From: Rick DeNatale <rick.denatale@...>
Date: 2006-09-26T06:12:41+09:00
Subject: Re: Riddle me this (a question about expressions)

On 9/25/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
> Hi --
>
> On Tue, 26 Sep 2006, Rick DeNatale wrote:
>
> > On 9/25/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
> >> Hi --
> >>
> >> On Mon, 25 Sep 2006, Rick DeNatale wrote:
> >>
> >> > On 9/24/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
> >> >>
> >> >> I am fairly certain that the return value can never be captured.  At
> >> >> least I can't figure out how to do it.
> >> >>
> >> >> In this:
> >> >>
> >> >>   def raise_value
> >> >>     x = raise
> >> >>   rescue
> >> >>     x
> >> >>   end
> >> >>
> >> >>   p raise_value   # nil
> >> >>
> >> >> I'm pretty sure that x is nil because of the thing where the parser
> >> >> sees an assignment and defines the variable.  I don't think an
> >> >> assignment ever actually takes place -- as witness the fact that:
> >> >>
> >> >>    x = raise || 1
> >> >>
> >> >> also leaves x as nil.
> >> >
> >> > But try:
> >> >
> >> > x = raise rescue nil || 1
> >> >
> >> > Of course this is a trivial example, but the point is that raise can
> >> > in fact have a value.
> >>
> >> Even more trivial:
> >>
> >>    x = raise rescue 1
> >>
> >> :-)  But I'd still say that the return value of the call to raise is
> >> not what's being captured in x.
> >
> > Well, from a syntax point of view, it sure LOOKs like the rvalue of
> > the assignment is the expression "raise rescue 1"
>
> It is; that expression evaluates to 1, and that's what x gets.  But I
> don't think the call to raise is, itself, returning 1.
>
>
> David
>
> --
>                    David A. Black | dblack@wobblini.net
> Author of "Ruby for Rails"   [1] | Ruby/Rails training & consultancy [3]
> DABlog (DAB's Weblog)        [2] | Co-director, Ruby Central, Inc.   [4]
> [1] http://www.manning.com/black | [3] http://www.rubypowerandlight.com
> [2] http://dablog.rubypal.com    | [4] http://www.rubycentral.org

Regardless of what happens under the covers with setjmp/longjmp, I
can't see anyway to explain the results syntactically other than
saying that the 'raise .. rescue ...' is an rvalue in the assigment.

The genesis of this thread was why the ruby compiler disallows:

     x = return y

but allows

     x = raise rescue 1

And some have expressed the opinion that both should be disallowed.

I can't agree. I think that the proper documentation of
Kernel#raise/Thread#raise should be that it causes a non-local return
unless it is rescued, in which case the value is the value of the
expression in the rescue.  That's how it works now.

Of course the final arbiter is Matz.


-- 
Rick