Re: [stack] unary functions from X to Y?

On Mon, Mar 9, 2009 at 5:58 AM, William Tanksley <[email protected]> wrote:
> Actually, a queue is a bad choice for the future structure. XY uses it
> because it wanted an easy to do 'dip', but 'dip' doesn't require a
> persistent structure. When used only to view and modify the future, XY
> treats its queue as a stack -- which is the correct approach.

XY doesn't need access to the end of the queue for dip either. It
could implement it using patterns:

; pdip { [a f] f / a } ;
1 2 4 [ + ] pdip
=> 3 4

Once you have dip is there any need for a retain stack or the ability
to modify the end of the queue?

> I think Forth's system is terrible, since it
> negates the benefits of easy factoring; but XY's system makes it hard to
> look as far into the future as Forth's system can. But what about
> partial continuations -- neither language provides those, although both
> come close.

XY provides partial continuations in the library. Since XY allows
complete control of the queue any sort of control structure becomes
possible. F is similar in that it provides an operator $. It expects a
quotation on the stack with stack effect ( stack queue -- stack queue
). It calls it with the current stack and queue, and replaces the
existing stack and queue with the result. The other queue/stack
manipulations are built on that. Too much flexibility?

Chris.