From: pjb@... (Pascal J. Bourguignon)
Date: 2008-04-18T18:15:10+09:00
Subject: Re: When are lambdas needed?

Stedwick <philip.brocoum@gmail.com> writes:

> 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?

Lambdas are anonymous function.

As long as you can give a name to all the functions, they're not
needed, you can just define (ie. name) a function, and give that
function by name instead of using lambda.

But that's the very point!  One gets fed up in having to come with
names for ALL the little functions we can encounter in a program.

Like if you went on a road, and gave a name to all the little peebles
you encounter, keeping with you a picture of each peeble with the
associated (unique!) name in your notebook.  What a big useless
notebook! 


Also, lambdas (closures actually) are equivalent to classes (objects).
So, as long as you can write a class (give it a name!), and
instanciate an object, everywhere you would use lambdas, they're not
needed.  But what a burden!

Compare:

------------------------------------------------------------------------
def example 
  array=[1,2,3,4]
  array.each{|i| print i," -> ",i+i,"\n" }
  array.each{|i| print i," -> ",i*i,"\n" }
  ...
  :done
end
------------------------------------------------------------------------

vs.

------------------------------------------------------------------------
def printDouble(i)
  print i," -> ",i+i,"\n"
end
def printSquare(i)
  print i," -> ",i*i,"\n"
end
...
class Array
  def callForEach(f)
     for i in 0..self.size()-1 do
        f.call(self[i])
     end
  end
end
def example 
  array=[1,2,3,4]
  array.callForEach(method(:printDouble))
  array.callForEach(method(:printSquare))
  :done
end
------------------------------------------------------------------------


and compare:

------------------------------------------------------------------------
def example 
  k=42
  array=[1,2,3,4]
  array.each{|i| print i," -> ",i+k,"\n" }
  array.each{|i| print i," -> ",i*k,"\n" }
  :done
end
------------------------------------------------------------------------

vs.

------------------------------------------------------------------------
class Adder
   def initialize(k)
      @k=k
   end
   def run(i)
      print i," -> ",i+@k,"\n"
   end
end

class Multiplier
   def initialize(k)
      @k=k
   end
   def run(i)
      print i," -> ",i*@k,"\n"
   end
end

class Array
  def sendRunWithEach(o)
     for i in 0..self.size()-1 do
        o.run(self[i])
     end
  end
end

def example 
  k=42
  array=[1,2,3,4]
  array.sendRunWithEach(Adder.new(k))
  array.sendRunWithEach(Multiplier.new(k))
  :done
end
------------------------------------------------------------------------


So if you have a lot of time, a lot of memory, and a lot of fingers,
perhaps you can do without lambda, but us poor mortals just need it.


-- 
__Pascal Bourguignon__