Exiting from a Kernel#loop - ruby

Question

In Kernel#loop documentation, there is an example that uses break in order to break out of the loop. Otherwise, the documentation talks about rasing a StopIteration error.
I tried them both:
i=0
loop do
puts i
i+=1
break if i==10
end
i=0
loop do
puts i
i+=1
raise StopIteration if i==10
end
The output is the same. Are there differences between the two approaches? I think that there should be, otherwise why bother to define an error class and all the managements that come with it?

Answer 1

break is a keyword in ruby, which terminates the most internal loop, whether it be loop, or for, and a few more (see here).
StopIteration is an exception, which is caught by Kernel.loop (see here).
So in your scenario, they are the same, but in a different scenario, they will act differsntly:
puts "first run"
for i in 0..20 do
puts i
break if i==10
end
puts "first done"
puts "second run"
for i in 0..20 do
puts i
raise StopIteration if i==10
end
puts "second done" # <= will not be printed
Here is a scenario where StopIteration can be utilized when break cannot:
puts "first run"
def first_run(i) # will not compile
puts i
break if i==10
end
i=0
loop do
first_run(i)
i+=1
end
puts "first done"
puts "second run"
def second_run(i)
puts i
raise StopIteration if i==10
end
i=0
loop do
second_run(i)
i+=1
end
puts "second done"
Here is another use-case, which uses the fact the Enumerator.next throws a StopIteration error when the Enumerator has reached the end:
enum = 5.times.to_enum
loop do
p enum.next
end
will print
0
1
2
3
4
Thanks Cary for this example.

Answer 2

There are two uses of the break keyword.
First: the break keyword, when in a block, causes the method that the block was passed to to return. If you pass an argument to break, the return value of the method will be that argument. Here is an example:
def a
puts 'entering method a'
yield
puts 'leaving method a'
end
result = a { break 50 }
puts result
This will print:
entering method a
50
Second: the break keyword can cause a while, until, or for loop to terminate. Here is an example:
i = 0
result =
while i < 5
i += 1
puts i
break 75 if i == 3
end
puts result
This will print:
1
2
3
75
Your example with Kernel#loop makes use of the first case, causing the loop method to return.
StopIteration is an exception which only works with Kernel#loop as far as I can tell. For example:
infinite_loop = Enumerator.new do |y|
i = 0
while true
y << i
i += 1
end
end
infinite_loop.each do |x|
puts x
raise StopIteration if x == 4
end
Fails because StopIteration is uncaught, however:
x = 0
loop.each do
puts x
x += 1
raise StopIteration if x == 4
end
Catches StopIteration and exits.