PLOGHELP PROCESSES                             Chris Mellish August 1983
                                  revised by Kathryn Seifert August 1986

How to create POP-11 processes which use Prolog procedures

Keywords: POP-11, mixed language programming, processes, prolog barrier


CONTENTS - (Use <ENTER> g to access sections)

 -- PROLOG PROCESSES
 -- EXAMPLE
 -- RELATED DOCUMENTATION


-- PROLOG PROCESSES ---------------------------------------------------

The interaction between Prolog and the POPLOG process mechanism (see
HELP * PROCESS and REF * PROCESS) is still only partially explored.  The
POP-11 procedure 'prolog_barrier_apply' is very important  in creating
POP-11 processes which use Prolog procedures (see
PLOGHELP * PROLOG_BARRIER_APPLY).

A call of 'prolog_barrier_apply' has the following format

    prolog_barrier_apply(<procedure:P> -> <result>;

The procedure P will be applied inside a prolog 'barrier' (see
PLOGHELP * PROLOG_BARRIER_APPLY).

The importance of 'prolog_barrier_apply' here is that the information
that is kept about processes will only include vital information about
the state of Prolog goals if the Prolog goals are invoked inside an
invocation of 'prolog_barrier_apply'. It is thus suggested that
'prolog_barrier_apply' is used at 'consproc' time whenever the process
created will involve Prolog procedures.


-- EXAMPLE ------------------------------------------------------------

Here is an example of using Prolog programs and processes to produce
"solution generators". A more complex example can be seen by
SHOWLIB * PARSEDEMO.

    :- prolog_language("prolog").

    man(fred).
    man(tom).
    man(harry).

    woman(mary).
    woman(jane).
    woman(hilary).

    :- prolog_language("pop11").


    ;;; The following procedure produces a "solution repeater" for
    ;;; any single argument Prolog predicate
    ;;;
    define solution_repeat(predicate);
        vars process, prolog_procedure, whattodo, var;
        ;;; Get value of Prolog predicate as POP-11 procedure
        prolog_valof(predicate,1) -> prolog_procedure;
        ;;; will insert this variable as the argument to the
        ;;; predicate
        prolog_newvar() -> var;

        ;;; This procedure will be run when the process is run.
        ;;; It has two frozen arguments - a Prolog variable
        ;;; which is the argument to the goal and the Prolog
        ;;; procedure to be run on it.
        procedure var prolog_procedure;
           vars contn;
           ;;; Make continuation for Prolog procedure
           ;;; See PLOGHELP * TERMSINPOP for an explanation of
           ;;; the procedure 'prolog_generalise'.
           ;;; When the procedure succeeds, the process suspends,
           ;;; leaving the value of the variable on the stack.
           ;;; When the procedure is resumed, the SUSPEND call
           ;;; returns, which causes the Prolog procedure to
           ;;; backtrack.
           procedure result;
                suspend(prolog_generalise(result),1)
           endprocedure(%var%) -> contn;
           ;;; Now call the Prolog procedure
           ;;; It won't return until it fails
           prolog_procedure(var,contn);
           ;;; If no more solutions, return TERMIN
           ksuspend(termin,1)
        endprocedure(%var,prolog_procedure%) -> whattodo;

        ;;; Now make a process from it. Don't forget to use
        ;;; PROLOG_BARRIER_APPLY

        consproc(0,prolog_barrier_apply(%whattodo%)) -> process;

        ;;; Return a closure of RUNPROC, which can simple be APPLYed

        runproc(%0,process%)
    enddefine;


    ;;; Now let's create some solution repeaters

    vars nextman, nextwoman;

    solution_repeat("man") -> nextman;
    solution_repeat("woman") -> nextwoman;

    ;;; Running a single repeater
    vars x;
    [] -> x;
    until x == termin do
        nextman() -> x;
        x=>
    enduntil;
    ** fred
    ** tom
    ** harry
    ** ^Z

    ;;; Start man repeater off again
    solution_repeat("man") -> nextman;

    ;;; Interleaving repeaters
    repeat 2 times
        nextman()=>
        nextwoman()=>
    endrepeat;
    ** fred
    ** mary
    ** tom
    ** jane


-- RELATED DOCUMENTATION ----------------------------------------------

HELP * PROCESS
 POP-11's process facility

PLOGHELP * MIXED_LANGUAGES
 Overview of interface between Prolog and other languages

PLOGHELP * PLOGTOPOP
 How to call POP-11 from Prolog

PLOGHELP * POPTOPLOG
 How to call Prolog from POP-11

PLOGHELP * PROLOG_BARRIER_APPLY
 POP-11 procedure which applies a procedure inside a Prolog 'barrier'

PLOGHELP * TERMSINPOP
 Facilities for manipulating Prolog terms and variables in POP-11

REF * PROCESS
 Detailed description of POP-11 processes