From: Robert Klemme <shortcutter@...>
Date: 2007-10-02T18:10:13+09:00
Subject: Re: Stupid error on blocks

2007/10/2, mortee <mortee.lists@kavemalna.hu>:
> Federico Brubacher wrote:
> > The names are all right but within the block I get for all the methods I
> > created my last element (element N) in my $ingredients_column variable .
> >
> > for column_name in $ingredients_columns do
> >
> >    define_method("sort_by_" + column_name.human_name) {  @ingredients =
> > Ingredient.find(:all, :order => "#{column_name.human_name  } DESC")}
> >
> > end
>
> I'm not really experienced, but I guess your problem is caused because
> the block you pass to define_method isn't evaluated at the time you pass
> it to define_method, but when you actually call the created methods.
> This means the local variables are evaluated at that time too.

That's part of the explanation. The other part is scoping because the
iteration variable in a for loop belongs to the outer scope and thus
values are overwritten all the time (see below).

> You may want to use eval instead if you want variables expanded at
> define-time, and their value be included in the body of the newly
> defined methods (the string you pass to eval gets interpolated first
> (variable references expanded), and then the resulting ruby code
> executed - thus the values effective just when eval is called get
> incorporated in the defined methods' bodies):
>
> for column_name in $ingredients_columns do
>   eval %Q[
>     def sort_by_#{column_name.human_name} {
>       @ingredients = Ingredient.find(
>         :all, :order => "#{column_name.human_name} DESC")
>     }
>   ]
> end
>
> If anyone knows of a better approach, please post it as I'd be
> interested too.

That's one way to do it.  Actually only a smaller change is needed
because the root cause in this case is the scoping rule of the
iteration variable in a "for" loop which is different than when
iterating with a block as this example demonstrates:

RKlemme@padrklemme1 ~
$ ruby/names.rb
1_foo
2_baz
1_bar
2_baz
1_baz
2_baz

RKlemme@padrklemme1 ~
$ cat ruby/names.rb
#!ruby

class Tester
  def self.create_methods_1(names)
    names.each do |name|
      define_method("m1_#{name}") do
        puts "1_#{name}"
      end
    end
  end
  def self.create_methods_2(names)
    for name in names do
      define_method("m2_#{name}") do
        puts "2_#{name}"
      end
    end
  end
end

method_names = %w{foo bar baz}
Tester.create_methods_1 method_names
Tester.create_methods_2 method_names

t=Tester.new

method_names.each do |n|
  t.send "m1_#{n}"
  t.send "m2_#{n}"
end

Kind regards

robert