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Handle exceptions in comparison ops. 

You can't compare logic vars, so just bunt.
This commit is contained in:
Simon Forman 2019-07-20 16:38:48 -07:00
parent 47ae7b2328
commit c676dce2f9
1 changed files with 30 additions and 12 deletions
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42
thun/thun.pl
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@ -22,6 +22,30 @@
:- dynamic func/3.
:- dynamic '≡'/2.
/*
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))).
/*
An entry point.
*/
@ -128,18 +152,12 @@ func(bool, [false|S], [false|S]) :- !.
func(bool, [_|S], [true|S]).
func(>, [A, B|S], [ true|S]) :- B > A.
func(>, [A, B|S], [false|S]) :- \+ B > A.
func(<, [A, B|S], [ true|S]) :- B < A.
func(<, [A, B|S], [false|S]) :- \+ B < A.
func(>=, [A, B|S], [ true|S]) :- B >= A.
func(>=, [A, B|S], [false|S]) :- \+ B >= A.
func(<=, [A, B|S], [ true|S]) :- B =< A.
func(<=, [A, B|S], [false|S]) :- \+ B =< A.
func(=, [A, B|S], [ true|S]) :- B =:= A.
func(=, [A, B|S], [false|S]) :- B =\= A.
func(<>, [A, B|S], [ true|S]) :- B =\= A.
func(<>, [A, B|S], [false|S]) :- B =:= A.
comparison_operator(>).
comparison_operator(<).
comparison_operator(>=).
comparison_operator(<=, =<).
comparison_operator(=, =:=).
comparison_operator(<>, =\=).
/*