Re: Pausable continuations

[Stefan Israelsson Tampe <stefan.itampe@gmail.com>]

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I have been working on improving the interface, And this works
(define gen
  (make-generator
   (lambda (x)
     (let lp ((i 0))
       (when (< i 20000000)
         (return x i)
         (lp (+ i 1)))))))

(define (test)
  (let ((g (gen)))
    (let lp ((s 0))
      (let ((i (next g)))
        (if (eq? i 'finished)
            s
            (lp (+ s i)))))))

This nice funktional interface runs with no measurable speed overhead
compared to old cases. Also this is essentially python generators and this
code runs in 0.3s where a python generator example runs in 0.5s, using
delimited continuation we are talking about 6-7s.

On Sun, Feb 13, 2022 at 11:31 AM Stefan Israelsson Tampe <
stefan.itampe@gmail.com> wrote:

> (define (f x)
>   (let lp ((i 0))
>     (if (< i 10)
>         (begin
>           (pk 'value-from-parent (pause x i))
>           (lp (+ i 1))))))
>
> (define (test-1)
>   (let ((x   (make-pause-stack))
>         (ret 0))
>     (let lp ((i 0))
>       (let-values (((k x) (resume x (- i))))
>         (cond
>          ((= k pause)
>           (pk 'value-from-child x)
>           (lp (+ i 1)))
>
>          ((= k parent)
>           (pk 'parent))
>
>          ((= k leave)
>           (pk 'leave))
>
>          ((= k child)
>           (pk 'child)
>           (f x)
>           (leave x)
>           (set! ret  i)))))
>
>
>     (pk 'finish x)
>     ret))
>
> On Sun, Feb 13, 2022 at 11:27 AM Mikael Djurfeldt <mikael@djurfeldt.com>
> wrote:
>
>> 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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