Ruby: An Interesting Block Pattern

Ruby has blocks, which enable all sorts of interesting idioms. I'm going to show one that will be familiar to Rails enthusiasts, but was new to me.

I was reading some code in a book, and it had the following:

def if_found(obj)
  if obj
    yield 
  else
    render :text => "Not found.", :status => "404 Not Found"
    false
  end    
end

Here's how you call it:

if_found(obj) do
    # We have a valid obj.  Render something with it.
end

The code in the block will only execute if the obj was found. If it wasn't found, the response will already have been taken care of.

I've been in the same situation in Python (using Pylons), and I coded something like:

def handle_not_found(obj):
    if not obj:
        return render_404_page()
    return None

Here's how you call it:

response = handle_not_found(obj)
if response:
    return response
# Otherwise, continue normally.

Pylons likes to return responses, whereas render in Ruby works as a side effect whose return value isn't important. However, that's not my point.

My point is that the Python code uses "if response:" whereas the Ruby code uses "if_found(obj) do". Python uses an explicit if statement, whereas Ruby hides the actual if statement in a block. Similarly, Rubyists tend to write "my_list.each do |i|..." (even though Ruby has a for statement), whereas Pythonistas use "for i in my_list".

Ok, now that I've totally made a mountain out of a molehill, please note that I'm not saying either is better than the other. I'm just saying it's interesting to note the difference.