Cognitive Complexity

Cognitive Complexity is a measure of how difficult a unit of code is to intuitively understand. Unlike Cyclomatic Complexity, which determines how difficult your code will be to test, Cognitive Complexity tells you how difficult your code will be to read and comprehend.

A method's cognitive complexity is based on a few simple rules:

• Code is not considered more complex when it uses shorthand that the language provides for collapsing multiple statements into one
• Code is considered more complex for each "break in the linear flow of the code"
• Code is considered more complex when "flow breaking structures are nested"

Further reading

• Cognitive Complexity docs
• Cognitive Complexity: A new way of measuring understandability

Classes should not assume that instance variables are set or present outside of the current class definition.

Good:

class Foo
  def initialize
    @bar = :foo
  end

  def foo?
    @bar == :foo
  end
end

Good as well:

class Foo
  def foo?
    bar == :foo
  end

  def bar
    @bar ||= :foo
  end
end

Bad:

class Foo
  def go_foo!
    @bar = :foo
  end

  def foo?
    @bar == :foo
  end
end

Example

Running Reek on:

class Dummy
  def test
    @ivar
  end
end

would report:

[1]:InstanceVariableAssumption: Dummy assumes too much for instance variable @ivar

Note that this example would trigger this smell warning as well:

class Parent
  def initialize(omg)
    @omg = omg
  end
end

class Child < Parent
  def foo
    @omg
  end
end

The way to address the smell warning is that you should create an attr_reader to use @omg in the subclass and not access @omg directly like this:

class Parent
  attr_reader :omg

  def initialize(omg)
    @omg = omg
  end
end

class Child < Parent
  def foo
    omg
  end
end

Directly accessing instance variables is considered a smell because it breaks encapsulation and makes it harder to reason about code.

If you don't want to expose those methods as public API just make them private like this:

class Parent
  def initialize(omg)
    @omg = omg
  end

  private
  attr_reader :omg
end

class Child < Parent
  def foo
    omg
  end
end

Current Support in Reek

An instance variable must:

• be set in the constructor
• or be accessed through a method with lazy initialization / memoization.

If not, Instance Variable Assumption will be reported.

Classes should not assume that instance variables are set or present outside of the current class definition.

Good:

class Foo
  def initialize
    @bar = :foo
  end

  def foo?
    @bar == :foo
  end
end

Good as well:

class Foo
  def foo?
    bar == :foo
  end

  def bar
    @bar ||= :foo
  end
end

Bad:

class Foo
  def go_foo!
    @bar = :foo
  end

  def foo?
    @bar == :foo
  end
end

Example

Running Reek on:

class Dummy
  def test
    @ivar
  end
end

would report:

[1]:InstanceVariableAssumption: Dummy assumes too much for instance variable @ivar

Note that this example would trigger this smell warning as well:

class Parent
  def initialize(omg)
    @omg = omg
  end
end

class Child < Parent
  def foo
    @omg
  end
end

The way to address the smell warning is that you should create an attr_reader to use @omg in the subclass and not access @omg directly like this:

class Parent
  attr_reader :omg

  def initialize(omg)
    @omg = omg
  end
end

class Child < Parent
  def foo
    omg
  end
end

Directly accessing instance variables is considered a smell because it breaks encapsulation and makes it harder to reason about code.

If you don't want to expose those methods as public API just make them private like this:

class Parent
  def initialize(omg)
    @omg = omg
  end

  private
  attr_reader :omg
end

class Child < Parent
  def foo
    omg
  end
end

Current Support in Reek

An instance variable must:

• be set in the constructor
• or be accessed through a method with lazy initialization / memoization.

If not, Instance Variable Assumption will be reported.

Duplication occurs when two fragments of code look nearly identical, or when two fragments of code have nearly identical effects at some conceptual level.

Reek implements a check for Duplicate Method Call.

Example

Here's a very much simplified and contrived example. The following method will report a warning:

def double_thing()
  @other.thing + @other.thing
end

One quick approach to silence Reek would be to refactor the code thus:

def double_thing()
  thing = @other.thing
  thing + thing
end

A slightly different approach would be to replace all calls of double_thing by calls to @other.double_thing:

class Other
  def double_thing()
    thing + thing
  end
end

The approach you take will depend on balancing other factors in your code.

A NilCheck is a type check. Failures of NilCheck violate the "tell, don't ask" principle.

Additionally, type checks often mask bigger problems in your source code like not using OOP and / or polymorphism when you should.

Example

Given

class Klass
  def nil_checker(argument)
    if argument.nil?
      puts "argument isn't nil!"
    end
  end
end

Reek would emit the following warning:

test.rb -- 1 warning:
  [3]:Klass#nil_checker performs a nil-check. (NilCheck)

A NilCheck is a type check. Failures of NilCheck violate the "tell, don't ask" principle.

Additionally, type checks often mask bigger problems in your source code like not using OOP and / or polymorphism when you should.

Example

Given

class Klass
  def nil_checker(argument)
    if argument.nil?
      puts "argument isn't nil!"
    end
  end
end

Reek would emit the following warning:

test.rb -- 1 warning:
  [3]:Klass#nil_checker performs a nil-check. (NilCheck)

A NilCheck is a type check. Failures of NilCheck violate the "tell, don't ask" principle.

Additionally, type checks often mask bigger problems in your source code like not using OOP and / or polymorphism when you should.

Example

Given

class Klass
  def nil_checker(argument)
    if argument.nil?
      puts "argument isn't nil!"
    end
  end
end

Reek would emit the following warning:

test.rb -- 1 warning:
  [3]:Klass#nil_checker performs a nil-check. (NilCheck)

This cop checks for uses of each_key and each_value Hash methods.

Note: If you have an array of two-element arrays, you can put parentheses around the block arguments to indicate that you're not working with a hash, and suppress RuboCop offenses.

Example:

# bad
hash.keys.each { |k| p k }
hash.values.each { |v| p v }
hash.each { |k, _v| p k }
hash.each { |_k, v| p v }

# good
hash.each_key { |k| p k }
hash.each_value { |v| p v }

This cop checks for uses of each_key and each_value Hash methods.

Note: If you have an array of two-element arrays, you can put parentheses around the block arguments to indicate that you're not working with a hash, and suppress RuboCop offenses.

Example:

# bad
hash.keys.each { |k| p k }
hash.values.each { |v| p v }
hash.each { |k, _v| p k }
hash.each { |_k, v| p v }

# good
hash.each_key { |k| p k }
hash.each_value { |v| p v }

This cop makes sure that accessor methods are named properly.

Example:

# bad
def set_attribute(value)
end

# good
def attribute=(value)
end

# bad
def get_attribute
end

# good
def attribute
end

This cop checks for unused block arguments.

Example:

# bad

do_something do |used, unused|
  puts used
end

do_something do |bar|
  puts :foo
end

define_method(:foo) do |bar|
  puts :baz
end

Example:

#good

do_something do |used, _unused|
  puts used
end

do_something do
  puts :foo
end

define_method(:foo) do |_bar|
  puts :baz
end

This cops checks the indentation of the first key in a hash literal where the opening brace and the first key are on separate lines. The other keys' indentations are handled by the AlignHash cop.

By default, Hash literals that are arguments in a method call with parentheses, and where the opening curly brace of the hash is on the same line as the opening parenthesis of the method call, shall have their first key indented one step (two spaces) more than the position inside the opening parenthesis.

Other hash literals shall have their first key indented one step more than the start of the line where the opening curly brace is.

This default style is called 'specialinsideparentheses'. Alternative styles are 'consistent' and 'align_braces'. Here are examples:

Example: EnforcedStyle: specialinsideparentheses (default)

# The `special_inside_parentheses` style enforces that the first key
# in a hash literal where the opening brace and the first key are on
# separate lines is indented one step (two spaces) more than the
# position inside the opening parentheses.

# bad
hash = {
  key: :value
}
and_in_a_method_call({
  no: :difference
                     })

# good
special_inside_parentheses
hash = {
  key: :value
}
but_in_a_method_call({
                       its_like: :this
                     })

Example: EnforcedStyle: consistent

# The `consistent` style enforces that the first key in a hash
# literal where the opening brace and the first key are on
# seprate lines is indented the same as a hash literal which is not
# defined inside a method call.

# bad
hash = {
  key: :value
}
but_in_a_method_call({
                       its_like: :this
                      })

# good
hash = {
  key: :value
}
and_in_a_method_call({
  no: :difference
})

Example: EnforcedStyle: align_braces

# The `align_brackets` style enforces that the opening and closing
# braces are indented to the same position.

# bad
and_now_for_something = {
                          completely: :different
}

# good
and_now_for_something = {
                          completely: :different
                        }

This cop looks for trivial reader/writer methods, that could have been created with the attr_* family of functions automatically.

Example:

# bad
def foo
  @foo
end

def bar=(val)
  @bar = val
end

def self.baz
  @baz
end

# good
attr_reader :foo
attr_writer :bar

class << self
  attr_reader :baz
end

This cop checks for trailing comma in array and hash literals.

Example: EnforcedStyleForMultiline: consistent_comma

# bad
a = [1, 2,]

# good
a = [
  1, 2,
  3,
]

# good
a = [
  1,
  2,
]

Example: EnforcedStyleForMultiline: comma

# bad
a = [1, 2,]

# good
a = [
  1,
  2,
]

Example: EnforcedStyleForMultiline: no_comma (default)

# bad
a = [1, 2,]

# good
a = [
  1,
  2
]