I'm trying to create a tolower implementation in prolog, but i keep getting an error and I'm not sure what it means.
?- tolower("HE",L).
ERROR: tolower/2: Undefined procedure: (+)/2
here is my implementation so far.
tolower([], _).
tolower([H|T], L):-
H + 32, tolower(T, L).
Let us see what H + 32 means:
?- write_canonical(H+32). +(_,32)
So you are invoking a predicate called +, with two arguments.
You have not defined such a predicate, hence you get an error.
To evaluate arithmetic expressions, use for example a constraint such as (#=)/2 for integers:
R #= H + 32
This is a relation, saying that R is equal to H + 32, evaluated as an integer expression.
Depending on your Prolog system, you may have to import a library to use (#=)/2.