From: "David A. Black" <dblack@...>
Date: 2009-02-07T05:00:29+09:00
Subject: Re: eval statement

Hi --

On Sat, 7 Feb 2009, Mike Gold wrote:

> Jonathan Wills wrote:
>>
>> a = 1
>> eval('a=2')
>> puts a
>>
>> This will print out '2', as I want.  However if I remove the first line,
>>
>> eval('a=2')
>> puts a
>
> The thing to remember is that local variables are always determined at
> parse time.
>
> The contents of 'a=2' is not seen by the parser when the file is read,
> so in the second case the 'a' in 'puts a' is determined to be a method
> call: "call method 'a', then call method puts".

Actually it can't tell whether it's a method or a variable, so you
get:

   undefined local variable or method `a' for main:Object (NameError)

> Constants and instance variables are different, as they are determined
> by their naming scheme and therefore behave more as you expected.
>
> irb(main):001:0> eval("@a = 2")
> => 2
> irb(main):002:0> @a
> => 2
> irb(main):003:0> eval("X = 3")
> => 3
> irb(main):004:0> X
> => 3

They're that way because of how they're scoped, but the assignments
themselves are handled by eval the same way as the local ones; that
is, there's no extra information available prior to the execution.


David

-- 
David A. Black / Ruby Power and Light, LLC
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