From: William James <w_a_x_man@...>
Date: 2009-02-07T12:35:48+09:00
Subject: Re: eval statement

Pascal J. Bourguignon wrote:

> 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

C:\>irb --prompt xmp
(def  f(e)
                    (a = 42)
                    (eval e)
                    (eval e)
                  end)
    ==>nil
(f "puts a ; e='puts a+1'")
42
43
    ==>nil
(f "p a ; e='p a+1'")
42
43
    ==>nil