Re: DSSSL extensions for XSL

Subject: Re: DSSSL extensions for XSL
From: christo@xxxxxxxxxxxxxxxxxx (Frank Christoph)
Date: Mon, 29 Sep 1997 17:05:43 +0900
> > What [is] "upwards-only" call/cc supposed to denote?  I'm aware of something
> > similar to "downwards-only" call/cc, namely the so-called call/ec
> > (call-with-escaping-continuation).  This might be called downwards-only
> > because the continuation can be passed down, but not up and out of the
> > protected expression.  Is this what you mean?
> An upwards-only continuation is one that can only be used to return to a
> stack frame in the current call chain. When the call/cc call that
> created a continuation has returned, the continuation can no longer be
> called.

Then that's call/ec.

I don't think it makes sense to call this "upwards-only", because then by
contrast the full-power call/cc would be "upwards-and-sideways", rather than
the obvious dual, "downwards": with call/cc you can copy the instances of the
stack to the heap and then pass control back and forth between the copies
(thought of as threads), which implies (to me) lateral motion (as is the case,
for example, with coroutines).

Returning "downwards" doesn't make much sense; if you take the direction to
refer to which way the continuation is passed, rather than which way the
the function returns, then it makes sense to say call/ec only allows passing
a continuation downwards, whereas call/cc allows passing a continuation upwards.


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