From: matt@... (matt neuburg)
Date: 2006-10-28T06:30:14+09:00
Subject: Re: What are closures, continuations?

Joe Ruby MUDCRAP-CE <joeat303@yahoo.com> wrote:

> One more question -- what are lambdas?

This is a term from LISP. It basically just means a function that you
can define and then pass around and call. In my previous example (about
closures), the thing in curly braces, "{puts var}", was effectively a
lambda.

var = 7
p = Proc.new {puts var}
def hereGoesNothing
  var = "yoho"
  yield
end
hereGoesNothing &p # => 7
var = 200
hereGoesNothing &p # => 200

###

def proc_maker
  var = 7
  Proc.new {puts var}
end
p = proc_maker
p.call # => 7

In the first example, I form the function, I assign it to a variable,
and hand it (twice) to a method (hereGoesNothing) which calls it
(yield). In the second example, I form the function and hand it out as
the result of a method (proc_maker), assign it to a variable, and call
it (call).

LISPers like to celebrate this ability (among others) to treat a
function just like any other Thing, passing it around and so forth, by
chanting a poem (with apologies to JRR Tolkien) whose last three lines
are:

  One Thing to name them all, One Thing to define them,
  One Thing to place them in environments and bind them,
In the Lambda Order they are all first-class.

m.


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