From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2008-08-19T15:13:30+09:00
Subject: Re: passing a variable from a block back to the method

On Tue, Aug 19, 2008 at 7:21 AM, James Dinkel <jdinkel@gmail.com> wrote:
> I have several files that I have to read, check the contents, and then
> write any changes.  I overloaded the File class to help me out with
> this:

> This is a huge waste if the files don't change (as they often don't).
> The problem I'm having is with the "# decide not to change anything" in
> the block and passing that back to the method so "# return if not
> changing anything" knows the answer.  I'm not sure if I can pass an
> instance variable around to do this?  I don't really know how that would
> work since I don't really have an instance of file to work with.
>
> Maybe someone has some ideas?

Untested, but what about:


 ----------------------------------------------------------
 class File

  def self.change(filename)

    # read the file, execute a block, then write the file
      File.open(filename, 'r+') do |file|
        lines = file.readlines
        new_lines = yield lines
        # return if not changing anything
        return unless new_lines
        file.pos = 0
        file.print new_lines
        file.truncate(file.pos)
      end
  end
 end
 ----------------------------------------------------------

 The method is called with:

 ----------------------------------------------------------
 File.change('awesomefile') do |contents|
  contents.collect! do |line|
    # magic happens
    # decide not to change anything --> the last statement evaluates to nil
    nil

    # when you want to return the new lines
  end
 end
 ----------------------------------------------------------

Of course you should change the logic in the block to not be within a
collect! when you want
to return nil.

Hope this helps,

Jesus.