Re: [stack] disallowing recursive definitions

On 12/3/08 3:19 AM, "William Tanksley, Jr" <[email protected]> wrote:

> Manfred Von Thun <[email protected] <mailto:m.vonthun%40latrobe.edu.au>
> > wrote:
> 
>> >  But as remember, continuation passing means giving
>> >  a procedure an additional parameter which represents the
>> >  rest of the program, and which will normally be executed
>> >  before the procedure returns. Landin¹s papers from the 60¹s
>> >  (and at least one much later) are good on this. Look for
>> >  ³History of continuations² or similar.
> 
> Joy doesn't have any functionality which is like this. Forth does,
> with its direct access to its return stack.
> 
Not so. In Joy all parameters are taken from the stack. Combinators
expect quotations, and quotations are also good as continuation
parameters. I have used it a number of times to implement
backtracking (in the only way I know: with continuations.) There
is one file in the Joy library

http://www.latrobe.edu.au/philosophy/phimvt/joy/plglib.joy
> 
which does what are variously called truth trees, semantic tableaux
or Wang¹s algorithm. Most of the programs in that file are annotated
as expecting

t  f  c  F

on the stack, with F on top. F is a formula in propositional logic which
the tree method tries to make true (or false)  by assigning truth to some
atoms (in t) and falsity to others (in f). The c parameter is the
continuation,
and it does all the hard work of the backtracking.

So, because Joy has quotations, no extra mechanism is needed to
implement continuations.
> 
John Cowan was right about the abandoned cont operator. It turned
out that quotations can do the job. However it may also be that a
different cont operator might make backtracking programs easier
to read.
> 
Incidentally, the mainstream languages which allow procedures as
continuations will also allow several continuations. That is true in
Joy too. I seem to remember that the implementation of Snobol
(and maybe of Icon) used two continuations: one for success,
the other for failure.

  - Manfred


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