arithmetic.pl

digit(D) --> [D],{member(D, `0123456789`)}.

digits([D]) --> digit(D).
digits([D|Ds]) --> digit(D), digits(Ds).

number_term(N) --> digits(D), {number_codes(N, D)}.

infix_op(add) --> `+`.
infix_op(sub) --> `-`.

expression(N) --> number_term(N).
expression(func(Operator, E1, E2)) --> 
    expression(E1), infix_op(Operator), expression(E2).