Class.new and local variables

[James Brister]

I'm trying to do something similar to the Camping 'R' class generator.
Can anyone explain to me how to get the following to print out from the
program below:

param is p and arg is this is a P
param is q and arg is this is a Q

(basically I want to capture the argument to the maker functions inside
the class they generate).

---- cut here --- cut here --- cut here ---

## Using this function causes a blank after "arg is"
def maker1(thearg)
Class.new do
@arg = thearg.dup
def self.argval
@arg
end
def doit(param)
puts "param is #{param} and arg is #{self.class.argval}"
end
end
end

## Using this function causes the same value to be printed out for all
classes
def maker2(thearg)
Class.new do
@@arg = thearg.dup
def doit(param)
puts "param is #{param} and arg is #{@@arg}"
end
end
end

## using this function causes a undefined 'local variable or method'
def maker3(thearg)
Class.new do
def doit(param)
puts "param is #{param} and arg is #{thearg}"
end
end
end

# change maker1 to maker2 or maker3
class P < maker1 'this is a P'
def something
doit('p')
end
end

# change maker1 to maker2 or maker3
class Q < maker1 'this is a Q'
def something
doit('q')
end
end

P.new.something
Q.new.something

 

[David A. Black]

Hi --

I'm trying to do something similar to the Camping 'R' class generator.
Can anyone explain to me how to get the following to print out from the
program below:

param is p and arg is this is a P
param is q and arg is this is a Q

(basically I want to capture the argument to the maker functions inside
the class they generate).

---- cut here --- cut here --- cut here ---

## Using this function causes a blank after "arg is"
def maker1(thearg)
Class.new do
@arg = thearg.dup
def self.argval
@arg
end
def doit(param)
puts "param is #{param} and arg is #{self.class.argval}"
end
end
end

Instance variables are strictly per object. The class you get back
from maker1 is not the same object as any of its subclasses.

## Using this function causes the same value to be printed out for all
classes
def maker2(thearg)
Class.new do
@@arg = thearg.dup
def doit(param)
puts "param is #{param} and arg is #{@@arg}"
end
end
end

Class variables are weird

## using this function causes a undefined 'local variable or method'
def maker3(thearg)
Class.new do
def doit(param)
puts "param is #{param} and arg is #{thearg}"
end
end
end

The def keyword starts a new scope.

However... the define_method method doesn't:

def maker3(thearg)
Class.new do
define_methoddoit) do |param|
puts "param is #{param} and arg is #{thearg}"
end
end
end

In this version, I'm using only code blocks (and no 'def's). Code
blocks do see the local variables that exist at the point where the
code block starts. So thearg is in scope. Also, the block you provide
to define_method becomes, basically, the body of the method. I define
it to take one argument (note: the parameter syntax and binding
semantics for this have changed in Ruby 1.9, but they've actually
changed in a way that makes blocks and methods more rather than less
congruent), and of course that argument is in scope too.


David

--
The Ruby training with D. Black, G. Brown, J.McAnally
Compleat Jan 22-23, 2010, Tampa, FL
Rubyist http://www.thecompleatrubyist.com

David A. Black/Ruby Power and Light, LLC (http://www.rubypal.com)

 

[James Brister]

Wow. I go get coffee and the solution arrives.... excellent! Thanks.

James

David A. Black wrote:

[ the solution to my problem ]