Hi,

I'm trying to understand this.

The example of a generator which you give below counts upwards, but I don't
see how the value of n is passed out of the generator.

Could you give another example of a generator which does pass out the
values, along with a usage case which prints out the values returned by the
generator?

Best regards,
Mikael

Den tors 10 feb. 2022 17:52Stefan Israelsson Tampe <stefan.itampe@gmail.com>
skrev:

> Consider a memory barrier idiom constructed from
> 0, (mk-stack)
> 1. (enter x)
> 2. (pause x)
> 3. (leave x)
>
> The idea is that we create a separate stack object and when entering it,
> we will swap the current stack with the one in the argument saving the
> current stack in x  and be in the 'child' state and move to a paused
> position in case of a pause, when pausing stack x, we will return to where
> after where entered saving the current position in stack and ip, and be in
> state 'pause' and when we leave we will be in the state 'leave and move
> to the old stack, using the current
> ip. At first encounter the function stack frame is copied over hence there
> will be a fork limited to the function only.
>
> This means that we essentially can define a generator as
> (define (g x)
>   (let lp ((n 0))
>     (if (< n 10)
>         (begin
>            (pause x)
>            (lp (+ n 1))))))
>
> And use it as
> (define (test)
>     (let ((x (mk-stack)))
>         (let lp ()
>            (case (enter x)
>                ((pause)
>                    (pk 'pause)
>                    (lp))
>                 ((child)
>                  (g x)
>                  (leave x))))))))
>
> A paused or leaved stack cannot be paused, an entered stack cannot be
> entered and one cannot leave a paused stack, but enter a leaved stack.
>
> Anyhow this idea is modeled like a fork command instead of functional and
> have the benefit over delimited continuations that one does not need to
> copy the whole stack and potentially speed up generator like constructs.
> But not only this, writing efficient prolog code is possible as well. We
> could simplify a lot of the generation of prolog code, speed it up and also
> improve compiler speed of prolog code significantly.
>
> How would we approach the  prolog code. The simplest system is to use
> return the
> alternate pause stack when succeeding things becomes very simple,
>
> x   = stack to pause to in case of failure
> cc = the continuation
>
> (<and> (x cc)  goal1 goal2)
>      :: (cc (goal1 (goal2 x))
>
> (<or >   (x cc)  goal1 goal2)
>     ::  (let ((xx (mkstack)))
>              (case (enter xx)
>                  ((child)
>                   (cc (goal2 xx)))
>
>                 ((pause)
>                  (cc (goal2 x)))))
>
> Very elegant, and we also can use some heuristics to store already made
> stacks when
> leaving a stack and reuse at the next enter which is a common theme in
> prolog,
>
> Anyhow we have an issue, consider the case where everythings
> succeds forever. Then we will blow the stack . There is no concept of tail
> calls here. So what you can do is the following for an <and>,
>
> (let ((xx (mk-stack)))
>     (case (enter xx)
>       ((child)
>        (goal1 x (lambda (xxx) (pause xx xxx)))
>
>       ((pause xxx)
>          (goal2 xxx cc))))
>
> This enable cuts so that a cutted and (and!) in kanren lingo will use
> (goal2 x cc)
>
> And we have tail calls!
>
>
> I have a non jitted version guile working as a proof of concept.
>
> The drawback with this is if a function uses a lot of stack, it will be a
> memory hog.
>
> WDYT?
>
>
>
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>
> .
>
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