From: Karl <threadhead@...>
Date: 2010-03-03T02:25:15+09:00
Subject: Re: Compute value inside define_method

On Mar 2, 9:37 am, Jesús Gabriel y Galán <jgabrielyga...@gmail.com>
wrote:
> On Tue, Mar 2, 2010 at 5:25 PM, Karl <threadh...@gmail.com> wrote:
> > On Mar 2, 9:15 am, Jesús Gabriel y Galán <jgabrielyga...@gmail.com>
> > wrote:
> >> On Tue, Mar 2, 2010 at 5:00 PM, Karl <threadh...@gmail.com> wrote:
> >> > I know there is a 'trick' for forcing a value to compute, but I can't
> >> > seem to remember or find it. The following routine needs to computer
> >> > the value of 'register += 1' and not insert it literally.
>
> >> > into.class_eval do
> >> >              define_method "#{dow}_#{tod}_time" do
> >> >                @registers.get_value( "#{register += 1}"to_i )
> >> >              end
> >> >    ...snip...
> >> > end
>
> >> > You can see my attempt above, but it doesn't work. Neither does
> >> > 'eval("register += 1")'
>
> >> > What's the trick? or should I approach the problem from a different
> >> > angle?
>
> >> What is register in your code above? Because this should work:
>
> >> irb(main):005:0> register = 0
> >> => 0
> >> irb(main):006:0> "#{register += 1}".to_i
> >> => 1
> >> irb(main):008:0> register
> >> => 1
> >> irb(main):009:0> "#{register += 1}".to_i
> >> => 2
>
> >> On the other hand, if register is already and int and the get_value
> >> method receives an int, why not just:
>
> >> @registers.get_value(register += 1)
>
> >> ?
>
> >> Jesus.
>
> > 'register' is an int.
>
> > The problem is one of scope. When this is wrapped in the define_method
> > it carries as a variable int object when it is included in a class.
> > Thus, when executed via MyClass.weekeday_morning_time two consecutive
> > times, 'register += 1' increments. I don't want it to increment when
> > called, I just need it to increment when it is *defining* the method.
>
> > This is part of a dynamic method creation loop, and the register
> > values are fixed. The dynamic method creation allows me to create
> > several hundred methods accessing consecutive registers without
> > writing all that code. So if I'm defining a method 'weekday_day_time'
> > it is 'get_value' from register 21. The 'register += 1' is just
> > incrementing the counter through the creation loop.
>
> > Make sense? I know, this is a bit obscure.
>
> It's not a problem of scope. The problem is that define_method
> receives a block that closes over the register variable. The block
> passed to define_method is not executed when the define_method is
> executed, but when the method is invoked, as you have observed. All
> defined methods carry the binding to the same register variable and so
> every invocation of one of those methods will increment the same
> variable. One way to avoid that is force the execution of evaluating
> the register variable when defining the method, for example like:
>
> irb(main):012:0> 3.times do |i|
> irb(main):013:1* instance_eval "def test_#{i}; #{i};end"
> irb(main):014:1> end
> => 3
> irb(main):015:0> test_0
> => 0
> irb(main):016:0> test_1
> => 1
> irb(main):018:0> test_2
> => 2
>
> The *eval method family can also receive a String, which is obviously
> not a closure, and so my #{i} above is executed when the instance_eval
> is executed. Maybe someone can come up with a cleaner solution.
> Translated to your problem (untested):
>
> into.class_eval do
>              instance_eval <<END
>                 def #{dow}_#{tod}_time
>                    @registers.get_value(#{register += 1})
>                 end
>              END
>    ...snip...
> end
>
> Hope this helps.
>
> Jesus.

That was it. I couldn't remember passing a string to class_eval forces
the computation of #{} strings.

Thanks.

If there's a better way, of course I'd love to know.