From: Brian Candler <B.Candler@...>
Date: 2007-05-18T21:49:58+09:00
Subject: Re: Coopting String interpolation

On Fri, May 18, 2007 at 05:05:07PM +0900, Robert Klemme wrote:
> >I'm working in a research
> >project and I need to do some "clever stuff" (that's the thing I can't
> >disclose) to all the Strings. This thing is "different" depending on
> >how strings are composed. All the ways I know for string composition
> >but string interpolation (<<, +, concat, gsub, etc.) can be overridden
> >redefining methods in the string class. I'm looking for a way to
> >intercept the string expansion to do my thing. Does ruby internally
> >call some overridable method to compose the strings used in a
> >interpolation?
> 
> Likely but also likely not accessible to pure Ruby code.  But in your 
> case (research project) it might be ok to hack the interpreter if you 
> need to catch all string interpolations.  Did you look into this yet?

Here's a simpler idea. Given that string interpolation calls to_s, you can
use this as your hook if you wrap your objects in a proxy object. You need
to ensure that operations like '+' also return an instance of this proxy
object, so you can do all the operations you want whilst keeping the string
wrapped. Then the only case that to_s is called is when the interpolation
takes place, which gives you the hook you're looking for (as long as you're
not calling to_s in any other context)

Example:

class WrapString
  def initialize(str)
    @str = str
  end

  def str
    @str
  end
  protected :str

  def +(other)
    self.class.new(str + (other.str rescue other.to_s))
  end
  # rinse and repeat

  # Here is your interpolation hook:
  def to_s
    str.to_s * 2
  end
end

a = WrapString.new("abc")
b = WrapString.new("def")

puts "Answer is #{a + b}"

a = WrapString.new("abc")
b = "def"

puts "Answer is #{a + b}"

The result of a + b is <WrapString @str="abcdef">, and then the
interpolation calls to_s which in this case just doubles it to
"abcdefabcdef"

There's probably a neater implementation of the above using method_missing
or a delegation pattern rather than enumerating all the methods of String,
but you get the idea.

Just a thought?

Brian.