From: Logan Capaldo <logancapaldo@...>
Date: 2006-09-26T10:44:56+09:00
Subject: Re: Riddle me this (a question about expressions)

On Tue, Sep 26, 2006 at 06:24:38AM +0900, 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 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.
> >>
> >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.
> 
> I still agree with you :-)  I find it interesting to look at what
> raise is actually doing/returning, but clearly x gets its 1 from that
> whole expression, and there's no way (that I can see) to drive a wedge
> between the calling of raise and the execution of the rescue clause.
> 
> >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.
> 
> Neither can I.  The rescue modifier is a special dispensation, but
> raise is still just a method call and shouldn't cause a syntax error.
> 
> 
> David
> 
I think a more interesting question is, whether or not raise _should_ be
a method. It is a method in, Smalltalk for instance, but Smalltalk also
reifies the callstack. I agree that a method should not cause a syntax
error (in that position), but I'm not 
so sure I agree with the idea that raise should be a method. But it
isn't my baby.

> -- 
>                   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