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comp.lang.ruby

Simulating single assignment in Ruby

Daniel Berger

5/18/2007 12:10:00 AM

Hi,

Is there a way to freeze instance variables in Ruby in order to make
them single assignment? I'm just wondering if there's a way to simulate
Erlang in this regard as a way to avoid side effects.

I don't want to simply not define (or undefine) a setter method because
you could still get at the instance variable via instance_variable_set.
Redefining instance_variable_set won't work either, because that method
is apparently not called when performing direct assignment of instance
variables. I tried calling the freeze method on the instance variables
directly but that didn't seem to work either.

Is it possible?

Regards,

Dan
7 Answers

Austin Ziegler

5/18/2007 12:55:00 AM

0

On 5/17/07, Daniel Berger <djberg96@gmail.com> wrote:
> Is there a way to freeze instance variables in Ruby in order to make
> them single assignment? I'm just wondering if there's a way to
> simulate Erlang in this regard as a way to avoid side effects.
>
> I don't want to simply not define (or undefine) a setter method
> because you could still get at the instance variable via
> instance_variable_set. Redefining instance_variable_set won't work
> either, because that method is apparently not called when performing
> direct assignment of instance variables. I tried calling the freeze
> method on the instance variables directly but that didn't seem to work
> either.

Do it differently:

require 'digest/md5'

class Module
def erl_accessor(*names)
names.each do |name|
var = "@_#{Digest::MD5.hexdigest(rand(65536).to_s + name.to_s)}"
define_method(name) { || instance_variable_get(var) }
define_method("#{name}=") { |v|
instance_variables.include? var and raise "Cannot change @#{name}."
instance_variable_set(var, v)
}
end
nil
end
end

class Foo; erl_accessor :bar, :baz; end

foo = Foo.new
foo.bar = 5

Since your actual instance variables aren't related to the names, you
can only access through accessors.

-austin
--
Austin Ziegler * halostatue@gmail.com * http://www.halo...
* austin@halostatue.ca * http://www.halo...feed/
* austin@zieglers.ca

enduro

5/18/2007 8:53:00 PM

0

Austin Ziegler schrieb:

> On 5/17/07, Daniel Berger <djberg96@gmail.com> wrote:
>
>> Is there a way to freeze instance variables in Ruby in order to make
>> them single assignment? I'm just wondering if there's a way to
>> simulate Erlang in this regard as a way to avoid side effects.
>
> class Module
> def erl_accessor(*names)
> [...]
> define_method("#{name}=") { |v|
> instance_variables.include? var and raise "Cannot change
> @#{name}."
> instance_variable_set(var, v)

What about Thread safety here?
I think you would have do lock before the variable is tested.
If another thread were to assign the same variable
just after the test and before the last line is executed,
then other assignment would be lost, wouldn't it?

I stumbled over this, because this issue also came up
with my proposal of a "growth-allowing freeze"-state of objects.

Regards
Sven

ara.t.howard

5/19/2007 3:32:00 PM

0


On May 17, 2007, at 6:10 PM, Daniel Berger wrote:

> Hi,
>
> Is there a way to freeze instance variables in Ruby in order to
> make them single assignment? I'm just wondering if there's a way to
> simulate Erlang in this regard as a way to avoid side effects.
>
> I don't want to simply not define (or undefine) a setter method
> because you could still get at the instance variable via
> instance_variable_set. Redefining instance_variable_set won't work
> either, because that method is apparently not called when
> performing direct assignment of instance variables. I tried calling
> the freeze method on the instance variables directly but that
> didn't seem to work either.
>
> Is it possible?
>
> Regards,
>
> Dan
>

cfp:~/src/ruby/ > cat a.rb
class Class
def fattr a, &b
define_method(a){ instance_eval &b }
end
end

class C
fattr(:a){ 40 }
fattr(:b){ a + 2 }
end

c = C.new
p c.b


cfp:~/src/ruby/ > ruby a.rb
42


with this approach there is simply no variable to modify - it exists
only through closure. i know someone could hack in some crazy
Binding.of_caller thing to munge the closure, but it would be very
painful.

kind regards.

-a
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama

enduro

5/19/2007 4:47:00 PM

0

ara.t.howard schrieb:

> class Class
> def fattr a, &b
> define_method(a){ instance_eval &b }
> end
> end
>
> class C
> fattr(:a){ 40 }
> fattr(:b){ a + 2 }
> end
> c = C.new
> p c.b
> # => 42
> with this approach there is simply no variable to modify - it exists
> only through closure.

But a method to modify:
class C
fattr(:b){ 17 }
end
p c.b # => 17

Or did I get something wrong?

Sven

ara.t.howard

5/19/2007 6:39:00 PM

0


On May 19, 2007, at 10:46 AM, Sven Suska wrote:

> ara.t.howard schrieb:
>
>> class Class
>> def fattr a, &b
>> define_method(a){ instance_eval &b }
>> end
>> end
>>
>> class C
>> fattr(:a){ 40 }
>> fattr(:b){ a + 2 }
>> end
>> c = C.new
>> p c.b # => 42
>> with this approach there is simply no variable to modify - it
>> exists only through closure.
>
> But a method to modify:
> class C
> fattr(:b){ 17 }
> end
> p c.b # => 17
>
> Or did I get something wrong?
>
> Sven
>

yes of course. and we also have 'remove_const :C' which allows us to
replace the class wholesale, and this line of reasoning extends all
the way up in ruby. still - my approach does, in fact, prevent
modifying instance vars; the fact that the class itself can stiff be
modified (by adding methods) may or may not be an issue. perhaps
enclosed attributes and C.freeze and (class << C;self;end).freeze
would be sufficient.

-a
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama

Daniel Berger

5/21/2007 8:16:00 AM

0

ara.t.howard wrote:
>
> On May 17, 2007, at 6:10 PM, Daniel Berger wrote:
>
>> Hi,
>>
>> Is there a way to freeze instance variables in Ruby in order to make
>> them single assignment? I'm just wondering if there's a way to
>> simulate Erlang in this regard as a way to avoid side effects.
>>
>> I don't want to simply not define (or undefine) a setter method
>> because you could still get at the instance variable via
>> instance_variable_set. Redefining instance_variable_set won't work
>> either, because that method is apparently not called when performing
>> direct assignment of instance variables. I tried calling the freeze
>> method on the instance variables directly but that didn't seem to work
>> either.
>>
>> Is it possible?
>>
>> Regards,
>>
>> Dan
>>
>
> cfp:~/src/ruby/ > cat a.rb
> class Class
> def fattr a, &b
> define_method(a){ instance_eval &b }
> end
> end
>
> class C
> fattr(:a){ 40 }
> fattr(:b){ a + 2 }
> end
>
> c = C.new
> p c.b
>
>
> cfp:~/src/ruby/ > ruby a.rb
> 42
>
>
> with this approach there is simply no variable to modify - it exists
> only through closure. i know someone could hack in some crazy
> Binding.of_caller thing to munge the closure, but it would be very painful.
>
> kind regards.

Ara, Austin - thanks for the respsonses. Both are interesting approaches.

However, I think I'm going to try digging into the opaque objects behind
the scenes via DL and see if I can't do something more directly. Perhaps
I'll contribute that back to evil.rb. :)

Regards,

Dan

Paul Brannan

5/23/2007 10:59:00 AM

0

I think what you want is a constant:

class Foo
def initialize
sc.const_set(:FOO, 42)
end

def foo
puts sc::FOO
end

def sc
class << self; self; end
end
end

Foo.new.foo #=> 42

(I wish it were possible to use just FOO instead of sc::FOO, but
constant lookup in ruby is static).

I like Ara's method too. I wouldn't have considered it.

Paul