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comp.lang.ruby

Why can't I define vars in caller's binding using eval?

Christopher J. Bottaro

5/22/2008 3:38:00 AM

In other words, why can't I do this?

def f(b)
eval("x = 10", b)
end

f(binding)
puts "x = #{x}"

Is there any way to make that code work (besides obviously setting x
to something before calling f).

Thanks.
3 Answers

Andrew Mitchell

5/23/2008 8:40:00 AM

0

Christopher J. Bottaro wrote:
> In other words, why can't I do this?
>
> def f(b)
> eval("x = 10", b)
> end
>
> f(binding)
> puts "x = #{x}"
>
> Is there any way to make that code work (besides obviously setting x
> to something before calling f).
>
> Thanks.

Worked for me.

Dios:~ andrewmitchell$ irb
>> def f(b)
>> eval("x=10", b)
>> end
=> nil
>> f(binding)
=> 10
>> puts "x = #{x}"
x = 10
=> nil
>>
--
Posted via http://www.ruby-....

Heesob Park

5/23/2008 9:14:00 AM

0

2008/5/23 Andrew Mitchell <amitchell@ttcent.com>:
> Christopher J. Bottaro wrote:
>> In other words, why can't I do this?
>>
>> def f(b)
>> eval("x = 10", b)
>> end
>>
>> f(binding)
>> puts "x = #{x}"
>>
>> Is there any way to make that code work (besides obviously setting x
>> to something before calling f).
>>
>> Thanks.
>
> Worked for me.
>
> Dios:~ andrewmitchell$ irb
>>> def f(b)
>>> eval("x=10", b)
>>> end
> => nil
>>> f(binding)
> => 10
>>> puts "x = #{x}"
> x = 10
> => nil
>>>
But it works only in irb.

Regards,

Park Heesob

Florian Gilcher

5/23/2008 10:20:00 AM

0

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As far as i know, this is because local variables have a lexical scope.

In my experience, the parser treats everything that was not mentioned
as a
local variable before a method call and the runtime doesn't check
whether
it is a variable.

This also explains why IRB works differently, as every statement is
compiled
as a single statement.

Because of this, it is also impossible to assume the existance of
local variables
when executing a Proc in a certain context. (without considering hefty
AST-Hacking
with ruby2ruby).


Regards,
Florian Gilcher

On May 23, 2008, at 11:13 AM, Heesob Park wrote:

> 2008/5/23 Andrew Mitchell <amitchell@ttcent.com>:
>> Christopher J. Bottaro wrote:
>>> In other words, why can't I do this?
>>>
>>> def f(b)
>>> eval("x = 10", b)
>>> end
>>>
>>> f(binding)
>>> puts "x = #{x}"
>>>
>>> Is there any way to make that code work (besides obviously setting x
>>> to something before calling f).
>>>
>>> Thanks.
>>
>> Worked for me.
>>
>> Dios:~ andrewmitchell$ irb
>>>> def f(b)
>>>> eval("x=10", b)
>>>> end
>> => nil
>>>> f(binding)
>> => 10
>>>> puts "x = #{x}"
>> x = 10
>> => nil
>>>>
> But it works only in irb.
>
> Regards,
>
> Park Heesob
>

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