From: Dan Doel <djd15@...>
Date: 2004-02-01T06:51:54+09:00
Subject: Re: No difference between .. and ... flip/flop operators?

Phil Tomson wrote:

>identical as in there is no difference in behavior between them?
>
Yes, that's what I meant.

As for all the stuff about what to replace it with and such (not just
the parent post, but many others)...

Some of the examples given can be done almost as easily without flip
flops, as long as you don't insist on doing things within a loop.
I realize this isn't the general case, but sometimes they turn out
that way, for instance:

> The Perl code that needs to be duplicated in Ruby is:
> while (<>) {
>     $in_header =   1  .. /^$/;
>     $in_body   = /^$/ .. eof();
> }

This can be done by something like this (borrowing from your modified code):

File.open("filename") do |f|
    str = f.readlines
    m = /\n\n/.match str
    in_header = m.pre_match
    in_body = m.post_match
end

This is as short as your flip flop code, and in fact, I can eliminate 1 
line.

I also cooked up a little magic in an attempt to reproduce flip flops on my
own. It, of course, doesn't work as well as real flip flops. Here's my code:

module Kernel
    def between(a, b, &blk)
        within = false
        proc do |l|
            within = true if a === l
            blk.call l if within
            within = false if b === l
        end
    end
end

With this, the following two pieces of code produce identical output:

lines.each { |l|
    puts l if l=~/BEGIN/ .. l=~/END/
}

lines.each &between(/BEGIN/, /END/ { |l|
    puts l
}

Further, you can technically chain together #between calls to make
nested flipflops, but it looks very ugly. It's not as general as
flip flops, nested or otherwise, either, so I'll let this stew for
a while and see if I can come up with any hideous code that makes
homebrew flip flops look nice.

As for the devil's advocate talking about Unix idioms as methods,
I'd point out that I can define my own #grep and use it, so you'd
have to look it up anyway. I cannot define my own Ruby syntax based
on other languages. Just a little devil's advocate back at you. :)

I, for one, don't have a strong opinion about whether flip flops
should be in the language. From the perl code posted, it looks like
that's where it's taken from. Matz is also considering taking out
some other perlisms, I believe (like certain $ variables), so
maybe this is something else he took from perl that he wishes he
didn't. However, I'd note that taking this out isn't a catastrophe.
Most languages don't have such an idiom, and they do fine. And,
maybe we can even figure out something to replace it that isn't
too bad from either side's perspective, given that Ruby is quite
able to be modified by itself for things like this.

Sorry for the long post.

- Dan