From: "Hal E. Fulton" <hal9000@...>
Date: 2002-09-12T06:39:49+09:00
Subject: if and unless

Well, the reason I like having both these
was expressed by someone else a day or two
ago. Too lazy to look it up now.

But to paraphrase (and over-generalize):

I use unless for exceptional or incidental logic, and if
for expected or mainstream logic.

Thus when I was working on a little program (which needs
to be refactored, <sigh>) to download a file and edit it,
I realized there was no need to re-upload the file if it
was not changed.

So I wrote:

  upload unless unchanged

So someone asked, why not:

  upload if changed

Well, since the whole point of downloading the file is
to edit it, I expect that 99% of the time the file will
be changed.

But it's not just that. Writing it this way (to me)
*emphasizes* that point.

I use 'unless' for "the road less travelled," to invoke
Robert Frost. (The analogy breaks down immediately. :)

In fact, I'd say that it's always just the branch that 
occurs less frequently; it's the one we consider weird
(or the one we want to ignore) even if it happens a lot.
But that's just me.

And this, of course, is separate from the question of
whether we use the pre- or post- form of 'if' (or 'unless').

Hal