Feature #12262: Anti-loop

• Status: Open
• Assignee: -
• Target version: -

[sawa (Tsuyoshi Sawada)]

The loop method continues by default, and requires the keyword break to escape. This is good when the continuing cases are the norm and the escaping cases are exceptional:

loop do
  ...
  if ...
    ...
  elsif ...
    ...
  elsif ...
    ...
    break # breaks on exceptional cases
  elsif ...
    ...
  else
    ...
  end
end

But when the continuing cases are exceptional and the escaping cases are the norm, the construction requires a lot of break, and it becomes cumbersome:

loop do
  ...
  if ...
    ...
    break # lot of breaks
  elsif ...
    ...
    break # lot of breaks
  elsif ...
    ...
    break # lot of breaks
  elsif ...
    ...
  else
    ...
    break # lot of breaks
  end
end

I actually see this use case a lot when user input is asked with validation on a command line script.

I request a loop-like method that works in the opposite way to loop, that is, it escapes (i.e., runs only once) by default, and requires a keyword to continue (perhaps next). The second code above would then be written like:

some_loop_like_method do
  ...
  if ...
    ...
  elsif ...
    ...
  elsif ...
    ...
  elsif ...
    ...
    next # continues on exceptional cases
  else
    ...
  end
end

[jwille (Jens Wille)]

You can make your last example work with loop by just adding a break at the end of the loop body. I don't think that warrants a new method.

[shevegen (Robert A. Heiler)]

I don't have any big pro or contra opinion, but there is one thing I am wondering:

Is this still called a loop in the second case? Because the default is to break
after the first run. And a loop implies to continue, until one ends it or? :-)

However had, what I might find interesting, is to have other means to force the
end of a loop. Like, "break" is used, but what if we could designate another
way to end a loop? If we could do that, then perhaps your suggestion might be
implied to work in the second case, because you could somehow specify that
loop would en when a "return nil" would be implied.

E. g. something like (the syntax does not work, it just is an example):

loop(break_on: nil) {
  if ...
    ...
  elsif ...
    ...
  elsif ...
    ...
    next # continues on exceptional cases
  else
    ...
  end
}   

Where the default would be a loop like:

loop(break_on: :break)

Which can be omitted. (The symbol :break would then default on the
keyword break).

Please consider this just as food-for-thought, I actually do not
really suggest it - I am just playing with the thought here. :)

(I myself probably would prefer "the simpler, the better" which
is why I do not suggest a change, but as said, I am neutral on
this suggestion.)