Re: [stack] disallowing recursive definitions

On Fri, Feb 29, 2008 at 5:36 AM, John Cowan wrote:

> William Tanksley, Jr scripsit:
>
> > I'm generally on Knuth's side on this one.
>
> Remember that Knuth thought Dijkstra was *mostly* right, and takes care to
> disclaim the notion that he is in favor of random gotos.  AFAIK, gotos are
> only used in TeX to compensate for Pascal's lack of a return statement.
>
> > But I don't see how tail recursion is a general goto statement. It
> > seems structured -- in particular, a tail recursion has only one entry
> > point.
>
> If you take an arbitrary tagbody (a list of statements and labels with
> gotos embedded in the statements), you can restructure it into a sequence
> of functions, one per label, where each function ends by tail calling the
> next function in sequence (corresponding to flowing through the label)
> and each goto is changed into a tail call.  This can be done entirely as
> a local transformation.
>
> So while it's true that tail calls have only one entry point, entry
> points and labels are basically the same thing from a Scheme perspective,
> something that neither Knuth nor Dijkstra deals with.  "Lambda: the
> ultimate GOTO."
>
>
I would say, rather: "continuations: the ultimate goto".  And I would mean
that in a good way:)

What you're describing is an equivalence between gotos and a sequence of
function calls, something which is not 100% possible without support for
recursion, and something which is not 100% efficient without tail-call
optimization, but I can't see how you can twist this around to indict
recursion.

I do sort of see, now, how the anaphoric (from hell?) quality of stack
programming languages does provide a lot of expressive power without the
need to resort to recursion.

--Joe


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