From: chr_news@... (Chistoph)
Date: 2001-11-04T03:30:21+09:00
Subject: [ruby-talk:24278] Re: examples of functional programming in Ruby

ptkwt@shell1.aracnet.com (Phil Tomson) wrote in message news:<4hME7.93667$HZ.3157029@sjcpnn01.usenetserver.com>...
> Would anyone be willing to share some  code snippets that show examples of 
> functional programming in Ruby?
> 
> Phil

Hi,

I have been absent from the ruby group for a while. Anyway here is an
Fibbonacci example from the ``what if Fixnum singleton classes
existed'' department.

Surprisingly enough the implementation is faster then a
straightforward recursive implementation.

class Integer
  def fib 
     return 1 if self < 2
     return (self-1).fib + (self-2).fib
  end
end

 
(using generators is much faster but this another matter)

-------------
require 'simple_rec.rb'

class PositiveRecursiveInteger
   def fib; pred.fib + pred.pred.fib end
   def Null.fib; 1 end
   def (Null.succ).fib; 1 end
end

p (31.to_posrec_int.fib) ==> #  2178309


-------------
# simple_rec.rb 
# A simple example of a class with recursive
# instance creation - not thread safe!

class Module
   public :remove_method
end
class Object
   def remove_singleton_method sym
      class << self; self end.remove_method sym 
   end
end



class PositiveRecursiveInteger < Numeric
   Null = self::new
   def Null.pred; raise ArgumentError 'negative predessor' end 
   def Null.succ;  __succ__  end
   Null.instance_eval { @to_i = 0 }
  
   attr_reader :succ, :pred, :to_i
   def to_s; @to_i.to_s end
   alias inspect to_s
  
   def initialize p;  @pred = p end
  
   private
   def __succ__
      remove_singleton_method :succ
      @succ = PositiveRecursiveInteger::new self
      def @succ.succ; __succ__ end
      return @succ
   end

   public 
   def coerce (other); [to_i, other] end
   def +(other); to_i + other end
   def -(other); to_i - other end
   def *(other); to_i* other end
   # etc  ... 
end


class Fixnum
   # this is rather slow ..
   def to_posrec_int 
      raise ArgumentError 'negative argument' if self < 0
      res = PositiveRecursiveInteger::Null
      times { res=res.succ }
      return res
   end
end