Thanks Joh! ... the explanation was very helpful!
Regards,
Kashyap
On Sun, May 12, 2019 at 3:23 PM Joh-Tob Schäg <johtob...@gmail.com> wrote:
> The docs:
> (later 'var . prg) -> varExecutes prg in a pipe'ed child process. The
> return value of prg will later be available in var. Note that later
> uses pr and rd to communicate the result, so prg should not write any
> data to standard output as a side effect.
>
> : (prog1 # Parallel background calculation of square numbers
> (mapcan '((N) (later (cons) (* N N))) (1 2 3 4))
> (wait NIL (full @)) )
> -> (1 4 9 16)
>
> It might help that this code the same.
> : (prog1 # Parallel background calculation of square numbers
> (mapcan '((N) (later (list) (* N N))) (1 2 3 4))
> (wait NIL (full @)) )
> -> (1 4 9 16)
>
> Or more departmentalized:
> (setq B (list ))
> -> (NIL)
> ('((N) (later B (prog1 (* N N) (wait 8000)))) 1)
> -> (NIL)
> B
> -> (NIL)
> (wait 8000)
> ..
> B
> -> (1)
>
> Later needs a store to place things. This code uses the following non
> obvious fact:
>
> (setq D (list))
> -> (NIL)
> (setq `D 8)
> -> 8
> D
> -> (8)
>
> or in a more general case:
> (setq U (range 1 20))
> (set (nth U 8) 0)
> U
> -> (1 2 3 4 5 6 7 0 9 10 11 12 13 14 15 16 17 18 19 20)
>
> What happens step by step:
> 1.0 prog1 evaluates (mapcan '((N) (later (list) (* N N))) (1 2 3 4))
> and stores the resulting expression
> 1.1 mapcan calls '((N) (later (list) (* N N))) with 1 through 4.
> 1.1.1 Each call of this lamda creates a new cell (lets call it temp)
> by calling (list) or (cons)
> 1.1.2 temp is set to be the place where return value of the inner
> expression which is calculated by another process is stored by later
> 1.1.3 the temp cell is returned with the result of the calculation still
> pending
> 1.2 mapcan takes these cells and links them destuctively in to one list
> 1.3 mapcan returns that list and the result it written in to @.
>
> 2.0 prog1 evaluates (wait NIL (full @)) ) where @ refers to the return
> value of the previous expression at this level of evaluation.
> 3.0 prog1 returns the value it stores. The value being the value that
> was also refered by @
>
> What is @?
> @ is the list of cells where (in which car) other process will put
> their values into.
>
> Why wait for the full list? This might get really ugly if the list
> becomes partially garbage and is written to. I am not sure this
> behaves since this is internal to the interpreter and "undefined
> behavior"
>
>
>
> On Sun, 12 May 2019 at 22:06, C K Kashyap <ckkash...@gmail.com> wrote:
> >
> > Hi Alex,
> > Could you please explain the role of (cons) as the first parameter to
> later? If I understand correctly, that does not seem to have a role in the
> the final list return values from the child processes.
> > Regards,
> > Kashyap
> >
> > On Sun, May 12, 2019 at 7:17 AM C K Kashyap <ckkash...@gmail.com> wrote:
> >>
> >> Yup ... that's the first edit I tried :)
> >> Regards,
> >> Kashyap
> >>
> >> On Sun, May 12, 2019 at 12:34 AM Alexander Burger <a...@software-lab.de>
> wrote:
> >>>
> >>> On Sat, May 11, 2019 at 10:09:27PM +0200, Alexander Burger wrote:
> >>> > (let Lst
> >>> > (mapcan
> >>> > '((File)
> >>> > (later (cons)
> >>> > (... download File ...)
> >>> > T ) )
> >>> > ...
> >>>
> >>> Note that instead of 'T' you may also return any non-NIL value, e.g the
> >>> downloaded size, or a list with statistical values.
> >>>
> >>> / A!ex
> >>>
> >>> --
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>
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