As an example of using the prolog event system supplied by the tcltk
library, we will return to our 8-queens example but now approaching
from the Prolog side rather than the Tcl/Tk side:
:- use_module(library(tcltk)).
setup :-
tk_new([name('SICStus+Tcl/Tk - Queens')], Tcl),
tcl_eval(Tcl, 'source queens.tcl', _),
tk_next_event(Tcl, Event),
( Event = next -> go(Tcl),
; closedown(Tcl)
).
closedown(Tcl) :-
tcl_delete(Tcl).
go(Tcl) :-
tcl_eval(Tcl, 'clear_board', _),
queens(8, Qs),
show_solution(Qs, Tcl),
tk_next_event(Tcl, Event),
( Event = next -> fail
; closedown(Tcl)
).
go(Tcl) :-
tcl_eval(Tcl, 'disable_next', _),
tcl_eval(Tcl, 'clear_board', _),
tk_next_event(Tcl, _Event),
closedown(Tcl).
This is the top-level fragment of the Prolog side of the 8-queens
example. It has three predicates: setup/0,
closedown/1, and go/1. setup/0 simply creates the
Tcl interpreter, loads the Tcl code into the interpreter using a call to
tcl_eval/3 (which also initialises the display) but then calls
tk_next_event/2 to wait for a message from the Tk side.
The Tk part that sends prolog_event-s to Prolog looks like this:
button .next -text next -command {prolog_event next}
pack .next
button .stop -text stop -command {prolog_event stop}
pack .stop
that is two buttons, one that sends the atom next, the
other that sends the atom stop. They are used to get the
next solution and to stop the program respectively.
So if the user presses the next button in the Tk window, then the
Prolog program will receive a next atom via a
prolog_event/tk_next_event pair, and the program can
proceed to execute go/1.
go/1 is a failure driven loop that generates 8-queens solutions
and displays them. First it generates a solution in Prolog and displays
it through a tcl_eval/3 call. Then it waits again for a Prolog
events via tk_next_event/2. If the term received on the
Prolog event queue is next, corresponding to the user pressing
the “next solution” button, then fail is executed and the next
solution found, thus driving the loop.
If the stop button is pressed then the program does some
tidying up (clearing the display and so on) and then executes
closedown/1, which deletes the Tcl interpreter and the
corresponding Tk windows altoegther, and the program terminates.
This example fragment show how it is possible for a Prolog program and a Tcl/Tk program to communicate via the Prolog event queue.