From: Robert Klemme <shortcutter@...>
Date: 2009-02-03T19:06:02+09:00
Subject: Re: Is this possible in Ruby

2009/2/2 Damjan Rems <d_rems@yahoo.com>:
>
> Just an idea which may come in handy. Can this be done and how in Ruby.
>
> def calc(what)
>  some_eval(what)
> end
>
> a=10
> b=12
> calc('a+b') # expression as parameters
> => 12   # would return
>
> a='aa'
> b='bb'
> calc('a+b')
> => 'aabb'
>
> This is of course strictly hypothetical. I would like the expression to
> be evaluated inside the method using variables which are local to the
> calling part.

As has been demonstrated you need the binding of the context in which
you want to evaluate your expression string.  I see two options.

1. explicitly, i.e.

def calc(what, bind)
  eval(what, bind)
end

but this is pointless as "calc" is just an alias for "eval".

2. implicitly

You can keep track of bindings by implementing a trace function (see
set_trace_func). Argument 4 is the current binding and you can place
them in a thread local stack, e.g.

# untested
set_trace_func lambda {|*a|
  case a.first
  when /call$/
    (Thread.current[:bindings] ||= []).push(a[4])
  when /return$/
    Thread.current[:bindings].pop
  end
}

def calc(s)
  eval(s, Thread.current[:bindings][-2])
end

Kind regards

robert

-- 
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