From: Mike <michaelst@...>
Date: 2007-03-08T07:10:08+09:00
Subject: Re: Setting default vales

Thank you for your answer.
But it looks like this is a real problem for Ruby to use such
construct.
probably there is some other way (I don't like to use @) to solve it.
Could I get a left side of "=" by some reference?
For example,
b = my_func(def_val)
Could I redefine "=" and get some information on "b"? And if b is
invalid I'll return "def_val" otherwise I'll return "b" itself.

Also, can I pass a "reference"/"pointer" into function?



On Mar 7, 8:55 pm, "Robert Dober" <robert.do...@gmail.com> wrote:
> On 3/7/07, Mike <michae...@gmail.com> wrote:
>
>
>
> > Hello,
>
> >    I want to have some function that will two parameters. First is the
> > variable name and second is a value.
> > Inside I will check:
> >    * if the variable is existed I do nothing
> >    * else I assign "value" to variable
>
> > It should be something like this but it is not working.
>
> > def set_default(name, val)
> >     if ! name then
> >         eval "#{name} = #{val}"
> >     end
> > end
>
> > set_default("b", "2")
> > puts "b = " + b
>
> The first deception will come from this little snippet I am afraid,
>  cat default.rb && echo "--------------" && ruby default.rb
> # vim: sts=2 sw=2 nu expandtab nowrap:
> a = 42
> def test_a
>   defined? a
> end
> p test_a
> --------------
> nil
>
> As you can see a is not visible inside the method.
> Let us try to find a different tool for that
>
> # vim: sts=2 sw=2 nu expandtab nowrap:
> a = 42
> test_a = proc {
>   defined? a}
>
> p test_a.call
> --------------
> "local-variable"
>
> Which is better, we could now try to implement what you want as a proc
> # vim: sts=2 sw=2 nu expandtab nowrap:
> a = 42
> default = proc { |name, value|
>   eval "defined? #{name} ? nil : #{name} = #{value}"}
>
> default.call :a, 43
> default.call :b, 43
> puts a
> puts b
> --------------
> 42
> def_proc.rb:9: undefined local variable or method `b' for main:Object
> (NameError)
> What is happening here?
> Well we cannot make b spring into live *outside* the proc scope.
>
> This all just to ask two questions
> Why using local variables and not instance variables? look at snippet (1) below
>
> Is the name ||= value syntax not good enough? I know it will overwrite
> nil and false values but more often than not it is quite good enough.
>
> If you really need what you want you will need some help from meta
> programming gurus. If I am not mistaken this can be done with things
> like binding_from_caller
> as can be found in Facets, also have a look at Maurico Fernandez' sitehttp://eigenclass.org/
>
> HTH.
> Robert
> (1)
> # vim: sts=2 sw=2 nu expandtab nowrap:
> @a = 42
> def default name, value
>   return if instance_variables.include? name
>   # There even is instance_variable_exists? in
>   #  1.8.6
>   instance_variable_set name, value
> end
> default "@a", 43
> default "@b", 43
> puts @a
> puts @b
> --------------
> 42
> 43
>
> --
> We have not succeeded in answering all of our questions.
> In fact, in some ways, we are more confused than ever.
> But we feel we are confused on a higher level and about more important things.
> -Anonymous