Re: [stack] language hierarchy

Don Groves <[email protected]> wrote:
> William Tanksley, Jr wrote:
> >> How about "self" as a call to its enclosing context?

> > It won't work for dynamic definitions, because there's no distinct
> > concept of "enclosing context". All you know is that you're inside a
> > quotation that's inside another quotation; you can't look ahead to see
> > which enclosure is the one being defined and/or executed. There are
> > several solutions:

> > 1. Provide special syntax for quoted definitions (perhaps make them
> > compiled).
> > 2. Provide recursion primitives like linrec, genrec, etc.
> > 3. Provide partial continuations.

> You're probably right on this but I'm not entirely convinced.  John said
> "explicit recursion" which I take to mean calling a function from within
> itself. This would seem to leave leave out 1. and 2.

#1 actually is the definition of explicit recursion -- at least for
any definition which allows using a pseudokeyword rather than naming
the invoked function directly.

> Maintaining a pointer to the current enclosure should be sufficient for
> self invocation. That would require another stack, of course, which may
> be worse than having a simple solution to the problem.

Hmm, in effect most concatenative languages keep something
functionally equivalent to a second stack -- the return addresses of
all calls. Along with each return address you'd be able to keep a
recursion address; both would be pushed at CALL time, and RET would
use the return address while RECURSE uses the recursion address.
That's not bad semantics... I'm almost tempted to play with it and see
what else can be done (can one of those fields be used for other
purposes?).

Of course, the stack usage is doubled or worse, but such, it seems, is life.

> And using the
> already existing recursive methods is more in line with concatenative
> theory anyway, ..., so I'll forget about it ;-)

Is that true? Are the *rec functions especially good with
concatenative languages? (I don't know.) I'm sure they work with any
functional language...

> Don

-Wm