The possible argument modes are:
+ The argument is instantiated, i.e. it is not a variable. ++ The argument is ground. - The argument is not instantiated, it must be a free variable without any constraints, especially it must not occur in any other argument and it cannot be a suspending variable. ? The mode is not known or it is neither of the above ones.mode/1 is an operator and accepts also comma-separated list of mode specifications in the form
mode p(+), q(-), r(++, ?).This operator binds less than comma, so the argument of mode/1 might have to be parenthesised when it is followed by other goals. Modes for built-in and external predicates are ignored. Modes are significant only for the first 15 arguments, for higher arguments the mode is always taken as ?.
NOTE: If the instantiation of the predicate call violates its mode declaration, no exception is raised and its behaviour is undefined.
Success: % code size: % no mode declarations [eclipse]: [append]. /home/eclipse/src/append.pl compiled 212 bytes in 0.03 seconds % mode for one argument decreases the code size [eclipse]: mode(append(++, ?, ?)), [append]. /home/eclipse/src/append.pl compiled 120 bytes in 0.00 seconds % modes for other arguments further decreases the size [eclipse]: mode(append(++, ++, -)), [append]. /home/eclipse/src/append.pl compiled 92 bytes in 0.00 seconds % size of the trail stack cygnus% cat p.pl p(f(1), [output]) :- !. p(f(2), [another_one]). test :- p(f(1), X), statistics(trail_stack_used, T1), writeln(T1). :- test. cygnus% eclipse [eclipse]: [p]. 16 /home/eclipse/p.pl compiled 540 bytes in 0.02 seconds % With modes the code is shorter and does less trailing [eclipse]: mode(p(++, -)), [p]. 12 /home/eclipse/p.pl compiled 408 bytes in 0.02 seconds % bad mode declaration: [eclipse]: mode(p(+)), [user]. p(a). user compiled 40 bytes in 0.00 seconds yes. [eclipse]: p(X). % call violates the mode declaration no (more) solution. Error: mode p(X). (Error 4). mode p(+), get_flag(p/1, mode, X). (Error 5). % equivalent to mode((p(+), get_flag(p/1, mode, X)))