Re: [stack] language hierarchy

On Dec 11, 2007, at 19:38 , William Tanksley, Jr wrote:

> Don Groves <[email protected]> wrote:
>> John Nowak wrote:
>>> This has the downside that functions cannot make use of explicit
>>> recursion (as hypot isn't defined until def is called, and hence the
>>> quote can't make use of it), but this restriction is in place anyway
>>> due to the type system.
>
>> 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.
>
> #3 is especially interesting, since PCs can implement recursion or
> backtracking equally easily. In a typefree language, it's very simple
> to implement; in a typechecked language, of course, one additionally
> has to ensure that the word which begins the partial continuation is
> exposed to the same stack as the words which call it.

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.

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


>
>> Don
>
> -Wm