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comp.lang.ruby

problem with accessors and dsl's

Tsunami Script

1/29/2009 10:30:00 PM

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 !
--
Posted via http://www.ruby-....
2 Answers

Joel VanderWerf

1/29/2009 10:38:00 PM

0

Tsunami Scripter 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"

--
vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407

David A. Black

1/29/2009 11:00:00 PM

0

Hi --

On Fri, 30 Jan 2009, Tsunami Scripter wrote:

> 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 ?

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

--
David A. Black / Ruby Power and Light, LLC
Ruby/Rails consulting & training: http://www.r...
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