[ruby-talk:4793] Re: closures

Stuart Zakon <[e-mail]> writes:

> Can somebody please explain what a closure is within the context of
> Ruby? (Simple explanation, simple example).

A closure is simply a chunk of code that "remembers" the context that
it originally appeared in. This is unusual, because in conventional
block-structured languages, once something goes out of scope it is
lost.

I Ruby, we can implement closures using blocks, and convert them into
objects (of class Proc) using the 'proc' method (there are also some
other ways).

So, here's an example:

    def times(n)
      proc { |val| n * val }
    end

Here's a method that takes one argument, n. It returns a closure; a
proc object that also takes an argument. The closure multiplies it's
argument by 'n'.

We can use this like this:

    t12 = times(12)
    t12.call(4)    #=> 48
    t12.call(6)  #=> 72

    t5 = times(5)
    t5.call(4)     #=> 20
    t5.call(6)     #=> 30


The key here is that the original parameter to times, n, is long gone
by the time the closure is called, but it is still available within
the closure.


Regards


Dave