Question

Many methods in Ruby array return an enumerator when invoked without parameters or blocks (index, keep_if, each, drop_while and many more).

• When is it appropriate to use methods in this form, as opposed to calling them with a block?

Answer 1

From the docs to Enumerator:

Most methods have two forms: a block form where the contents are
evaluated for each item in the enumeration, and a non-block form which
returns a new Enumerator wrapping the iteration.

This allows you to chain Enumerators together. For example, you can
map a list’s elements to strings containing the index and the element
as a string via:

puts %w[foo bar baz].map.with_index {|w,i| "#{i}:#{w}" }
# => ["0:foo", "1:bar", "2:baz"]

An Enumerator can also be used as an external iterator. For example,
Enumerator#next returns the next value of the iterator or raises
StopIteration if the Enumerator is at the end.

e = [1,2,3].each   # returns an enumerator object.
puts e.next   # => 1
puts e.next   # => 2
puts e.next   # => 3
puts e.next   # raises StopIteration

I’m sorry for copy-paste, but I couldn’t explain better.