Why does `Enumerable` have `first` but not `last`?

Question

Enumerable has first:

(3..5).to_enum.first
# => 3

but it does not have last:

(3..5).to_enum.last
# => NoMethodError: undefined method `last' for #<Enumerator: 3..5:each>

Why is that?

Answer 1

It is because not all enumerable objects have the last element.

The simplest example would be:

[1, 2, 3].cycle

# (an example of what cycle does)
[1,2,3].cycle.first(9) #=> [1, 2, 3, 1, 2, 3, 1, 2, 3]

Even if the enumerator elements are finite, there is no easy way to get the last element other than iterating through it to the end, which would be extremely inefficient.