From: David Masover <ninja@...>
Date: 2010-07-29T12:11:09+09:00
Subject: Re: hard drive backup program in 100% pure ruby

On Wednesday, July 28, 2010 01:06:56 pm travis.ml-ruby-talk@subspacefield.org 
wrote:
> Just figured I'd mention that I'm working on this:
> 
> http://www.subspacefield.org/security/hdb/

Looks interesting...

> I think that I'll run into needing some features from the File or
> FileUtil classes (such as mapping from filename to inode numbers)

Really? Check out File#stat.

> My background is python, and here were my reactions to ruby:
> 
> 1) Assignment is weird.  Both names point to the same object,
>    unless it's a "small" type (fits in a register).

I don't think so. For example:

irb(main):001:0> 5.instance_variable_set :@foo, 'five'
=> "five"
irb(main):002:0> 3.instance_variable_get :@foo
=> nil
irb(main):003:0> 5.instance_variable_get :@foo
=> "five"
irb(main):004:0> 

In other words, the "small" types point to the same object. As an 
implementation detail, Ruby likely does store them as plain old numbers in a 
register, but you don't have to care about that.

>    That really confused me for a bit.  Fix is easy - always use dup in
>    your initializers.

What? Why?

I mean, I guess -- there's certainly precedent for it. But most of the time, 
when I'm passing something to an initializer, I'm not keeping a reference 
around -- or if I am, it's because I actually want the object to be shared.

> 2) I originally wrote some fancy set operations (like Set class but
>    not using a hash) using array operators like -=.

Careful. Remember:

a -= b

always expands to:

a = a - b

In other words, you're duping your entire array every single time you do that. 
Is that what you want? You could always do array.delete_if (or array.reject, 
if you want the duping behavior) -- those take blocks which let you set your 
own comparison.

I don't know offhand how any of the builtin array operations compare objects, 
or what you need to override. There are at least four builtin equivalence 
operators, which all do slightly different things:

a == b
a === b
a.eql?(b)
a.equals?(b)

And I don't know offhand what each of those do, but you should look them up 
before overriding.

> 3) What's the syntax for class method constructors?

...what?

> I.E. I want four
> different ways to create metadata entries, some of which have overlapping
> signatures, so I can't use a single initialzer method.

Are you sure there's no way your initializer can tell what it's dealing with? 
For example, say you ultimately expect a string:

def initialize anything
  @foo = anything.to_s
end

And while I don't ordinarily encourage type checking, I think it's OK if 
you're dealing with option arguments:

def initialize options
  if options.kind_of? Hash
    @a = options[:a]
    @b = options[:b]
  else
    @a = options.to_s
  end
end

>    Currently I do this by having an empty initializer and doing something
> like: a = Whatever.new().create_from_string(string_args)

By the way: The parens are optional in Ruby. Even in the DataMapper code, 
where they're encouraged, I don't think I ever see the empty parens like that.

That said, there's nothing stopping you from doing this:

class Whatever
  class << self
    def create_from_string(string_args)
      new.tap{|obj| obj.create_from_string(string_args)}
    end
  end
  def create_from_string_args(string_args)
    ...
  end
end

That, or just do your initialization inside the body of 'create_from_string' 
-- or accept a single standardized format (like an options hash) in the 
initialize method, and just convert the specialized argument (like the 
argument to create_from_string) into something more generic.

Remember, there's no special "new" syntax, so there's absolutely nothing 
stopping you from making as many factory methods as you like. (Perl is the 
same way, and I've always wondered why more languages aren't.)



There's a limit to how much advice I can give you, though, without a little 
more details about what you're trying to do. For example, what is it that 
you're trying to put into set-like behavior, and under what circumstances? 
What kind of object are you trying to create_from_string, and what are the 
possible ways you want of initializing it?