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

On Thu, 17 Apr 2008 20:50:18 -0500, zuo peng wrote:

> the lambda expression came from the lisp community, it's kind of
> abstraction of procedures.
> in oo languages it's usually difficult to extend methods than extend
> data structures.
> ruby or other modern programming languages did a great job to mix them
> together. well , back to the topic, imo to fully understand the power of
> lambda(or abstraction), one has to look into the lisp/scheme world :)

Lambda comes from lambda calculus. The fact that LISP uses them is just a 
happy coincidence.

But let's compare LISP and Ruby for a moment.

In LISP, any time you want to pass an unnamed ad-hoc function to another 
function, you define it like (lambda (var1 var2) (code)), so you wind up 
using the lambda macro a lot. You might have
(mapcar #'(lambda (x) (* x x)) '(1 2 3 4))
which returns 
(1 4 9 16)

In Ruby, you do this kind of thing a lot too. Only we have syntactic 
sugar that makes things a little nicer in many cases. The equivalent code 
to the LISP example is
[1 2 3 4].map{|x| x*x}
so you just used a lambda, but didn't have to type the keyword.

There's one thing to know though. Kernel#proc and Kernel#lambda have 
slightly different semantics when it comes to the return, next, and break 
keywords. Kernel#lambda does things one way, and Kernel#proc or a bare 
block do things the other way. So really, Ruby uses lambdas a lot (maybe 
even more than LISP) but we just don't use the term very much, mostly 
because we've shortened the syntax.

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