From: Akimichi Tatsukawa <akimichi_tatsukawa@...>
Date: 2008-04-18T18:40:45+09:00
Subject: Re: When are lambdas needed?

On Fri, 18 Apr 2008 06:35:08 +0900
Stedwick <philip.brocoum@gmail.com> wrote:

> I have seen many tutorials on the Internet explaining where lambdas
> CAN be used, such as clever multiplication functions, but when are
> they actually NEEDED?
> 
> Sure, I can take a lambda and "pass it around" so to speak, but I can
> call a function from anywhere too, right?
> 
> Can somebody give me an extremely useful, NOT complicated, example of
> when lambdas are the absolute perfect solution to a problem?
> 
> Thanks!
> 

How about using lambda as representing recursive type and 
implemeting lazy evaluation in easygoing way.


class Natural
 def initialize
   @succ = lambda {Natural.new}
 end

 def take(maximum)
   [1] + if maximum == 1
           []
         else
           @succ.call.take(maximum - 1)
         end
 end

 def accumulate(num, result = 0)
   if num == 0
      result
   else
      accumulate(num - 1, result + 1)
   end
 end

 def summate(maximum) 
   if maximum == 1
     1
   else
     accumulate(maximum) + summate(maximum - 1)
   end
 end
 
 def filter(index = 1,result = [], &block) 
   num = accumulate(index)
   if yield(num) == true
     result << num
     filter(index + 1,result, &block)
   else
     result
   end
 end
end


irb(main):040:0> n = Natural.new
#<Natural:0xb7a970ec @succ=#<Proc:0xb7a9c2a4@(irb):3>>

irb(main):041:0> n.take(100)
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]

irb(main):042:0> n.accumulate(100)
100

irb(main):043:0> n.filter {|i|  i < 10}
[1, 2, 3, 4, 5, 6, 7, 8, 9]

irb(main):045:0> n.summate(100)
5050

--
Akimichi Tatsukawa