Build math & comparision functions.

thun/gnu-prolog/meta-math.pl

@@ -0,0 +1,70 @@
+/*
+
+To handle comparision operators the possibility of exceptions due to
+insufficiently instantiated arguments must be handled.  First try to make
+the comparison and set the result to a Boolean atom.  If an exception
+happens just leave the comparison expression as the result and some other
+function or combinator will deal with it.  Example:
+
+    func(>,  [A, B|S], [C|S]) :- catch(
+            (B > A -> C=true ; C=false),
+            _,
+            C=(B>A)  % in case of error.
+            ).
+
+To save on conceptual overhead I've defined a term_expansion/2 that sets
+up the func/3 for each op.
+*/
+
+term_expansion(comparison_operator(X), (func(X, [A, B|S], [C|S]) :-
+    F =.. [X, B, A], catch((F -> C=true ; C=false), _, C=F))).
+
+% I don't use Prolog-compatible op symbols in all cases.
+term_expansion(comparison_operator(X, Y), (func(X, [A, B|S], [C|S]) :-
+    F =.. [Y, B, A], catch((F -> C=true ; C=false), _, C=F))).
+
+% Likewise for math operators, try to evaluate, otherwise use the
+% symbolic form.
+
+term_expansion(math_operator(X), (func(X, [A, B|S], [C|S]) :-
+    F =.. [X, B, A], catch(C is F, _, C=F))).
+
+term_expansion(math_operator(X, Y), (func(X, [A, B|S], [C|S]) :-
+    F =.. [Y, B, A], catch(C is F, _, C=F))).
+
+
+% Symbolic math.  Compute the answer, or derivative, or whatever, later.
+math_operator(+).
+math_operator(-).
+math_operator(*).
+math_operator(/).
+math_operator(mod).
+
+
+comparison_operator(>).
+comparison_operator(<).
+comparison_operator(>=).
+comparison_operator(<=, =<).
+comparison_operator(=, =:=).
+comparison_operator(<>, =\=).
+
+
+expand_op(Op, Term) :- Op, expand_term(Op, Term).
+
+print_o(Stream, Op) :-
+    findall(Term, expand_op(Op, Term), List),
+    maplist(writeln(Stream), List).
+
+writeln(Stream, Thing) :-
+    portray_clause(Stream, Thing),
+    nl(Stream).
+
+do :-
+    open(`math.pl`, write, Stream),
+    print_o(Stream, math_operator(Op)),
+    print_o(Stream, comparison_operator(Op)),
+    print_o(Stream, comparison_operator(Op, Po)),
+    close(Stream),
+    halt.
+
+