Forum: Ruby problem with accessors and dsl's

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Tsunami S. (Guest)
on 2009-01-30 00:32
I've created a class using the following code :

class CommandHandler

    attr_accessor :command,:arguments

    def initialize(command=nil,arguments=nil,what=nil)
        @command = command
        @arguments = arguments
        @what = what
        @code = nil
    end

    def perform(arguments,what)
        @code.call(arguments,what) if !@code.nil?
    end

end

and a class named CommandProcessor , which , among other methods has the
following :

def add_handler(&block)
   handler = CommandHandler.new
   handler.instance_eval &block
   @map[handler.command] = handler
end

so I can add handlers in a DSL-like way . Here is a sample of code in
action :

add_handler do
    command=("get")
    code=(lambda do |arguments,what|
         begin
             @mech.get(arguments.join(""))
         return @mech.page.body
             rescue
                return "#{arguments} could not be loaded"
             end
         end)
     end
end

However , by the time the code reaches @map[handler.command] = handler ,
if I print the handler object , all of it's attributes appear as nil . I
specifically added

command=("...")
and code=("...")

so that the interpreter wouldn't have to use "heuristics" to deduce what
I was trying to say . Why isn't the code working as it should ?

Thanks !
Joel VanderWerf (Guest)
on 2009-01-30 00:41
(Received via mailing list)
Tsunami S.er wrote:
> so I can add handlers in a DSL-like way . Here is a sample of code in
> action :
>
> add_handler do
>     command=("get")

This is interpreted as local variable assignment. You can use the
following syntaxes instead:

     self.command = "get"

or (defining #command differently)

     command "get"
David A. Black (Guest)
on 2009-01-30 01:03
(Received via mailing list)
Hi --

On Fri, 30 Jan 2009, Tsunami S.er wrote:

>        @code = nil
>
>    command=("get")
>
> However , by the time the code reaches @map[handler.command] = handler ,
> if I print the handler object , all of it's attributes appear as nil . I
> specifically added
>
> command=("...")
> and code=("...")
>
> so that the interpreter wouldn't have to use "heuristics" to deduce what
> I was trying to say . Why isn't the code working as it should ?

See Joel's answer; I just wanted to add the gloss that the interpreter
doesn't really use heuristics to figure out what x = y means. It
*always* interprets it as an local variable assignment. If it used
more elaborate heuristics, it might pick up on your non-use of the
syntactic sugar and decide it was a method call. But it's very
literal-minded about that form of expression.

I wouldn't sweat the "DSL" aspects of it, if by that you mean the
business of not having to specify a receiver. There's no real
advantage to that unless you're trying to create a full-blown
self-describing toolset (and even then, I think the "stealth"
instance_eval is not as great an idea as it might sometimes seem).


David

--
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