From: Tom Cloyd <tomcloyd@...>
Date: 2009-02-07T18:50:57+09:00
Subject: Re: eval statement

David A. Black wrote:
> Hi --
>
> On Sat, 7 Feb 2009, Mike Gold wrote:
>
>> Mike Gold wrote:
>> If all assignments were the same, this would print '3':
>>>
>>>   eval("a = 3", binding)
>>>   puts a
>>>
>>
>> Or even better,
>>
>>  eval("a = 3", binding)
>>  p local_variables  #=> ["a"]
>>  p a  #=> undefined local variable or method `a'
>>
>> We see that the local exists, but because the parser has not seen the "a
>> = ..." syntax, we can't access it.
>
> In 1.9 you get:
>
> []
> -:3:in `<main>': undefined local variable or method `a' for
> main:Object (NameError)
>
> so I guess that's been changed. The point about rb_method_missing (as
> the source of that error message) is interesting, though since it's be
> handled by a handler for non-existent methods, and within that handler
> it determines that it also isn't a local variable, I think it's
> reasonable to say that it can't tell -- or, perhaps more accurately,
> it *can* tell that it is neither a (viable) method call nor a
> variable, but it can't tell which you meant (hence the wording of the
> error message).
>
> There's also this interesting difference. In 1.8, given this code:
>
> eval("a=3")
> p a rescue p "undef"
> eval("p a")
>
> you get this output:
>
> "undef"
> 3
>
> whereas in 1.9 the output for the same code is:
>
> "undef"
> -:3:in `eval': undefined local variable or method `a'
>
> So the business of having eval always return to the same scope seems
> to have been done away with. (I always thought of it as similar to the
> phenomenon of having to be drunk again to remember what you did when
> drunk :-)
That's known as "state dependent learning" (but maybe you already know 
that!) - an interesting phenomenon, for sure. ~t.
>
>
> David
>


-- 

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Tom Cloyd, MS MA, LMHC - Private practice Psychotherapist
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