From: wegzumir <wegzumir@...>
Date: 2006-01-27T13:38:45+09:00
Subject: Re: Pass References To Methods As Arguments?

On 1/26/06, Austin Ziegler <halostatue@gmail.com> wrote:
> On 26/01/06, wegzumir <wegzumir@gmail.com> wrote:
> > Hey all.  I am trying to determine the Ruby syntax for passing a
> > reference to a method. I want to actually pass the reference itself as
> > an argument to another method.
>
> There is no way to do that directly in Ruby.
>
> > Consider the following simplified Perl example. It features two
> > subroutine functions and another subroutine that is capable of calling
> > any subroutine passed to it as a reference.
> >
> >   sub print_unix_line {
> >     print shift, "\n"
> >   }
> >
> >   sub print_windows_line {
> >     print shift, "\015\012";
> >   }
> >
> >   sub call_a_routine {
> >     my ($routine, $argument) = @_;
> >     $routine->($argument);
> >   }
> >
> >   &call_a_routine(\&print_unix_line, 'Hello, unix World!');
> >   &call_a_routine(\&print_windows_line, 'Hello, Windows World!');
>
> Your example really isn't a good one, but:
>
> 1.
>
>   print_unix_line = lambda { |arg| print arg + "\n" }
>   print_wind_line = lambda { |arg| print arg + "\015\012" }
>
>   def call_a_routine(routine, argument)
>     routine[argument]
>     # or: routine.call(argument)
>   end
>
>   call_a_routine(print_unix_line, "Hello, unix.")
>   call_a_routine(print_wind_line, "Hello, Windows.")
>
> 2.
>
>   class Routines
>     def print_unix_line(arg)
>       print arg + "\n"
>     end
>
>     def print_wind_line(arg)
>       print arg + "\015\012"
>     end
>   end
>
>   def call_a_routine(routine, argument)
>     @@routine ||= Routines.new
>     @@routine.__send__(routine, argument)
>   end
>
>   call_a_routine(:print_unix_line, "Hello, unix.")
>   call_a_routine(:print_wind_line, "Hello, Windows.")
>
> As far as the poorness of the example you have, you don't need to do
> anything special with Ruby to print properly:
>
>   puts "Hello Ruby, World"
>
> What exactly are you really trying to do?
>
> -austin
> --
> Austin Ziegler * halostatue@gmail.com
>                * Alternate: austin@halostatue.ca
>
>

Yes, my apologies for the example! I was actually trying to strip down
the problem to its barest essentials by making up a hypothetical
scenario.

The whole reason for this (the non hypothetical reason) is because of
the excellent computer science concepts expressed in the book
"Higher-Order Perl". I've used some of those concepts in my Perl code
in the past and I wanted to be able to express the same ideas in Ruby
(and of course be completely open to learning the more Ruby way of
doing the same things instead). In particular, one of the examples at
the beginning of the book is a recursive directory traverser that
accepts references to subroutines for arguments that are mapped as
handlers for what to do when a directory is first opened, when each
file is processed, and once the directory has been finished with.

Many thanks for the two solutions!