From: Jim Weirich <jweirich@...>
Date: 2003-12-27T13:57:55+09:00
Subject: Re: Closures and Letrec

Charles Comstock wrote:
> Is there a way to create recursive anonymous closures in ruby?  For instance
> if I write
> even = proc {|x| x == 0 ? true : odd(x-1)}
> odd = proc {|x| x == 1 ? true : even(x-1)}
> 
> In this case even definitely doesn't know about odd, and actually doesn't even
> know about even.  

Actually, even does know about even because Ruby sees an assignment to 
even.  To let even know about odd, do this ...

   odd = nil
   even = proc {|x| x == 0 ? true  : odd.call(x-1) }
   odd  = proc {|x| x == 0 ? false : even.call(x-1)}

By assigning to odd, ruby now knows that "odd" is a variable, not a 
method name.  (Notice I changed the definition of odd to avoid infinite 
recursion for the false case.)

-- 
-- Jim Weirich       jweirich@one.net      http://onestepback.org
-----------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct,
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)