Lint-UnmodifiedReduceAccumulator.md

Pattern: Unmodified reduce/inject accumulator

Issue: -

Description

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.

Examples

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

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

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

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

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

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

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

Further Reading

• RuboCop - Lint/UnmodifiedReduceAccumulator