From: badboy <badboy@...>
Date: 2009-02-07T04:38:00+09:00
Subject: Re: eval statement

Mike Gold schrieb:
> 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".
> 
> 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
> 
be careful when using eval in irb. It can lead to hasty conclusions (is 
this correct english? Google Translate sucks =D) which are not correct 
in a pure ruby environment (because under the hood irb uses lots of evil 
evals)