From: Mauricio Fernandez <mfp@...>
Date: 2006-05-13T18:03:21+09:00
Subject: Re: Making sense of the various Ruby "eval"s

On Sat, May 13, 2006 at 05:28:19PM +0900, Eli Bendersky wrote:
> ts wrote:
> >>>>>> "E" == Eli Bendersky <eliben@gmail.com> writes:
> > 
> > E> What have I missed ?
> > 
> >  The evilness of eval.
> > 
> > Guy Decoux
> 
> Guy,
> 
> I would really appreciate a serious answer. The Ruby documentation is 
> bad, there's no need to tell you that, and I'm trying to understand 
> things in depth, hence my questions.
> 
> I understand that eval can be mis-used, like many other advanced 
> features.
> 
> However, I much prefer a serious to-the-point answer. You have cleared 
> out a lot for me, and I thank you for that. What is left is my last 
> question:
> 
> Is block eval the same as single-quoted-string eval ?

$ cat string.rb 
def foo
  instance_eval %{
    387 8sbf fy"(/�%" /�/% "� �� sdug sdf
  }
end

puts "I can't foresee any problem."

# much later, in code far, far away
#foo

$ ruby string.rb 
I can't foresee any problem.
$ cat block.rb 
def foo
  instance_eval do
    387 8sbf fy"(/�%" /�/% "� �� sdug sdf
  end
end

puts "I can't foresee any problem."

# much later, in code far, far away
#foo

$ ruby block.rb 
block.rb:4: syntax error, unexpected tINTEGER, expecting kEND
    387 8sbf fy"(/�%" /�/% "� �� sdug sdf
         ^
block.rb:4: Invalid char `\302' in expression
block.rb:4: Invalid char `\267' in expression
block.rb:4: unterminated string meets end of file
block.rb:4: syntax error, unexpected tSTRING_END, expecting tSTRING_CONTENT or tREGEXP_END or tSTRING_DBEG or tSTRING_DVAR


Another difference:

$ cat eval.rb 
def foo_string(str)
  eval str
  instance_eval "var"
end

def foo_block(str)
  eval str
  instance_eval { var }
end

puts foo_string("var = 'some value'")
puts foo_block("var = 'some value'")

$ ruby eval.rb 
some value
eval.rb:8:in `foo_block': undefined local variable or method `var' for main:Object (NameError)
        from eval.rb:8:in `foo_block'
        from eval.rb:12


-- 
Mauricio Fernandez  -   http://eigenclass.org   -  singular Ruby