From: pjb@... (Pascal J. Bourguignon)
Date: 2009-02-07T10:24:32+09:00
Subject: Re: eval statement

badboy <badboy@heartofgold.co.cc> writes:

> Jonathan Wills schrieb:
>> Trying to switch a project of mine from python to ruby.  Love ruby so
>> far, but I'm finding that the eval statement is a bit more difficult to
>> use than python's exec statement.  Here is one of the problems I'm
>> having, RUBY_VERSION=1.9.1
>> 
>> 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
>> 
>> r.rb:2:in `<main>': undefined local variable or method `a' for
>> main:Object (NameError)
>> 
>> 
>> Now if in the first case eval can change a variable in its scope, why
>> can't it also create a variable in that same scope?  I may also not be
>> totally clear on the way ruby handles scoping, which might be part of
>> what I am having trouble understanding.
>> 
>> Any help understanding the eval statement would be appreciated.  I will
>> probably have more questions about it, but I'll leave at this for now.
>> 
>> thanks,
>> -Jonathan
> why do you want to use eval?
> eval is evil and in most cases not needed in Ruby

Why are you saying that eval is evil?

In know that in Common Lisp, EVAL has properties that make its use
dubious in most cases.  Mostly it's because it works in the global
environment.

But this is not the case of eval in Ruby. Since there's no compiler,
they can execute eval in the local lexical environment.  So is there
remaining any evilness I don't know?

irb(main):001:0> (def  f(e)
                    (a = 42)
                    (eval e)
                    (eval e)
                  end)
nil
irb(main):006:0> (f "puts a")
42
42
nil
irb(main):007:0> (f "puts a ; e=\"puts a+1\"")
42
43
nil

-- 
__Pascal Bourguignon__