I am working on writing a BASIC interpreter in Prolog. DCGs are a little tricky, which is why I am having trouble here today.
Here is my grammar.
bool --> [true].
bool --> [false].
is_num_char(AD) :- AD = '.'; (atom_codes(AD, [D]), D >= 48, D =< 57).
number --> [].
number --> is_num_char, number.
quotes_on_atom(S) :- atom_chars(S, ['"' | C]), last(C, '"').
string --> quotes_on_atom.
literal --> bool; number; string.
variable --> + literal.
assignment --> string, ['='], expr.
equal --> expr, ['=='], expr.
not_equal --> expr, ['!='], expr.
if --> [if], expr, [then], expr.
for_decl --> [for], assignment, [to], number, [step], number.
for_next --> [next], integer.
expr --> literal; variable; assignment;
equal; not_equal; if; for_decl; for_next.
Here is my main goal:
main :-
% expected: [for, [i, '=', 0], to, '5', step, '1']
trace, phrase(expr, [for, i, '=', '0', to, '5', step, '1']).
Here is the error I get:
uncaught exception: error(existence_error(procedure,is_num_char/2),number/0).
A stack trace revealed this:
The debugger will first creep -- showing everything (trace)
1 1 Call: phrase(expr,[for,i,=,'0',to,'5',step,'1']) ?
2 2 Call: expr([for,i,=,'0',to,'5',step,'1'],_335) ?
3 3 Call: literal([for,i,=,'0',to,'5',step,'1'],_335) ?
4 4 Call: bool([for,i,=,'0',to,'5',step,'1'],_335) ?
4 4 Fail: bool([for,i,=,'0',to,'5',step,'1'],_335) ?
4 4 Call: number([for,i,=,'0',to,'5',step,'1'],_335) ?
4 4 Exit: number([for,i,=,'0',to,'5',step,'1'],[for,i,=,'0',to,'5',step,'1']) ?
3 3 Exit: literal([for,i,=,'0',to,'5',step,'1'],[for,i,=,'0',to,'5',step,'1']) ?
2 2 Exit: expr([for,i,=,'0',to,'5',step,'1'],[for,i,=,'0',to,'5',step,'1']) ?
2 2 Redo: expr([for,i,=,'0',to,'5',step,'1'],[for,i,=,'0',to,'5',step,'1']) ?
3 3 Redo: literal([for,i,=,'0',to,'5',step,'1'],[for,i,=,'0',to,'5',step,'1']) ?
4 4 Redo: number([for,i,=,'0',to,'5',step,'1'],[for,i,=,'0',to,'5',step,'1']) ?
5 5 Call: is_num_char([for,i,=,'0',to,'5',step,'1'],_446) ?
5 5 Exception: is_num_char([for,i,=,'0',to,'5',step,'1'],_459) ?
4 4 Exception: number([for,i,=,'0',to,'5',step,'1'],_335) ?
3 3 Exception: literal([for,i,=,'0',to,'5',step,'1'],_335) ?
2 2 Exception: expr([for,i,=,'0',to,'5',step,'1'],_335) ?
1 1 Exception: phrase(expr,[for,i,=,'0',to,'5',step,'1']) ?
uncaught exception: error(existence_error(procedure,is_num_char/2),number/0)
{trace}
It seems that is_num_char is being passed the whole list of tokens, along with something else. I do not understand why this is happening, given that the number rule accepts only one argument. Additionally, it's odd that the token list unifies with number to begin with. It should be unified with for_decl instead. If you are knowledgeable about Prolog DCGs please let me know what I am doing wrong here.
