From: Robert Klemme <shortcutter@...>
Date: 2009-04-25T17:10:39+09:00
Subject: Re: hygienic startup file

On 25.04.2009 00:04, Thomas Hafner wrote:
> Robert Klemme <shortcutter@googlemail.com> wrote/schrieb <9e3fd2c80904240015h11f3376l424a24457691b0c0@mail.gmail.com>:
> 
>> The question is: why do you need constants if you want to hide them?
> 
> First I defined constants rather than variables simply because each of
> them actually is constant rather than variable: the value never
> changes after assignment. In C++ I would add a const qualifier to the
> local variables, e.g. to prevent from unintentionally overriding it.
> In Haskell ther's no need for a const keyword, because all definitions
> are immutable per se, which is regarded a feature in this purely
> functional programming language.

Keep in mind that Ruby constants are not really constant:

robert@fussel ~
$ ruby19 -e 'X=1;puts X;X=2;puts X'
1
-e:1: warning: already initialized constant X
2

Generally, Ruby is less strict with these things ("private" is another 
example).

> But in Ruby there are no local constants analogical to local
> variables, so when I simply define constants I unintentionally export
> them, too. (Freeze is no option, because it freezes the object hold by
> the variable, not the variable itself.)
> 
> Every alternative has its own tradeoff:
> 1. If I define simple constants, I pollute the namespace of the
> loading code.
> 2. If I define constants within class << Object.new, the code is
> somehow ugly.

This is also a kind of inappropriate solution because you create an 
instance just to get at its singleton class.  This is not needed (see 
below).

> 3. If I define local variables, I don't benefit from the fact that
> they are actually constant.
> 
> So currently alternative #3 is best for me, but if somebody shows me a
> way to make #2 less ugly (get away from class << Object.new), that
> would probably my favorite.

You can use an anonymous module for this:

robert@fussel ~
$ ruby19 <<EOF
 > Module.new do
 > TEST = 123
 > puts "Test is #{TEST}"
 > end
 > puts "Is there TEST?"
 > puts TEST
 > EOF
Test is 123
Is there TEST?
-:6:in `<main>': uninitialized constant TEST (NameError)

robert@fussel ~
$

Kind regards

	robert

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/