From: Ken Bloom <kbloom@...>
Date: 2008-04-18T14:10:19+09:00
Subject: Re: When are lambdas needed?

On Thu, 17 Apr 2008 16:58:44 -0500, Joel VanderWerf wrote:

> Avdi Grimm wrote:
>> On Thu, Apr 17, 2008 at 5:35 PM, Stedwick <philip.brocoum@gmail.com>
>> wrote:
>>>  Can somebody give me an extremely useful, NOT complicated, example of
>>>  when lambdas are the absolute perfect solution to a problem?
>> 
>> Every time you use a block in ruby you're using a lambda implicitly.
> 
> Implicitly, but not really. I wonder, was the original question more
> like "what are blocks for?" or "yes, I know what blocks are for, but why
> are there also these Proc objects, which are instantiated with the
> lambda construct?"
> 
> If the latter, then one answer is: when you need the program state that
> is encapsulated in a block to persist outside of the method in which it
> is used. For example, this happens in event-based frameworks: a setup
> method in a client class registers lambdas as event handlers; the
> framework doesn't need to know the classes or methods of the client
> class, hence loose coupling. As a bonus, these handlers can refer to
> shared program state (local variables) in the setup method.

There are about a zillion ways to get and use a Proc object in Ruby

def foo
  yield 1
end

def bar &baz
  #baz is a Proc object, and you can pass
  #that proc object to another method that takes a block
  foo &baz
end

a=lambda{|x| x*x}
#a is a Proc object

b=proc{|x| x*x}
#b is a Proc object

c=Proc.new{|x| x*x}
#c is a Proc object

c.call 2
=>4
#you can also use #call on a Proc object

c[2]
=>4
#or you can "index" it like an Array (which is just a synonym for #call)

The difference between lambda and the other ways of getting a Proc object 
is that lambda treats return, next, and break a little bit differently.

In other languages (specifically LISP) use of LAMBDA is the most common 
way to create closures. In Ruby, it's one of the rarest.

--Ken

-- 
Ken (Chanoch) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/