Remember that a Prolog "character" is an atom of length 1.

However, this predicate also accepts SWI-Prolog strings of length 1 on first position.

:- begin_tests(char_code).

test(ok1,[true(Truly)]) :- char_code('a',X), Truly = (X == 97).
test(ok2,[true(Truly)]) :- char_code("a",X), Truly = (X == 97).
test(ok3,[true(Truly)]) :- char_code(C,97) , Truly = (C == a).

test(ouch1,[error(type_error(_,_))])     :- char_code(aa,_).
test(ouch2,[error(type_error(_,_))])     :- char_code('',_).
test(ouch3,[error(type_error(_,_))])     :- char_code(12,_).
test(ouch4,[error(type_error(_,_))])     :- char_code(_,a).
test(ouch5,[error(type_error(_,_))])     :- char_code(_,"a").
test(ouch6,[error(instantiation_error)]) :- char_code(_,_).

% It's not not quite relational:

test(weird1,[true(Truly)]) :- XS="a", char_code(XS,C), char_code(XA,C), Truly = (XS\==XA).

:- end_tests(char_code).

rt(char_code) :- run_tests(char_code).