Why and how to use lazy evaluation in Ruby?

[ccniuj]

ref: Ruby 2.0 Enumerable::Lazy
ref: Ruby 2.0 Works Hard So You Can Be Lazy
ref: Functional Programming and Ruby by Pat Shaughnessy

What is lazy evaluation?

Lazy evaluation, or call-by-need, means the value of an variable is evaluated when it is needed, which is a opposite concept of call-by-name, or eager evaluation.

Why be lazy anyway?

For example, you want to map throw an array of integer to time 3 and plus 3:

arr = [1, 2, 3]
arr.map{ |i| i*3 }.map{ |i| i+3 }

Here is a problem: arr will be looped twice, since map is called one after one. As we can see, this expression actually generates 2 results whenever map is called. However, we only need the last result. How can we only get the last result without generating the intermediate result? Use lazy:

arr = [1, 2, 3]
arr.lazy.map{ |i| i*3 }.map{ |i| i+3 }.to_a

How to use ruby lazy?

Above example showcases one way to use lazy:

[1, 2, 3].lazy
# => #<Enumerator::Lazy: [1, 2, 3]>

And this lazy object can be chained by enumerable methods.

[Ricardo-Paul]

So good explanation. Thanks a lot