jvoorhis comments on Ruby's Hidden do {} while () Loop

"Ruby's Hidden do {} while () Loop": comment by Dr Nic

Wed, 13 Jun 2007 19:04:18 NZST

Hmm, I don’t think I use the following syntax barely enough:

@foo =
  begin
    ... do something ...
  end

"Ruby's Hidden do {} while () Loop": comment by Sam Smoot

Wed, 13 Jun 2007 13:38:07 NZST

Just a warning, but I heard tell on comp.lang.ruby once upon a time that this (post-conditional-loops) was to be removed in 2.0 in favor of flow-control within your loop.

That was probably the better part of a year ago though, and hell if I can find the thread.

So I’ve avoided it since then, but maybe prematurely, since comp.lang.ruby seems loaded with threads on the construct with nary a whisper of deprecation. Count me as a fan of the post-conditional loop.

"Ruby's Hidden do {} while () Loop" by jvoorhis

Wed, 13 Jun 2007 10:13:00 NZST

I found the following snippet while reading the source for Tempfile#initialize in the Ruby core library:


begin
  tmpname = File.join(tmpdir, make_tmpname(basename, n))
  lock = tmpname + '.lock'
  n += 1
end while @@cleanlist.include?(tmpname) or
  File.exist?(lock) or File.exist?(tmpname)

At first glance, I assumed the while modifier would be evaluated before the contents of begin...end, but that is not the case. Observe:


>> begin
?>   puts "do {} while ()" 
>> end while false
do {} while ()
=> nil

As you would expect, the loop will continue to execute while the modifier is true.


>> n = 3
=> 3
>> begin
?>   puts n
>>   n -= 1
>> end while n > 0
3
2
1
=> nil

While I would be happy to never see this idiom again, begin...end is quite powerful. The following is a common idiom to memoize a one-liner method with no params:


def expensive
  @expensive ||= 2 + 2
end

Here is an ugly, but quick way to memoize something more complex:


def expensive
  @expensive ||=
    begin
      n = 99
      buf = "" 
      begin
        buf << "#{n} bottles of beer on the wall\n" 
        # ...
        n -= 1
      end while n > 0
      buf << "no more bottles of beer" 
    end
end