Neat trick - 'with' and unnamed objects + 'with' proposals

[eao197]

On Tue, 24 Apr 2007 17:03:07 +0400, Bill Baxter  
<dnewsgroup at billbaxter.com> wrote:

> Denis Golovan wrote:
>>> Or perhaps analogous to how constructors are called 'this()' and have a
>>> 'this' pointer, we'd have a 'with' pointer so that the meaning of  
>>> 'this' in a class wouldn't be shadowed:
>>    +1. That feature is useful sometimes.
>
> You say "is" as if it exists in some other language that you've used?  
> Does it? And if so what's the syntax?

Such behaviour can easily be implemented in Ruby. This is very simple  
demonstration:

   # Just a demo class.
   class Demo
     attr_reader :shown
     attr_reader :focused
   end

   # Yet another demo class.
   class AnotherDemo
     attr_reader :title
   end

   # A globe function to be called with 'with' keyword.
   def some_global_function( obj )
     p obj
   end

   # This is implementation of 'with'.
   def with( obj, &blk )
     # 1: introduce a new method 'with' into object 'obj'.
     class <<obj
       def with; self; end
     end

     # 2: run the specified block on 'obj' context.
     # Because now 'obj' has method 'with' it can be
     # called from block body.
     begin
       obj.instance_eval &blk
     ensure
       # 3: method 'with' no more needed.
       class <<obj
         remove_method( :with )
       end
     end

     obj
   end

   # This is usage of 'with'
   d = with( Demo.new ) {
     @shown = false
     @focused = true
     # Content of new object of class Demo must be shown here.
     some_global_function( with )
   }
   a = with( AnotherDemo.new ) {
     @title = 'Hello, World'
     # Content of new object of class AnotherDemo must be shown here.
     some_global_function( with )
   }

   # Simple debug output.
   puts "d: shown: #{d.shown}, focused: #{d.focused}"
   puts "a: title: #{a.title}"

That program prints:

#<Demo:0x2c7d448 @shown=false, @focused=true>
#<AnotherDemo:0x2c7d3f8 @title="Hello, World">
d: shown: false, focused: true
a: title: Hello, World

Such implementation of 'with' assumes that obj hasn't method 'with'. But  
this limitation can be removed via aliasing.

-- 
Regards,
Yauheni Akhotnikau