Is there a reason that we cannot iterate on "reverse Range" in ruby?

I tried to iterate backwards with using a Range and each:

(4..0).each do |i|
  puts i
end
==> 4..0

Iteration through 0..4 writes the numbers. On the other Range r = 4..0 seems to be ok, r.first == 4, r.last == 0.

It seems to be strange to me that the construct above does not produce the expected result. What is the a reason for that? What are the situations when this behaviour is reasonable?

A range is just that: something defined by its start and end, not by its contents. "Iterating" over a range doesn't really make sense in a general case. Consider, for example, how you would "iterate" over the range produced by two dates. Would you iterate by day? by month? by year? by week? It's not well-defined. IMO, the fact that it's allowed for forward ranges should be viewed as a convenience method only.

If you want to iterate backwards over a range like that, you can always use downto:

$ r = 10..6
=> 10..6

$ (r.first).downto(r.last).each { |i| puts i }
10
9
8
7
6

Here are some more thoughts from others on why it's tough to both allow iteration and consistently deal with reverse-ranges.

How about (0..1).reverse_each which iterates the range backwards?

Iterating over a range in Ruby with each calls the succ method on the first object in the range.

$ 4.succ
=> 5

And 5 is outside the range.

You can simulate reverse iteration with this hack:

(-4..0).each { |n| puts n.abs }

John pointed out that this will not work if it spans 0. This would:

>> (-2..2).each { |n| puts -n }
2
1
0
-1
-2
=> -2..2

Can't say I really like any of them because they kind of obscure the intent.

Another way is (1..10).to_a.reverse