From: Srijayanth Sridhar <srijayanth@...>
Date: 2009-05-06T13:49:20+09:00
Subject: Re: a DSL + scope problem

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Thanks Brian,

All of these help. Eventually I would like those using the DSL to define
their own helper methods.

Jayanth

On Tue, May 5, 2009 at 4:02 PM, Brian Candler <b.candler@pobox.com> wrote:

> Srijayanth Sridhar wrote:
> > Basically what the DSL lets them do is define rules and associated
> > blocks on
> > certain regexes which all gets put into a table. This has simplified
> > things
> > a lot, and all they do is create rules. Its all working fine, but now I
> > want
> > to add another feature. I want to define some common methods in the DSL
> > class(like formatting strings a certain way etc), but I want this to be
> > available to the people who define the rules and the blocks and use the
> > DSL.
> > Ideally this is easily accomplished by passing self into the blocks. But
> > I
> > don't want to burden these chaps with what self is and so on, and
> > generally
> > speaking passing self seems a little ugly. Is there any other method you
> > guys can suggest?
> >
> > The DSL is being instance_eval-ed...
>
> If you are doing obj.instance_eval { ... } then 'self' is already obj.
>
> If these helper methods are pre-defined ones, then just define them
> within obj's class. The user should be able to call them. If not,
> provide a cut-down example which demonstrates the problem.
>
> If you want the user to be able to define their own helper methods
> within the DSL, this is also possible. You may find "define_method"
> helpful, which can define a method within a class, given a block.
>
> irb(main):002:0> def helper(name, &blk)
> irb(main):003:1> (class << self; self; end).class_eval {
> define_method(name,&blk) }
> irb(main):004:1> end
> => nil
> irb(main):005:0> helper(:test) { puts "hello!" }
> => #<Proc:0xb7ccbed0@(irb):5>
> irb(main):006:0> test()
> hello!
> => nil
>
> Also, note that class_eval also sets the context for a regular "def". So
> if the object you are instance_eval'ing within is actually a module or a
> class, and you use class_eval instead, users can just stick 'def'
> statements in the DSL to define methods within that class.
>
> irb(main):001:0> class Foo; end
> => nil
> irb(main):002:0> Foo.class_eval %{def test; puts "hi"; end}
> => nil
> irb(main):003:0> Foo.new.test
> hi
> => nil
>
> But if you are instance_eval'ing within an instance of Foo, then you
> want to define methods within the singleton class of that instance,
> which I think is best done using define_method as shown first.
> --
> Posted via http://www.ruby-forum.com/.
>
>

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