Metaprogramming Conventions


#1

Recently, there was a post on news:comp.lang.ruby[1] by Avdi G., who
asked if there were any conventions when using metaprogramming. He gave
this example:

require ‘memoize’
require ‘validate’

class Foo
attr_accessor :bar
memoize :bar
validate :bar do |value|
value.include? “Johnny Walker Black Label”
end
end

Disregard the fact that you rarely want to memoize a writer method.

How do I know if memoize and validate play nicely together?

There has already been a proposal that would make it very easy to add
metaprogramming capabilities without treading on each other’s toes[2],
but it would be a very big step.

A simpler solution would be to add a special method definition (an
advice), that had a `super’ that called the previous definition of
that method.

class Foo
attr_accessor :bar

 advice bar
   '[' + super + ']'
 end

 advice bar
   '{' + super + '}'
 end

end

foo = Foo.new
foo.bar = ‘baz’
foo.bar #=> {[baz]}

That way, the user could choose in which order the wrapper methods
should be called.

Cheers,
Daniel S.

[1]
http://groups.google.com/group/comp.lang.ruby/browse_thread/thread/e5be2e3b8a9d9614/6b3ab3e56f3a82cd
[2] http://www.rcrchive.net/rcr/show/321


#2

Hi –

On Sun, 23 Apr 2006, Daniel S. wrote:

validate :bar do |value|
would be a very big step.

called.
“super” is not the best term, though: it means “next highest up in the
method lookup path” (i.e., in a different class or module), whereas
you’re describing something that’s lexically above but semantically on
the same level. It’s more an alias-related operation than a super
one, though not exactly that either.

Also, it seems a little fragile to me. It would introduce a whole set
of constraints on the order of code and even the order of
file-loading.

David


David A. Black (removed_email_address@domain.invalid)
Ruby Power and Light, LLC (http://www.rubypowerandlight.com)

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