# Kernal::binding problem

**URL:** <https://www.ruby-forum.com/t/kernal-binding-problem/166255>\
**Category:** Ruby\
**Created:** [April 23, 2009, 8:31am UTC](https://www.ruby-forum.com/t/kernal-binding-problem/166255 "2009-04-23T08:31:49Z")\
**Posts on this page:** 7\
**Page:** 1

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**Author:** ![tbcox23](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b9bd4f/32.png) [@tbcox23](https://www.ruby-forum.com/u/tbcox23)\
**Post date:** [April 23, 2009, 8:31am UTC](https://www.ruby-forum.com/t/kernal-binding-problem/166255/1 "2009-04-23T08:31:49Z")

</div>

Trying this for the first time, and, as usual, not quite getting it.

Everything I can find on kernal::binding tells me that I just call it  
and will get back a binding object which can be passed in a call to  
eval. But…it doesn’t work for me.

In my main, I have a module required which contains a method I’m trying  
to call. For reasons irrelevant to this current problem, I’m using an  
eval of a string to do it, and want the method to execute in the  
environment of the main program. So…

eval( “moduleName.methodName”, binding)

Causes _crash_ due to by attempt to access a variable which exists in  
the calling environment but not in the module. In spite of passing the  
binding, the method knows nothing of the variable. It’s not working.

All examples I can find just do something like…

def meth\_x  
y = 1  
return binding  
end

Then ‘y’ is accessed using the binding. Well, if one can return a  
binding to initialize a variable -

z = meth\_x  
eval( “y”, z ) # =\> “1”

Then why can’t I simply pass ‘binding’ in an eval call FROM the calling  
environment?

I must be missing something, to state the obvious.

Can someone explain?

Thanks,

t.

–

```auto
Tom C., MS MA, LMHC - Private practice Psychotherapist
Bellingham, Washington, U.S.A: (360) 920-1226
<< removed_email_address@domain.invalid >> (email)
<< TomCloyd.com >> (website)
<< sleightmind.wordpress.com >> (mental health weblog)

```

---

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**Author:** ![tbcox23](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b9bd4f/32.png) [@tbcox23](https://www.ruby-forum.com/u/tbcox23)\
**Post date:** [April 23, 2009, 9:10am UTC](https://www.ruby-forum.com/t/kernal-binding-problem/166255/2 "2009-04-23T09:10:29Z")

</div>

Tom C. wrote:

> return binding  
> end
> 
> Then ‘y’ is accessed using the binding. Well, if one can return a  
> binding to initialize a variable -
> 
> z = meth\_x  
> eval( “y”, z ) # =\> “1”
> 
> Then why can’t I simply pass ‘binding’ in an eval call FROM the calling  
> environment?

You’ve got something like this, right?

module M  
def M.meth  
p x  
end  
end

x = 1  
eval “M.meth”, binding # Undefined variable x

The binding that you pass into eval here only affects variables that are  
unbound in the string. In this case there are none. The x in M.meth is a  
bound variable in the scope of the method. Compare:

module M  
def M.meth arg  
p arg  
end  
end

x = 1  
eval “M.meth(x)”, binding # ==\> 1

Also:

x = 1  
eval %{  
p x # ==\> 1  
def foo  
p defined?(x)  
end  
foo # ==\> nil  
}, binding

Of course, it’s quite possible I didn’t understand the question…

What are you trying to do?

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**Author:** ![tbcox23](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b9bd4f/32.png) [@tbcox23](https://www.ruby-forum.com/u/tbcox23)\
**Post date:** [April 23, 2009, 3:44pm UTC](https://www.ruby-forum.com/t/kernal-binding-problem/166255/3 "2009-04-23T15:44:57Z")

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On Thu, Apr 23, 2009 at 3:09 AM, Joel VanderWerf  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Tom C. wrote:

> module M  
> unbound in the string. In this case there are none. The x in M.meth is a  
> bound variable in the scope of the method.

And to stress the obvious

eval “some string”

which uses the current binding, is exactly equivalent to

eval “some string”, binding

Assuming that binding is a call to Kernel#binding and not a local  
variable.

So

eval “M.meth”  
and  
eval “M.meth”, binding

will both do the same thing, including throwing any exceptions.

–  
Rick DeNatale

Blog: [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)  
Twitter: [http://twitter.com/RickDeNatale](http://twitter.com/RickDeNatale)  
WWR: [http://www.workingwithrails.com/person/9021-rick-denatale](http://www.workingwithrails.com/person/9021-rick-denatale)  
LinkedIn: [http://www.linkedin.com/in/rickdenatale](http://www.linkedin.com/in/rickdenatale)

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**Author:** ![tbcox23](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b9bd4f/32.png) [@tbcox23](https://www.ruby-forum.com/u/tbcox23)\
**Post date:** [April 24, 2009, 3:35am UTC](https://www.ruby-forum.com/t/kernal-binding-problem/166255/4 "2009-04-24T03:35:06Z")

</div>

Rick DeNatale wrote:

> > > calling environment but not in the module. In spite of passing the binding,
> 
> eval “M.meth”  
> and  
> eval “M.meth”, binding
> 
> will both do the same thing, including throwing any exceptions.

So, how do I get M.meth to execute in the namespace of my main? I  
thought ‘binding’ packaged up that namespace and I could then pass it to  
the method. Wrong, apparently.

I want M.meth to have access to all variables in main’s namespace -  
well, instance variables, anyway. How do I do that?

t.

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**Author:** ![tbcox23](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b9bd4f/32.png) [@tbcox23](https://www.ruby-forum.com/u/tbcox23)\
**Post date:** [April 24, 2009, 5:42am UTC](https://www.ruby-forum.com/t/kernal-binding-problem/166255/5 "2009-04-24T05:42:02Z")

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On Thu, Apr 23, 2009 at 6:33 PM, Tom C. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> > > > 
> > > 
> > > You’ve got something like this, right?
> > 
> > will both do the same thing, including throwing any exceptions.
> 
> So, how do I get M.meth to execute in the namespace of my main? I thought  
> ‘binding’ packaged up that namespace and I could then pass it to the method.

No, it packages it so you can pass it to eval, which is useful if you  
want to call eval from a different point than where you capture the  
binding.

> I want M.meth to have access to all variables in main’s namespace - well,  
> instance variables, anyway. How do I do that?

If you want to access instance variables from the calling scope inside  
a method call (on a different object), you could pass self from the  
calling point as an argument to the method, and use instance\_eval in  
the method. E.g.:

irb(main):001:0\> module M  
irb(main):002:1\> def M.meth  
irb(main):003:2\> end  
irb(main):004:1\> end  
=\> nil  
irb(main):005:0\> module M  
irb(main):006:1\> def M.meth(target)  
irb(main):007:2\> puts target.instance\_eval { @foo }  
irb(main):008:2\> end  
irb(main):009:1\> end  
=\> nil  
irb(main):010:0\> class C  
irb(main):011:1\> def initialize(val)  
irb(main):012:2\> @foo = val  
irb(main):013:2\> end  
irb(main):014:1\> def test  
irb(main):015:2\> M.meth(self)  
irb(main):016:2\> end  
irb(main):017:1\> end  
=\> nil  
irb(main):018:0\> c = C.new(1)  
=\> #\<C:0x2bbd440 @foo=1\>  
irb(main):019:0\> c.test  
1  
=\> nil

---

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**Author:** ![tbcox23](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b9bd4f/32.png) [@tbcox23](https://www.ruby-forum.com/u/tbcox23)\
**Post date:** [April 24, 2009, 5:51am UTC](https://www.ruby-forum.com/t/kernal-binding-problem/166255/6 "2009-04-24T05:51:23Z")

</div>

Christopher D. wrote:

> > > > >
> > > 
> > > which uses the current binding, is exactly equivalent to  
> > > eval “M.meth”, binding  
> > > want to call eval from a different point than where you capture the  
> > > the method. E.g.:  
> > > irb(main):009:1\> end  
> > > irb(main):018:0\> c = C.new(1)  
> > > =\> #\<C:0x2bbd440 @foo=1\>  
> > > irb(main):019:0\> c.test  
> > > 1  
> > > =\> nil

Thanks! That’s very interesting, and is exactly what I was looking for.  
I was beginning to think it wasn’t possible. I would not likely soon  
have found this solution on my own, so I’m very grateful for your  
assistance. Thanks for taking the time to respond.

t.

–

```auto
Tom C., MS MA, LMHC - Private practice Psychotherapist
Bellingham, Washington, U.S.A: (360) 920-1226
<< removed_email_address@domain.invalid >> (email)
<< TomCloyd.com >> (website)
<< sleightmind.wordpress.com >> (mental health weblog)

```

---

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**Author:** ![tbcox23](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/b9bd4f/32.png) [@tbcox23](https://www.ruby-forum.com/u/tbcox23)\
**Post date:** [April 24, 2009, 3:30am UTC](https://www.ruby-forum.com/t/kernal-binding-problem/166255/7 "2009-04-24T03:30:59Z")

</div>

Joel VanderWerf wrote:

> > the environment of the main program. So…  
> > y = 1  
> > calling environment?  
> > eval “M.meth”, binding # Undefined variable x  
> > Yes,
> 
> The binding that you pass into eval here only affects variables that  
> are unbound in the string.  
> Man. Never read THAT anywhere else. Seems like essential information to  
> associate with ‘binding’ method documentation, but it doesn’t seem to  
> be, anywhere I’ve looked.  
> eval “M.meth(x)”, binding # ==\> 1  
> Interesting example. But it puzzles me. Why isn’t ‘x’ considered bound  
> in the scope of self? It’s then passed in to M.meth as an argument, so  
> why is ‘binding’ needed at all in this case? I don’t understand why you  
> did it this way - can you explain?  
> […]
> 
> What are you trying to do?

I’m trying to execute a module method in the namespace of my main  
program which calls it. In outline:

I have a class which requires two files, each of which contains a  
module. I then include each module:

```
utilname = '../lib/setnet/Utils_' + File.basename( name ) + '.rb'

```

#‘name’ passed in when class in instantiated  
if File.exist?( utilname ) # it may not exist  
require utilname  
include Utils\_db  
end  
require ‘…/lib/setnet/Utilities’ # will always exist  
include Utils

While I haven’t found any authority who says this, it appears that  
‘include’ can only occur at the top of a class definition. I only just  
figured this out, so I have about an hour+ experience with ‘include’.

I thought the ‘include’ would essentially move my module methods into my  
main, as if they were physically there already. But that’s not  
happening. I still have my namespace isolation problem:

When I call a module method from my main, it executes fine, until it  
tries to access a variable which would be plainly available of the  
module code were physically residing my class. It then crashes, as it  
finds only ‘nil’.

I don’t know how to fix this. I thought using ‘include’ would do it, but  
it seems to have no effect.

I don’t want to just pass the method the data it needs, since in theory  
I won’t know what that is. I want the method to take care of itself - to  
just have access to the calling program’s namespace, which would solve  
all problems.

Can anyone suggest a solution?

Tom