From: Jacob Fugal <lukfugl@...>
Date: 2006-05-10T02:18:30+09:00
Subject: Re: code snippet: can it be done better/shorter?

On 5/9/06, Robert Dober <robert.dober@gmail.com> wrote:
> On 5/9/06, Jacob Fugal <lukfugl@gmail.com> wrote:
> > An expression of the form 'a || b' is equivalent to this:
> >
> >   a ? a : b
> Which is unfortunately not true because of  possible side effects, in
> our case however there are none and it was a *nice way* to explain
> things, but watch out for method calls.

True. I should have said "essentially equivalent". :) For those
unclear what Robert means here, consider this contrived code:

  $ cat > test.rb

  $a_called = false
  def a
    if $a_called
      puts "Called again."
      return false
    else
      puts "Called once."
      $a_called = true
      return true
    end
  end

  x = a ? a : "default"
  puts "x is #{x.inspect}."

  $a_called = false

  x = a || "default"
  puts "x is #{x.inspect}."

  x = a || "default"
  puts "x is #{x.inspect}."

  $ ruby test.rb
  Called once.
  Called again.
  x is false.

  Called once.
  x is true.

  Called again.
  x is "default".

Using the ternary form, the method a gets called multiple times; using
the || form, it only gets called once and the value from that call is
reused. So a || b is more closely equivalent to:

  (tmp = a) ? tmp : b

But this is nowhere near as nice looking, and may still be not *quite* there. :)

Jacob Fugal