Node:Evaluating Prolog expressions from Tcl, Previous:Evaluating Tcl expressions from Prolog, Up:Evaluation functions
The Tcl interpreters created through the SICStus Prolog Tcl/Tk library have been extended to allow calls to the underlying Prolog system.
To evaluate a Prolog expression in the Prolog system from a Tcl
interpreter, the new prolog Tcl command is invoked. It has the
following form:
prolog PrologGoal
where PrologGoal is the printed form of a Prolog goal. This
causes the goal to be executed in Prolog. It will be executed in the
user module unless it is prefixed by a module name. Execution is
always deterministic.
The return value of the command either of the following:
"1", if execution succeeded,
"0", if execution failed,
If succeeded (and "1" was returned) then any variable in
PrologGoal that has become bound to a Prolog term will be returned
to Tcl in the Tcl array named prolog_variables with the variable
name as index. The term is converted to Tcl using the same conversion as
used for Tcl commands (see Command Format). As a special case the
values of unbound variables and variables with names starting with
_, are not recorded and need not conform to the special command
format, this is similar to the threatment of such variables by the
Prolog top-level.
An example:
test_callback(Result) :-
tcl_new(Interp),
tcl_eval(Interp,
'if {[prolog "foo(X,Y,Z)"] == 1} \\
{list $prolog_variables(X) \\
$prolog_variables(Y) \\
$prolog_variables(Z)}',
Result),
tcl_delete(Interp).
foo(1, bar, [a, b, c]).
When called with the query:
| ?- test_callback(Result).
will succeed, binding the variable Result to:
"1 bar {a b c}"
This is because execution of the tcl_eval/3 predicate causes the
execution of the prolog command in Tcl, which executes
foo(X, Y, Z) in Prolog making the following bindings:
X = 1, Y = bar, Z = [a, b, c]. The bindings are
returned to Tcl in the associative array prolog_variables where
prolog_variables(X) is "1", prolog_variables(Y) is "bar", and
prolog_variables(Z) is "a b c". Then Tcl goes on to execute the
list command as
list "1" "bar" "a b c"
which returns the result
"1 bar {a b c}"
(remember: nested lists magically get represented with curly brackets)
which is the string returned in the Result part of the Tcl call,
and is ultimately returned in the Result variable of the
top-level call to test_callback(Result).
If an error occurs during execution of the prolog Tcl command, a
tcl_error/2 exception will be raised. The message part of the
exception will be formed from the string Exception during Prolog
execution: appended to the Prolog exception message. An example is
the following:
?- tcl_new(T), tcl_eval(T, 'prolog wilbert', R).
which will print
{TCL ERROR: tcl_eval/3 - Exception during Prolog execution: wilbert existence_error(wilbert,0,procedure,user:wilbert/0,0)}
at the Prolog top-level, assuming that the predicate wilbert/0 is
not defined on the Prolog side of the system. (This is a
tcl_error exception containing information about the underlying
exception, an existence_error exception, which was caused by
trying to execute the non-existent predicate wilbert.)