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comp.lang.ruby

Delegating a method, with arguments

djberg96

11/19/2004 5:46:00 AM

Hi all,

I'm trying to write a class where I want to delegate some instance
methods as FileTest class methods. For example:

require "forwardable"
class Foo < String
extend Forwardable
def_delegator(:FileTest,:directory?)
def initialize(path)
@path = path
end
end

This doesn't work, however, since FileTest.directory? requires an
argument. What I would like is something like the following fictional
syntax:

def_delegator(:FileTest,:directory?(@path))

I don't want to create an instance of File and delegate to that
instance because I don't know if @path will actually be valid or not
(among other reasons).

So, would something like this be possible?

Regards,

Dan
5 Answers

T. Onoma

11/19/2004 6:10:00 AM

0

On Friday 19 November 2004 12:48 am, Daniel Berger wrote:
| Hi all,
|
| I'm trying to write a class where I want to delegate some instance
| methods as FileTest class methods. For example:
|
| require "forwardable"
| class Foo < String
| extend Forwardable
| def_delegator(:FileTest,:directory?)
| def initialize(path)
| @path = path
| end
| end

Not sure I understand, but is this what you are trying to do?

require "forwardable"
class Foo < String
extend Forwardable
def directory?
FileTest.directory?(@path)
end
def initialize(path)
@path = path
end
end


?

T.

Zach Dennis

11/19/2004 6:37:00 AM

0

Daniel Berger wrote:

> Hi all,
>
> I'm trying to write a class where I want to delegate some instance
> methods as FileTest class methods. For example:
>
> require "forwardable"
> class Foo < String
> extend Forwardable
> def_delegator(:FileTest,:directory?)
> def initialize(path)
> @path = path
> end
> end


If you said that def_delegator always in the form of: def_delegator(
class, method, arg1, arg2, arg3, etc... )

Then I think you could do: def_delegator( :FileTest, :directory? , :arg1 )

Or you could do something like: def_delegator( :FileTest,
:directory?=>[arg1] )


Zach

Robert Klemme

11/19/2004 2:13:00 PM

0


"Daniel Berger" <djberg96@hotmail.com> schrieb im Newsbeitrag
news:6e613a32.0411182145.45683083@posting.google.com...
> Hi all,
>
> I'm trying to write a class where I want to delegate some instance
> methods as FileTest class methods. For example:
>
> require "forwardable"
> class Foo < String
> extend Forwardable
> def_delegator(:FileTest,:directory?)
> def initialize(path)
> @path = path
> end
> end
>
> This doesn't work, however, since FileTest.directory? requires an
> argument. What I would like is something like the following fictional
> syntax:
>
> def_delegator(:FileTest,:directory?(@path))
>
> I don't want to create an instance of File and delegate to that
> instance because I don't know if @path will actually be valid or not
> (among other reasons).
>
> So, would something like this be possible?

This is easy but doesn't keep arity:

class Module
def def_delegator(instance_var, *syms)
syms.each do |m|
class_eval "def #{m}(*a,&b) #{instance_var}.#{m}(*a,&b) end"
end
end
end

class Foo
attr_accessor :bar
def_delegator("@bar", :foo)
end

class Bar
def foo() "yes" end
end

>> f=Foo.new
=> #<Foo:0x101a4030>
>> f.bar = Bar.new
=> #<Bar:0x1019ecc0>
>> f.foo
=> "yes"

With more effort and the additional info of the target class you can
determine the target method's arity and use that. However, the only
benefit is that clients who ask your delegator method for it's arity see
the real method's arity.

Kind regards

robert

djberg96

11/19/2004 6:56:00 PM

0

"trans. (T. Onoma)" <transami@runbox.com> wrote in message news:<200411190110.05257.transami@runbox.com>...
> On Friday 19 November 2004 12:48 am, Daniel Berger wrote:
> | Hi all,
> |
> | I'm trying to write a class where I want to delegate some instance
> | methods as FileTest class methods. For example:
> |
> | require "forwardable"
> | class Foo < String
> | extend Forwardable
> | def_delegator(:FileTest,:directory?)
> | def initialize(path)
> | @path = path
> | end
> | end
>
> Not sure I understand, but is this what you are trying to do?
>
> require "forwardable"
> class Foo < String
> extend Forwardable
> def directory?
> FileTest.directory?(@path)
> end
> def initialize(path)
> @path = path
> end
> end
>
>
> ?
>
> T.

I knew I could do that, but I was trying to find a shortcut rather
than define each method from FileTest manually. However, the syntax
I'm proposing isn't going to save much in terms of keystrokes, so
maybe it isn't worth it after all.

Regards,

Dan

Mauricio Fernández

11/19/2004 7:39:00 PM

0

On Sat, Nov 20, 2004 at 03:58:10AM +0900, Daniel Berger wrote:
> > | I'm trying to write a class where I want to delegate some instance
> > | methods as FileTest class methods. For example:
> > |
> > | require "forwardable"
> > | class Foo < String
> > | extend Forwardable
> > | def_delegator(:FileTest,:directory?)
> > | def initialize(path)
> > | @path = path
> > | end
> > | end
> >
> > Not sure I understand, but is this what you are trying to do?
> >
> > require "forwardable"
> > class Foo < String
> > extend Forwardable
> > def directory?
> > FileTest.directory?(@path)
> > end
> > def initialize(path)
> > @path = path
> > end
> > end
> I knew I could do that, but I was trying to find a shortcut rather
> than define each method from FileTest manually. However, the syntax
> I'm proposing isn't going to save much in terms of keystrokes, so
> maybe it isn't worth it after all.

It can be done cleanly under 1.9 with Object#instance_eval and
Module#define_method; the syntax would look like
def_delegator(:FileTest, :directory?){ @path }

Under 1.8, you can easily have something like
def_delegator(:FileTest, :directory?, :directory?, "@path")
(2nd :directory? for compatibility with forwardable.rb) using
Module#module_eval.

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