From: Bob Hutchison <hutch@...>
Date: 2007-04-13T21:14:01+09:00
Subject: Re: Idiom wanted (now hiring!)


On 13-Apr-07, at 2:02 AM, Jonathan wrote:

> Is there a cool way to do this without calling the function twice?:
>
> a = func(b) unless func(b).nil? (AKA)
> a = func(b) if func(b)
>


In lisp there are libraries of utilities that provide what is called  
'anaphoric' versions of control structures etc. They are  
simultaneously widely used and frowned upon. Paul Graham is one of  
the primary sources of code for this.

Ruby can't really do what lisp does, but here is a hacky  
approximation and some test cases. You'll see why they are frowned  
upon in the test cases (lisp doesn't do any better than ruby at  
avoiding errors).

Actually, what follows is the output of 'irb --prompt-mode xmp' so,  
should you want to actually use this stuff, you'll have to clean up  
the output. I don't think I recommend using it in Ruby by the way.

Cheers,
Bob


module Anaphoric
   def ifa(v)
     if v then
       @it = v
       yield
     end
   end

   def ifa2(v)
     if v then
       yield(v)
     end
   end
end
     ==>nil

include Anaphoric
     ==>Object

ifa (2 + 3 + 4) do
   puts "1 -- it #{@it}"
end
1 -- it 9
     ==>nil

ifa2 (2 + 3 + 4) do | it |
   puts "2 -- it #{it}"
end
2 -- it 9
     ==>nil

ifa (2 + 3 + 4) do
   puts "3a -- it #{@it}"
   ifa (99 + 2 + 3 + 4) do
     puts "3b -- it #{@it}"
   end
   puts "3c -- it #{@it}"
end
3a -- it 9
3b -- it 108
3c -- it 108
     ==>nil

ifa2 (2 + 3 + 4) do | it |
   puts "4a -- it #{it}"
   ifa2 (99 + 2 + 3 + 4) do | it |
     puts "4b -- it #{it}"
   end
   puts "4c -- it #{it}"
end
4a -- it 9
4b -- it 108
4c -- it 108
     ==>nil

ifa2 (2 + 3 + 4) do | it |
   puts "5a -- it #{it}"
   ifa2 (99 + 2 + 3 + 4) do | it2 |
     puts "5b -- it #{it2}"
   end
   puts "5c -- it #{it}"
end
5a -- it 9
5b -- it 108
5c -- it 9
     ==>nil




> -- 
> Posted via http://www.ruby-forum.com/.
>

----
Bob Hutchison                  -- tumblelog at <http:// 
www.recursive.ca/so/>
Recursive Design Inc.          -- <http://www.recursive.ca/>
xampl for Ruby                 -- <http://rubyforge.org/projects/xampl/>