Class: RuboCop::Cop::Lint::UnmodifiedReduceAccumulator

Relationships & Source Files
Super Chains via Extension / Inclusion / Inheritance
Class Chain:
self, ::RuboCop::Cop::Base, ::RuboCop::ExcludeLimit, NodePattern::Macros, RuboCop::AST::Sexp
Instance Chain:
Inherits: RuboCop::Cop::Base
Defined in: lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb


Looks for reduce or inject blocks where the value returned (implicitly or explicitly) does not include the accumulator. A block is considered valid as long as at least one return value includes the accumulator.

If the accumulator is not included in the return value, then the entire block will just return a transformation of the last element value, and could be rewritten as such without a loop.

Also catches instances where an index of the accumulator is returned, as this may change the type of object being retained.

For the purpose of reducing false positives, this cop only flags returns in reduce blocks where the element is the only variable in the expression (since we will not be able to tell what other variables relate to via static analysis).


# bad
(1..4).reduce(0) do |acc, el|
  el * 2

# bad, may raise a NoMethodError after the first iteration
%w(a b c).reduce({}) do |acc, letter|
  acc[letter] = true

# good
(1..4).reduce(0) do |acc, el|
  acc + el * 2

# good, element is returned but modified using the accumulator
values.reduce do |acc, el|
  el << acc

# good, returns the accumulator instead of the index
%w(a b c).reduce({}) do |acc, letter|
  acc[letter] = true

# good, at least one branch returns the accumulator
values.reduce(nil) do |result, value|
  break result if something?

# good, recursive
keys.reduce(self) { |result, key| result[key] }

# ignored as the return value cannot be determined
enum.reduce do |acc, el|
  x = foo(acc, el)

Constant Summary

::RuboCop::Cop::Base - Inherited


Class Attribute Summary

::RuboCop::Cop::Base - Inherited

.gem_requirements, .lint?,

Returns if class supports autocorrect.


Override if your cop should be called repeatedly for multiple investigations Between calls to on_new_investigation and on_investigation_end, the result of processed_source will remain constant.

Class Method Summary

::RuboCop::Cop::Base - Inherited


List of cops that should not try to autocorrect at the same time as this cop.



.callbacks_needed, .cop_name, .department,

Returns an url to view this cops documentation online.


Call for abstract Cop classes.


Override and return the Force class(es) you need to join.


Returns true if the cop name or the cop namespace matches any of the given names.


Register a version requirement for the given gem name.


::RuboCop::ExcludeLimit - Extended


Sets up a configuration option to have an exclude limit tracked.


Instance Attribute Summary

Instance Method Summary

::RuboCop::Cop::Base - Inherited


Adds an offense that has no particular location.


Adds an offense on the specified range (or node with an expression) Unless that offense is disabled for this range, a corrector will be yielded to provide the cop the opportunity to autocorrect the offense.


Called before any investigation.


Configuration Helpers.

#cop_name, #excluded_file?,

This method should be overridden when a cop’s behavior depends on state that lives outside of these locations:


Gets called if no message is specified when calling add_offense or add_global_offense Cops are discouraged to override this; instead pass your message directly.


Alias for Base#cop_name.


Called after all on_…​


Called before all on_…​


Called instead of all on_…​


There should be very limited reasons for a Cop to do it’s own parsing.


Called between investigations.

#relevant_file?, #target_rails_version, #target_ruby_version, #annotate, #apply_correction, #attempt_correction,

Reserved for Cop::Cop.


Called to complete an investigation.

#correct, #current_corrector,

Reserved for Commissioner:

#current_offenses, #currently_disabled_lines, #custom_severity, #default_severity, #disable_uncorrectable, #enabled_line?, #file_name_matches_any?, #find_message, #find_severity, #range_for_original, #range_from_node_or_range,

Actually private methods.


::RuboCop::Cop::AutocorrectLogic - Included

::RuboCop::Cop::IgnoredNode - Included

Constructor Details

This class inherits a constructor from RuboCop::Cop::Base

Instance Method Details

#acceptable_return?(return_val, element_name) ⇒ Boolean (private)

Determine if a return value is acceptable for the purposes of this cop If it is an expression containing the accumulator, it is acceptable Otherwise, it is only unacceptable if it contains the iterated element, since we otherwise do not have enough information to prevent false positives.

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 191

def acceptable_return?(return_val, element_name)
  vars = expression_values(return_val).uniq
  return true if vars.none? || (vars - [element_name]).any?


#accumulator_index?(node, accumulator_name)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 78

def_node_matcher :accumulator_index?, <<~PATTERN
  (send (lvar %1) {:[] :[]=} ...)

#allowed_type?(parent_node) ⇒ Boolean (private)

Exclude begin nodes inside a dstr from being collected by return_values

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 199

def allowed_type?(parent_node)

#block_arg_name(node, index) (private)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 159

def block_arg_name(node, index)

#check_return_values(block_node) (private)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 142

def check_return_values(block_node)
  return_values = return_values(block_node.body)
  accumulator_name = block_arg_name(block_node, 0)
  element_name = block_arg_name(block_node, 1)
  message_opts = { method: block_node.method_name, accum: accumulator_name }

  if (node = returned_accumulator_index(return_values, accumulator_name, element_name))
    add_offense(node, message: format(MSG_INDEX, message_opts))
  elsif potential_offense?(return_values, block_node.body, element_name, accumulator_name)
    return_values.each do |return_val|
      unless acceptable_return?(return_val, element_name)
        add_offense(return_val, message: format(MSG, message_opts))

#element_modified?(node, element_name)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 83

def_node_search :element_modified?, <<~PATTERN
    (send _receiver !{:[] :[]=} <`(lvar %1) `_ ...>)               # method(el, ...)
    (send (lvar %1) _message <{ivar gvar cvar lvar send} ...>)     # el.method(...)
    (lvasgn %1 _)                                                  # el = ...
    (%RuboCop::AST::Node::SHORTHAND_ASSIGNMENTS (lvasgn %1) ... _) # el += ...


[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 104

def_node_search :expression_values, <<~PATTERN
    (%RuboCop::AST::Node::VARIABLES $_)
    (%RuboCop::AST::Node::EQUALS_ASSIGNMENTS $_ ...)
    (send (%RuboCop::AST::Node::VARIABLES $_) :<< ...)
    $(send _ _)
    (dstr (begin {(%RuboCop::AST::Node::VARIABLES $_)}))

#lvar_used?(node, name)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 93

def_node_matcher :lvar_used?, <<~PATTERN
    (lvar %1)
    (lvasgn %1 ...)
    (send (lvar %1) :<< ...)
    (dstr (begin (lvar %1)))
    (%RuboCop::AST::Node::SHORTHAND_ASSIGNMENTS (lvasgn %1))

#on_block(node) Also known as: #on_numblock

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 115

def on_block(node)
  return unless node.body
  return unless reduce_with_block?(node)
  return unless node.argument_list.length >= 2



Alias for #on_block.

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 122

alias on_numblock on_block

#potential_offense?(return_values, block_body, element_name, accumulator_name) ⇒ Boolean (private)

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 176

def potential_offense?(return_values, block_body, element_name, accumulator_name)
  !(element_modified?(block_body, element_name) ||
    returns_accumulator_anywhere?(return_values, accumulator_name))


[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 70

def_node_matcher :reduce_with_block?, <<~PATTERN
    (block (call _recv {:reduce :inject} ...) args ...)
    (numblock (call _recv {:reduce :inject} ...) ...)

#return_values(block_body_node) (private)

Return values in a block are either the value given to next, the last line of a multiline block, or the only line of the block

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 128

def return_values(block_body_node)
  nodes = [block_body_node.begin_type? ? block_body_node.child_nodes.last : block_body_node]

  block_body_node.each_descendant(:next, :break) do |n|
    # Ignore `next`/`break` inside an inner block
    next if n.each_ancestor(:block).first != block_body_node.parent
    next unless n.first_argument

    nodes << n.first_argument


#returned_accumulator_index(return_values, accumulator_name, element_name) (private)

Look for an index of the accumulator being returned, except where the index is the element. This is always an offense, in order to try to catch potential exceptions due to type mismatches

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 167

def returned_accumulator_index(return_values, accumulator_name, element_name)
  return_values.detect do |val|
    next unless accumulator_index?(val, accumulator_name)
    next true if val.method?(:[]=)

    val.arguments.none? { |arg| lvar_used?(arg, element_name) }

#returns_accumulator_anywhere?(return_values, accumulator_name) ⇒ Boolean (private)

If the accumulator is used in any return value, the node is acceptable since the accumulator has a chance to change each iteration

[ GitHub ]

# File 'lib/rubocop/cop/lint/unmodified_reduce_accumulator.rb', line 183

def returns_accumulator_anywhere?(return_values, accumulator_name)
  return_values.any? { |node| lvar_used?(node, accumulator_name) }