🔒 Understanding Closures in Ruby: A Powerful Yet Elegant Feature

April 14, 2025

If you’ve been working with Ruby for a while, you’ve likely encountered blocks , procs , and lambdas. But beneath these familiar constructs lies a concept that gives them real power: closures.

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So, What Is a Closure?

A closure is a function or block of code that captures and remembers the environment in which it was defined — even when it’s executed outside of that scope.

In Ruby, closures allow us to write cleaner, more flexible, and more powerful code.

Let’s take a simple example:

def multiplier(factor)
  ->(n) { n * factor }
end

times_two = multiplier(2)
times_five = multiplier(5)

puts times_two.call(10) # => 20
puts times_five.call(10) # => 50

Even though factor is a local variable inside the multiplier method, the lambda returned by it retains access to factor — that’s the closure in action.

Why It Matters in Real Projects

Closures are essential for:

Encapsulating behavior without polluting the global or class namespace Creating flexible APIs , like Rails scopes or middleware stacks Delaying execution — think of callbacks or background jobs Functional programming patterns , like currying or chaining

You’ve likely seen it in Rails:

class User < ApplicationRecord
  scope :active, -> { where(active: true) }
end

That lambda is a closure — capturing no external variables in this case, but still a complete, standalone chunk of logic.

Bonus: Closures & State

Ruby closures can also maintain internal state in clever ways:

def counter
  count = 0
  -> { count += 1 }
end

my_counter = counter
puts my_counter.call # => 1
puts my_counter.call # => 2

That count variable is private to the closure and persists between calls.

Final Thoughts

Closures are one of Ruby’s most elegant features — blending the best of functional and object-oriented programming.

If you’re building tools, APIs, or just want to reduce duplication in your codebase, understanding closures isn’t optional — it’s a superpower.

Have you used closures in an interesting way in your Ruby projects? Let’s connect and share ideas!