Question

I am trying to write a closure in Ruby. This is the code written in Python:

def counter():
    x = 0
    def increment(y):
        nonlocal x
        x += y
        print(x)
    return increment

Is there a “nonlocal” equivalent in Ruby so I can access and make changes to the variable x from inside increment?

Answer 1

The nonlocal keyword tells Python which variables to capture. In Ruby, you don’t need such a keyword: all variables are captured unless explicitly mentioned otherwise.

So, the Ruby equivalent to your Python code translates almost directly:

counter = -> {
  x = 0
  ->y {
    x += y
    puts x
  }
}

i = counter.()

i.(2)
# 2

i.(3)
# 5

It would probably be more idiomatic to use a method for counter, though:

def counter
  x = 0
  ->y {
    x += y
    puts x
  }
end

i = counter

i.(2)
# 2

i.(3)
# 5