Re: Cretinous Stack Groups

[GLS@MIT-AI]

A cretin must contain a certain amount of state; it has the control
stack and binding pdls for a process, and possibly much other stuff.
Here I will not talk at all about the binding disciplines, but only about
the control.
Let one slot of the state of a cretin be known as the "resumer".
This slot may be empty, or may point to another cretin, in which
case that cretin is presumably in a suspended state, waiting for a value
to carry on with.
Now when one cretin (a) wants to transfer control to another (b), it must
pass a value to it; it may or may not also want to pass a resumption
point.  If it (a) does, it may want the resumption point to be itself (a),
its own resumer (the resumer of a), or yet a third party (c).
Now if a does choose to pass a resumer to b, then it will be stored
in b's resumer slot for later reference.  The resumer slot of b must
be empty for this to occur (stipulation, to avoid losing information
and allow error-checking).  It is permissible, however, not to pass
b a resumption point when b'd resumer slot is empty; in this situation
b must be smart enough not to use its resumer slot.
We thus propose primitives of this sort:
(CRETIN-GO value cretin resumption)
	This transfers control to <cretin>, giving it <value>,
and passing it <resumption> to be put in its resumer slot.
Error if <cretin>'s resumer slot is not empty.
(CRETIN-GO value cretin)
	Transfers control to <cretin>, passing no resumption point.
(CRETIN-JRST value cretin)
	Transfers control to <cretin>, passing it my resumer as a resumption
point, and also clearing my resumer slot.  If I have no resumer (my slot
is already empty), then <cretin>'s slot will be made empty also (?).
(CRETIN-PUSHJ value cretin)
	Transfer control to <cretin>, passing myself as the resumption
point.
(CRETIN-POPJ value)
	Transfer control to the cretin in my resumption slot (error if
none), passing no resumption point.
(MY-RESUMER)
	Returns my resumer, and as a side-effect clears my resumer slot.
Useful for weird hacks.
(HIS-RESUMER cretin)
	Returns the resumer slot of <cretin>.
(CLOBBER-HIS-RESUMER cretin resumption)
	Clobbers <cretin>'s resumer slot.  NIL (i.e. ()) means make empty.
CLOBBER-MY-RESUMER??

Some of these are incredibly dirty, and others are more disciplined.
It is not hard to show that if things are initialized properly,
and only CRETIN-PUSHJ and CRETIN-POPJ are used, then nothing bad can
happen - no errors should be able to occur undetected.  Maybe
this also holds if CRETIN-JRST is added - this is intended as the
equivalent of the "JRST hack".  The otherrs allow for completely
general hair, such as may be necessary for time-slicing or distributed
processing kinds of deals.
Now transferring control to another cretin suspends the current one,
putting it into a state that awaits a value.  When eventually someone
else transfers to it, that value is returned as the value of the call
to CRETIN-PUSHJ or whatever.  Multiple-value versions of all of
the above can be worked out.