From: Jacob Fugal <lukfugl@...>
Date: 2006-05-10T00:40:37+09:00
Subject: Re: code snippet: can it be done better/shorter?

On 5/9/06, Krekna Mektek <krekna@gmail.com> wrote:
> A bit late, but I was reading this post again, but your code is
> actually not the same, if I understood  you right.
>
> because from is assigned "Unknown" if it's an empty string.
> And from || "Unknown" results in true if from is defined, so the value
> "Unknown" is not assigned to it. Right?

The value "Unknown" will not be assigned to from, but it doesn't need
to be. An expression of the form 'a || b' is equivalent to this:

  a ? a : b

or more verbosely:

  if a then a else b end

So, if from evaluates true in a boolean context -- which it will
unless a is nil or false -- the value of a is used as the value of the
expression, not the value 'true'. And if a evaluates false in a
boolean context, then the value of b is used.

So the expression 'from || "Unknown"' will return either the value of
from, or the value "Unknown" if from has from has a value of nil or
false. And since from has a value of nil iff REG did not match, the
value "Unknown" will be used iff REG does not match. When REG does
match, the value of the capture from the regular expression will be
used (even if that value is "", since an empty string is a true value
in Ruby).

Jacob Fugal