Loses scope?

ruby-talk
1

Anybody know why this fails?

a = 32
class A
def go
  a
end
end
puts A.new.go # raises with "a not defined"

?
Thanks.
-r

···

--
Posted via http://www.ruby-forum.com/.

2

Because both "class" and "def" keywords create a new scope. They are
not closures, they don't see their surrounding scope. Consider this
other way, which doesn't create new scopes:

irb(main):001:0> a = 3
=> 3
irb(main):008:0> Test = Class.new do
irb(main):009:1* define_method(:go) do
irb(main):010:2* puts a
irb(main):011:2> end
irb(main):012:1> end
=> Test
irb(main):013:0> Test.new.go
3
=> nil

Jesus.

···

On Fri, Dec 18, 2009 at 11:06 PM, Roger Pack <rogerpack2005@gmail.com> wrote:

Anybody know why this fails?

a = 32
class A
def go
a
end
end
puts A.new.go # raises with "a not defined"

?

3

You may want to call class_eval and define_method with blocks instead.

···

On 12/18/2009 2:06 PM, Roger Pack wrote:

Anybody know why this fails?

a = 32
class A
  def go
   a
  end
end
puts A.new.go # raises with "a not defined"

4

Jesús Gabriel y Galán wrote:

Because both "class" and "def" keywords create a new scope. They are
not closures, they don't see their surrounding scope. Consider this
other way, which doesn't create new scopes:

That I did not know. Thanks!
-r

···

--
Posted via http://www.ruby-forum.com/\.

5

puts A.new.go # raises with "a not defined"

You may want to call class_eval and define_method with blocks instead.

Hmm. Perhaps an example using class_eval would help me understand that
better (define_method works great, BTW).
Thanks!
-r

···

--
Posted via http://www.ruby-forum.com/\.

6

Ah thanks for posting that, I'm rather new to ruby and that was
confusing me.

···

On Dec 18, 3:27 pm, Roger Pack <rogerpack2...@gmail.com> wrote:

Jesús Gabriel y Galán wrote:

> Because both "class" and "def" keywords create a new scope. They are
> not closures, they don't see their surrounding scope. Consider this
> other way, which doesn't create new scopes:

7

irb(main):001:0> class A
irb(main):002:1> end
=> nil
irb(main):005:0> a = 3
=> 3
irb(main):006:0> A.class_eval do
irb(main):007:1* define_method(:go) do
irb(main):008:2* puts a
irb(main):009:2> end
irb(main):010:1> end
=> #<Proc:0xb7dbffd0@(irb):7>
irb(main):011:0> A.new.go
3
=> nil

When you already have the class A created and you open it to add the
'go' method, you can use class_eval, which receives a block. Blocks
are closures, so they see their surrounding scope, and thus, the
variable 'a'.

Jesus.

···

On Sun, Dec 20, 2009 at 3:56 AM, Roger Pack <rogerpack2005@gmail.com> wrote:

puts A.new.go # raises with "a not defined"

You may want to call class_eval and define_method with blocks instead.

Hmm. Perhaps an example using class_eval would help me understand that
better (define_method works great, BTW).
Thanks!

8

Jesús Gabriel y Galán wrote:

irb(main):006:0> A.class_eval do
irb(main):007:1* define_method(:go) do

Many thanks and merry Christmas.
-r

···

--
Posted via http://www.ruby-forum.com/\.

9

Jesús Gabriel y Galán wrote:

irb(main):001:0> class A
irb(main):002:1> end
=> nil
irb(main):005:0> a = 3
=> 3
irb(main):006:0> A.class_eval do
irb(main):007:1* define_method(:go) do
irb(main):008:2* puts a
irb(main):009:2> end
irb(main):010:1> end
=> #<Proc:0xb7dbffd0@(irb):7>
irb(main):011:0> A.new.go
3
=> nil

Just curious... is there any difference between using define_method and
def in this case? It seems they achieve the same effect. Why would one
be preferred over the other?

A = Class.new

=> A

A.class_eval do

?> define_method(:test) do
?> puts "Testing!"

    end
  end

=> #<Proc:0xb72bdc40@(irb):3>

A.new.test

Testing!
=> nil

B = Class.new

=> B

B.class_eval do

?> def test

      puts "Testing!!"
    end
  end

=> nil

B.new.test

Testing!!
=> nil

Thanks.

Jeff

···

--
Posted via http://www.ruby-forum.com/\.

10

Jesús Gabriel y Galán wrote:

irb(main):001:0> class A
irb(main):002:1> end
=> nil
irb(main):005:0> a = 3
=> 3
irb(main):006:0> A.class_eval do
irb(main):007:1* define_method(:go) do
irb(main):008:2* puts a
irb(main):009:2> end
irb(main):010:1> end
=> #<Proc:0xb7dbffd0@(irb):7>
irb(main):011:0> A.new.go
3
=> nil

Just curious... is there any difference between using define_method and
def in this case? It seems they achieve the same effect. Why would one
be preferred over the other?

A = Class.new

=> A

A.class_eval do

?> define_method(:test) do
?> puts "Testing!"

end

end

=> #<Proc:0xb72bdc40@(irb):3>

A.new.test

Testing!
=> nil

B = Class.new

=> B

B.class_eval do

?> def test

  puts &quot;Testing\!\!&quot;
end

end

=> nil

B.new.test

Testing!!
=> nil

The difference is that def starts a new scope, it's not a closure, so
it doesn't see its surrounding scope. In the following case (as well
as the original case by the OP), the local variable 'a' is seen in the
block passed to define_method, but not to the body of the 'def'
keyword, since it's not a closure:

irb(main):001:0> class A; end
=> nil
irb(main):002:0> a = 3
=> 3
irb(main):003:0> A.class_eval do
irb(main):004:1* define_method(:go) do
irb(main):005:2* puts a # this a is the a outside the class_eval
irb(main):006:2> end
irb(main):007:1> end
=> #<Proc:0xb788d530@(irb):4>
irb(main):008:0> A.new.go
3
=> nil
irb(main):009:0> A.class_eval do
irb(main):010:1* def no_go
irb(main):011:2> puts a #this one is undefined
irb(main):012:2> end
irb(main):013:1> end
=> nil
irb(main):014:0> A.new.no_go
NameError: undefined local variable or method `a' for #<A:0xb7870584>
  from (irb):11:in `no_go'
  from (irb):14

Hope this helps,

Jesus.

···

On Thu, Dec 24, 2009 at 6:59 AM, Jeff Shantz <rubyforum@jeffshantz.com> wrote:
  from :0