From: Robert Klemme <shortcutter@...>
Date: 2005-10-20T23:40:47+09:00
Subject: Re: DbC, const and freeze.

Btw: this posting of Ryan was truncated on the news side. Does anybody
else have it truncated at their news server, too?

2005/10/20, Ryan Leavengood <leavengood@gmail.com>:
> On 10/19/05, John Carter <john.carter@tait.co.nz> wrote:
> >
> > For example, look at this hole...
> > irb
> > irb(main):001:0> a = [[1,2],[3,4]]
> > => [[1, 2], [3, 4]]
> > irb(main):002:0> a.freeze
> > => [[1, 2], [3, 4]]
> > irb(main):003:0> a << [9]
> > RuntimeError: can't modify frozen array
> >         from (irb):3:in `<<'
> >         from (irb):3
> >         from :0
> > irb(main):004:0> a[0]<<1
> > => [1, 2, 1]
> > irb(main):005:0> a
> > => [[1, 2, 1], [3, 4]]
> > irb(main):006:0>
> >
> > Even though "a" was frozen, I still could modify the things "a" contained.
>
> Fair enough. How about this:
<snip/>

Apart from the fact that deep_unfreeze is missing...

I still think that the concept of constness doesn't fit Ruby well. 
There are too many places where you can fiddle with state where you
normally (i.e. in other PL's) aren't allowed to.  Adding #unfreeze
wouldn't help either because it would break the contract of freeze
that is used today - IOW code may break.

In some cases where modification of an argument must be prevented a
delegation based approach may work.

I wouldn't go as far as to state that const is overrated but the fact
that C++ needed to introduce keyword "mutable" shows that it's not
about simply freezing state of an instance.  You rather want to freeze
observed state... Which brings up an idea....

module Kernel
private
  def const(obj)
    meth = {}
    sc = class<<obj;self;end
    obj.methods.grep(/=$/).each do |m|
      meth[m]=obj.method m
      sc.send(:define_method, m) { raise "ConstError" }
    end

    begin
      yield obj
    ensure
      meth.each do |m,mm|
        sc.send(:define_method, m, &mm)
      end
    end
  end
end

>> Test = Struct.new(:name, :val)
=> Test
>> obj = Test.new "foo", "bar"
=> #<struct Test name="foo", val="bar">
>> obj.name = "hello"
=> "hello"
>> const obj do
?>   obj.name = "new name"
>> end
RuntimeError: ConstError
        from (irb):8:in `name='
        from (irb):24
        from (irb):12:in `const'
        from (irb):23
        from :0
>> p obj.name
"hello"
=> nil
>> obj.name="new 2"
=> "new 2"
>> p obj.name
"new 2"
=> nil

This can easily be extended to multiple instances...  Maybe
modification of instance_variable_set does the job, too.

> Does anyone else love that name? ;)

Can you do #deep_think, too? We then could have #deep_thought?... :-)

Kind regards

robert