Extend quesion


#1

The following doesn’t quite do what I would expect:

module A
def foo
“A::foo”
end
end
module B
def foo
“B::foo”
end
end
class Demo
def to_a
self.extend(A)
end
def to_b
self.extend(B)
end
def initialize
self.extend(A)
end
end

d = Demo.new
puts d.foo #=> A::foo
d.to_b
puts d.foo #=> B::foo
d.to_a
puts d.foo #=> B::foo (but I expected A::foo)

OK, I can kind of see why this happended; B’s foo 'hide’s A’s foo
method. But
I’m not sure why it didn’t happen in the second ‘puts’ above (ie. why
wouldn’t it just keep printing A::foo every time?). I think the answer
is that
A has already been mixed-in so it’s not really mixed-in again (true?).

And how would I go about making this work so that it prints:

A::foo
B::foo
A::foo

?

Phil


#2

DÅ?a Streda 15 Február 2006 03:23 Phil T. napísal:

I think the answer is that A has already been mixed-in so it’s not really
mixed-in again (true?).

Quite so.

And how would I go about making this work so that it prints:

A::foo
B::foo
A::foo

?

Make the message printed an instance attribute and change it? duck

Mixins aren’t plain supposed to do that. They are to add methods, not
rewrite
them at whim. If you -have- to do this via metaprogramming, use
singleton
methods. Especially in non-trivial code, heavy use of #extend to change
behaviour over and over again would get really messy.

David V.


#3

In article removed_email_address@domain.invalid,
David V. removed_email_address@domain.invalid wrote:

B::foo
A::foo

?

Make the message printed an instance attribute and change it? duck

Mixins aren’t plain supposed to do that. They are to add methods, not rewri=
te=20
them at whim.

Yeah, I just thought it would be a nice, fairly easy solution.

If you -have- to do this via metaprogramming, use singleton=20
methods.

Yeah, I could probably do something like that by redefining the
method…

(BTW: Using extend as above makes foo a singleton method )

Especially in non-trivial code, heavy use of #extend to change=20
behaviour over and over again would get really messy.

The only way to really get it to work would be to remove_method before
doing
the extend. The problem with that is that remove_method doesn’t work in
that
context (removing singleton methods).

I should probably consider some sort of delegation pattern instead. One
fairly easy way would be to use method_missing and then change the
target
object of the message (though I want to avoid overuse of method_missing
because

  1. it can make things difficult to debug and 2) I might need it for
    another
    purpose in this application. )

Phil


#4

Phil T. wrote:

not rewri= te=20
them at whim.

Yeah, I just thought it would be a nice, fairly easy solution.

If you -have- to do this via metaprogramming, use singleton=20
methods.

Yeah, I could probably do something like that by redefining the
method…

Or use the strategy / state pattern.

One fairly easy way would be to use method_missing and then change
the target object of the message (though I want to avoid overuse of
method_missing because 1) it can make things difficult to debug and
2) I might need it for another purpose in this application. )

I think you can use SimpleDelegator and change the target in between.
Didn’t test it myself though.

Kind regards

robert

#5

DÅ?a Streda 15 Február 2006 08:48 Phil T. napísal:

The only way to really get it to work would be to remove_method before
doing the extend. The problem with that is that remove_method doesn’t work
in that context (removing singleton methods).

Not even in the singleton class?

David V.


#6

In article removed_email_address@domain.invalid,
David V. removed_email_address@domain.invalid wrote:

D=C5=88a Streda 15 Febru=C3=A1r 2006 08:48 Phil T. nap=C3=ADsal:

The only way to really get it to work would be to remove_method before
doing the extend. The problem with that is that remove_method doesn’t wo=
rk
in that context (removing singleton methods).

Not even in the singleton class?

I couldn’t get it to work. Maybe there’s some way.

However, I’ve come up with another way of doing what I was trying to do.
What I needed was a Simulation functionality (where the code is run) and
a
Translation functionality (where the code translated to another
language).
Since the user doesn’t need to switch from one to the other in the same
run it
makes sense to inherit from a different class based on a commandline
argument(or mixin a different module). something along the lines of
this
very simplified example:

class A #could be modules too
def foo
“A::foo”
end
end
class B
def foo
“B::foo”
end
end

superclass = if ARGV[0] == “B”
B
else
A
end

class Demo < superclass
#could use include module instead of inheritance of superclass
end

d = Demo.new
puts d.foo

ruby demo.rb #=> A::foo (default)
ruby demo.rb B #=> B::foo

This should do what I want and it doesn’t lead to extra calls as with
delegation/method_missing schemes which could slow things down
a little bit.

Phil


#7

In article removed_email_address@domain.invalid,
David V. removed_email_address@domain.invalid wrote:

rk
Since the user doesn’t need to switch from one to the other in the same r=
un
it makes sense to inherit from a different class based on a commandline
argument(or mixin a different module).

I’ll agree with Robert K.'s previous post that you probably want to=20
delegate to a strategy in this case. Much less… well… weird. -And- you=
=20
can switch at your leisure too.

Conditional inheritance considered weird?

At one point or another much of what we now consider common practice was
thought to be weird. :wink:

Phil


#8

DÅ?a Å tvrtok 16 Február 2006 01:33 Phil T. napísal:

in that context (removing singleton methods).
argument(or mixin a different module).
I’ll agree with Robert K.'s previous post that you probably want to
delegate to a strategy in this case. Much less… well… weird. -And-
you
can switch at your leisure too.

David V.


#9

2006/2/16, Phil T. removed_email_address@domain.invalid:

wo=3D
What I needed was a Simulation functionality (where the code is run) and a

Conditional inheritance considered weird?

At one point or another much of what we now consider common practice was
thought to be weird. :wink:

Well, certainly. I can remember that we had a discussion about
implementing role pattern by changing inheritance. But since it
doesn’t work as one would like I live with that and try different
approaches. IMHO it’s also more fun to try to get the best out of what
one has than to try to change the rules of the game all the time. :slight_smile:
And Ruby is so extremely flexible that there’s almost nothing that
can’t be done.

Maybe we can make a variant of the extends approach work. Here’s how:
do not use extend directly but another method that simply stores the
module in a member variable. Then override method_missing to delegate
to methods of this module by rebinding them to self (is that possible?
need to try that).

Kind regards

robert


#10

DÅ?a Å tvrtok 16 Február 2006 20:23 Phil T. napísal:

Conditional inheritance considered weird?

At one point or another much of what we now consider common practice was
thought to be weird. :wink:

By all means, play around with uncommon concepts. Except I’m more likely
to
use established patterns that do the same. Matter of taste? Certainly.
Up to
the point where one way doesn’t work and the other does - at the end of
the
day, it’s usefulness that matters.

In the specific case of doing so only once, conditional inheritance does
roughly what delegating to a strategy would. But in the first posted
example,
the latter approach gives you a few more points of flexibility that were
needed to achieve the expected behaviour.

Maybe there is a problem that is indeed most elegantly solved by runtime
conditional inheritance. Considering that delegation can technically
replace
inheritance, as seen in prototype-based OO programming languages, I
personally doubt it. But this wasn’t such a problem.

David V.


#11

I see that you’ve already got a solution, but how about something like
this?

module A
def foo
“A::foo”
end
end
module B
def foo
“B::foo”
end
end
class Demo
def to_a
@available_modules[:A] = Object.new.extend(A) unless
@available_modules[:A]
@current_module = @available_modules[:A]
end
def to_b
@available_modules[:B] = Object.new.extend(B) unless
@available_modules[:B]
@current_module = @available_modules[:B]
end
#Or more generically
def to_module(mod)
mod_sym = mod.name.to_sym
@available_modules[mod_sym] = Object.new.extend(mod) unless
@available_modules[mod_sym]
@current_module = @available_modules[mod_sym]
end
def method_missing(sym, *args)
@current_module.send(sym, *args)
end
def initialize
@available_modules = {A.name.to_sym, Object.new.extend(A)}
@current_module = [A.name.to_sym]
end
end