create test sources
  def self.source(code : String, path = "source.cr")
    Source.new(code, path)
  end

  # Helper to expect issues
  def self.expect_issue(rule, code)
    source = Source.new(code)
    rule.test(source)
    source.issues.empty?.should be_false
  end

  # Helper to expect no issues
  def self.expect_no_issue(rule, code)
    source = Source.new(code)
    rule.test(source)
    source.issues.should be_empty
  end
end

# Usage in specs
describe MyRule do
  it "reports violations" do
    rule = MyRule.new
    Ameba.expect_issue rule, %(
      def bad_code
      end
    )
  end
end

Packaging and Distribution

Creating a Reusable Rule Package

# shard.yml
name: ameba-company-rules
version: 1.0.0

description: |
  Company-specific Ameba rules for Crystal projects.
  Enforces coding standards and best practices.

authors:
  - Company DevTools <devtools@company.com>

crystal: ">= 1.0.0"
license: MIT

development_dependencies:
  ameba:
    github: crystal-ameba/ameba
    version: ~> 1.6.0

# Optional: Add to targets for binary
targets:
  ameba-company:
    main: src/cli.cr

Distribution Strategy

# Option 1: As a shard dependency
# In user's shard.yml
development_dependencies:
  ameba:
    github: crystal-ameba/ameba
  ameba-company-rules:
    github: company/ameba-company-rules

# Option 2: As vendored rules
# Copy rule files to project's lib/ameba-rules/
# Include in custom ameba binary

# Option 3: As a plugin
# Create standalone executable that extends ameba

Custom Ameba Binary

# bin/ameba-custom.cr
require "ameba/cli"
require "../lib/ameba-company-rules/src/ameba-company-rules"

# Rules are automatically discovered
Ameba::CLI.run

Build and distribute:

crystal build bin/ameba-custom.cr -o bin/ameba-custom
# Distribute binary or build from source

Real-World Rule Examples

Preventing N+1 Queries

module Ameba::Rule::Custom
  class PreventNPlusOne < Base
    properties do
      description "Detects potential N+1 query patterns"
    end

    MSG = "Potential N+1 query: accessing association in loop"

    def test(source)
      @in_loop = false
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::While | Crystal::Call)
      if node.is_a?(Crystal::Call) && node.name.in?("each", "map")
        @in_loop = true
        node.block.try(&.accept(self))
        @in_loop = false
        return false  # Don't visit block again
      end
      true
    end

    def test(source, node : Crystal::Call)
      return unless @in_loop

      # Detect association access patterns
      if looks_like_association?(node)
        issue_for node, MSG
      end
    end

    private def looks_like_association?(node)
      # Simplified detection
      node.name.in?("user", "posts", "comments") &&
        node.obj != nil
    end
  end
end

Enforcing API Versioning

module Ameba::Rule::Custom
  class ApiVersioning < Base
    properties do
      description "API controllers must be versioned"
    end

    MSG = "API controller must be in a versioned namespace (e.g., V1::)"

    def test(source)
      return unless source.path.includes?("/api/")
      AST::NodeVisitor.new self, source
    end

    def test(source, node : Crystal::ClassDef)
      return unless node.name.to_s.ends_with?("Controller")

      unless has_version_namespace?(node)
        issue_for node.name, MSG
      end
    end

    private def has_version_namespace?(node)
      # Check if class name includes version (V1::, V2::, etc.)
      node.name.to_s.matches?(/V\d+::/)
    end
  end
end

When to Use This Skill

Use the ameba-custom-rules skill when:

• Enforcing project-specific coding standards not covered by built-in rules
• Detecting domain-specific anti-patterns or code smells
• Validating business logic constraints in code
• Creating organization-wide linting standards
• Migrating from another language and enforcing new patterns
• Preventing specific bugs that have occurred in production
• Ensuring consistency across microservices
• Teaching team members about code quality through automated feedback
• Enforcing architectural decisions (e.g., layer boundaries)
• Standardizing error handling, logging, or monitoring patterns

Best Practices

1. Start simple - Begin with basic rules before tackling complex AST traversals
2. Test thoroughly - Write comprehensive specs covering edge cases
3. Provide clear messages - Error messages should explain what's wrong and suggest fixes
4. Make rules configurable - Use properties for thresholds and options
5. Document your rules - Include description and examples in properties block
6. Use specific node types - Overload test for specific AST nodes, not generic traversal
7. Consider performance - Avoid complex operations in hot paths; cache results when possible
8. Follow naming conventions - Use descriptive rule names that match their purpose
9. Provide fix suggestions - When possible, explain how to resolve the issue
10. Scope appropriately - Only check relevant files (use source.path checks)
11. Handle nil safely - Use try(&.) when accessing potentially nil AST properties
12. Avoid false positives - Better to miss some cases than flag correct code
13. Version your rules - Track rule versions and breaking changes
14. Keep rules focused - One rule should check one thing (Single Responsibility)
15. Integrate with CI - Ensure custom rules work in automated environments

Common Pitfalls

1. Overly broad matching - Catching too many cases and producing false positives
2. Not handling nil - AST nodes may have nil properties causing crashes
3. Ignoring visibility - Checking private methods when only public API matters
4. Complex visitor logic - Making traversal code hard to understand and maintain
5. Missing edge cases - Not testing unusual but valid code patterns
6. Poor error messages - Vague messages that don't help developers fix issues
7. Hard-coded values - Not making thresholds and options configurable
8. Checking generated code - Flagging auto-generated files that shouldn't be changed
9. Performance issues - Complex rules that slow down analysis significantly
10. Dependency conflicts - Using Ameba as regular dependency instead of development_dependencies
11. Not using properties - Hard-coding configuration instead of using properties block
12. Incomplete testing - Not testing disabled state, edge cases, or configuration
13. Tight coupling - Rules that depend on other rules or specific file structures
14. Unclear scope - Rules that apply to wrong files or contexts
15. Version incompatibility - Not testing against multiple Ameba versions

Resources

• Ameba Custom Rule Tutorial
• Ameba Internals Documentation
• Ameba API Documentation
• Crystal AST Documentation
• Crystal Compiler Source
• Ameba GitHub Repository
• Example Custom Rules