What is the advantage of creating an enumerable object using to_enum in Ruby?

enumeration, ruby

Solution

Most built-in methods that accept a block will return an enumerator in case no block is provided (like `String#each_char` in your example). For these, there is no reason to use `to_enum`; both will have the same effect.

A few methods do not return an Enumerator, though. In those case you might need to use `to_enum`.

# How many elements are equal to their position in the array?
[4, 1, 2, 0].to_enum(:count).each_with_index{|elem, index| elem == index} #=> 2

As another example, `Array#product`, `#uniq` and `#uniq!` didn't use to accept a block. In 1.9.2, this was changed, but to maintain compatibility, the forms without a block can't return an `Enumerator`. One can still "manually" use `to_enum` to get an enumerator:

require 'backports/1.9.2/array/product' # or use Ruby 1.9.2+
# to avoid generating a huge intermediary array:
e = many_moves.to_enum(:product, many_responses)
e.any? do |move, response|
  # some criteria
end

The main use of `to_enum` is when you are implementing your own iterative method. You typically will have as a first line:

def my_each
  return to_enum :my_each unless block_given?
  # ...
end

Problem

Why would you create a proxy reference to an object in Ruby, by using the to_enum method rather than just using the object directly? I cannot think of any practical use for this, trying to understand this concept & where someone might use it, but all the examples I have seen seem very trivial. For example, why use: ``` "hello".enum_for(:each_char).map {|c| c.succ } ``` instead of ``` "hello".each_char.map {|c| c.succ } ``` I know this is a very simple example, does anyone have any real-world examples?

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