From: Igor Pirnovar <gooigpi@...>
Date: 2009-04-06T03:10:56+09:00
Subject: Re: Function as Parameter?

Tomas Na wrote:

> To make this happen I figured I would pass in a callback. 
> Searching around, I couldn't find a lot of info about 
> callbacks, but I was reading about passing a symbol. I may 
> have got this wrong, but it looks like if I pass in 
> :function_name then it will be a symbol that I can call.

Symbol is not really what you think. It is merely a special code/name 
for "defined data" items (classes, variables and  methods (or functions 
as you refer to them)) which are stored in Ruby's symbol table. As such, 
namely as names, symbols are just like strings, they only are much more 
efficiently handled by Ruby than strings. To distinguish between a 
defined ruby item and its symbol the colon (:) is used before the name. 
Passing a symbol of a method (function) around is just like passing its 
name in the string form.

In order to execute a command as a string or a symbol you can use the {{ 
method }} method.

  def my_test
    puts "in my_test function"
  end
  puts :my_test           # -> my_test
  method(:my_test).call   # -> in my_test function
  method("my_test").call  # -> in my_test function
  var = "scope of creation"
  test_fu_lambda = lambda { puts 'Test: %s' % [ var ] }
  test_fu_lambda.call     # -> Test: scope of creation

Another way to pass a symbol to an iterator as an executable block is by 
preceding it with the ampersand (&), which effectively converts the 
method represented by the symbol to a proc! In Ruby, executable code is 
passed around in procs, lambdas and Method objects. There is no such 
thing as a function (method) pointer in Ruby. However a subtle 
differences between these objects of executable code exist, and you 
should learn about them. Especially you should know about the 
differences between procs and lambdas. The most important differences 
are in the way these executable objects are called (invoked) and the way 
their respective {{ return }} mechanisms behave. In general procs use {{ 
yield }} invocation semantics but lambdas use the method invocation 
semantics. Hence, procs are associated with block behaviour, whereas 
lambdas are closer to method (function) behaviour.

Yet another important thing to understand is that procs and lambdas are 
closures, but method objects are not. Closures are important because 
they encapsulate in them all variables from the  scope in which they are 
defined, which remain intact even when a proc or a lambda is executed in 
a totally different scope.

You can not expect to learn all you need to know about this from a 
single post on a forum, however, for you to continue working on your 
program it should suffice to know that you need to convert a method 
(function) you wish to pass to your LoginForm object to a lambda, and in 
it, that is in LoginForm object, invoke it with the {{ call }} method 
the i.e.; something like {{ test_fu_lambda.call }} as you have seen in 
the example above.

If {{ LoginForm }} is your own class you created, you need to change the 
invocation of your test function so it utilizes the {{ call }} method as 
shown above. Try the following:

  var = "I am a variable from the scope where test_fu_lambda was 
created"
  test_fu_lambda = lambda { puts 'Test: %s' % [ var ] }
  LoginForm.new(xaApp,'TestApp', test_fu_lambda)

Alternatively you can create a Method object of any of your methods and 
pass it in instead. However, in this case your Method object will not 
hold on to the variables from the scope in which it is defined:

  def TestFunc()
    puts 'Test: %s' % [ var ]
  end
  ...
  var = "this time not seen in my_fu_obj"
  my_fu_obj = method(:TestFunc)
  LoginForm.new(xaApp,'TestApp', my_fu_obj)

In both cases you have to change the invocation of the executable 
objects you are passing to your {{ LoginForm }} to utilize the {{ *.call 
}} syntax.

Indeed, what David wrote is the Ruby way of doing things, but sometimes 
converting methods or blocks to executable objects is more appropriate.
-- 
Posted via http://www.ruby-forum.com/.