Ruby Enumerator::Lazy: Efficient Lazy Evaluation for Large Collections

Problem: Memory‑hungry Collections

When you call eager methods like map or select on a large range, Ruby builds an intermediate array that holds every element. For a range of millions of numbers or an infinite sequence, this can quickly exhaust available memory or never finish.

Thesis: Defer Work Until You Need It

Ruby’s Enumerator::Lazy wraps an enumerable and postpones operations such as map, select, reject, and flat_map until a terminal method (e.g., force, first, each) is called. The result is a streaming pipeline that processes items one at a time, keeping memory usage constant.

Lazy Enumerator Basics

• Requires Ruby 2.0 or newer – ruby -v will show the version.
• Call .lazy on any enumerable: (1..).lazy creates a lazy enumerator over the infinite range.
• Chain lazy operators: select { |n| n.even? }, map { |n| n * 2 }, take(10).
• Terminal methods trigger evaluation: force returns a real array, each iterates without building an array.

Worked Example: First 10 Even Numbers from an Infinite Range

# Ensure Ruby 2.0+ is running
# irb or a script
lazy = (1..).lazy.select { |n| n.even? }.take(10)
puts lazy.force

Output:

2
4
6
8
10
12
14
16
18
20

Notice that no array of all odd numbers was created; the pipeline stops after the tenth even number is found.

Trade‑offs & Limitations

• Per‑element overhead: Each lazy operation adds a small function call cost, which can be noticeable in tight loops.
• Side‑effects are delayed: A puts inside a select block only runs when the terminal method is invoked. This can hide bugs if you expect immediate output.
• Non‑lazy methods break the chain: Calling sort, max, or minmax forces the entire sequence into memory, defeating the benefit.
• Version requirement: Ruby < 2.0 does not support .lazy and will raise NoMethodError.

Actionable Guidance

1. Use .lazy when dealing with large or infinite enumerables and you only need a subset or a streaming result.
2. Keep the pipeline free of early‑forcing methods; if you need a sorted result, materialize the data first.
3. Test side‑effects by adding a puts inside the block and observe that it executes only during force or each.
4. Measure memory usage with tools like ps or Ruby’s ObjectSpace to confirm that the lazy pipeline keeps memory low.
5. Document the lazy behavior in code comments so future maintainers understand that the chain is deferred.

In short, Enumerator::Lazy is a powerful tool for Ruby developers who need to process large data streams efficiently. Apply it judiciously, keep an eye on side‑effects, and you’ll avoid the pitfalls of eager evaluation.