determine if something is a list and not chars

[mvolkmann]

I know I can use the list_si predicate in library(is) to determine if something is a list, but I have a case where I'd like to treat lists of character atoms different from other kinds of lists. So I wrote this:

:- use_module(library(si)).

is_list(X) :- chars_si(X), !, fail.
is_list(X) :- var(X), !, fail.
is_list([_|Xs]) :- is_list(Xs).
is_list([]).

Seems pretty straightforward, but when I load this and enter is_list([foo, bar]). it says false.
Removing the first clause fixes it, but I don't understand why I can't keep the first clause.
Entering chars_si([foo, bar]). gives false as expected, so I didn't think that clause would be used.

It seems to me that I should be able to replace the first two clauses with these:

is_list(X) :- \+ chars_si(X).
is_list(X) :- \+ var(X).

But that doesn't work either. I'm stumped on this.

[UWN]

Removing the first clause fixes it, but I don't understand why I can't keep the first clause.

Because chars_si([]) succeeds. Seems you are already quite heavy into cuts.

First you need a good name for your relation, maybe list_but_not_chars_si/1. And then you need to clarify

?- list_but_not_chars_si([]).
   true % maybe this
|  false. % or this

[mvolkmann]

I copied that code from here: https://www.swi-prolog.org/pldoc/man?predicate=is_list/1.
It wasn't actually my idea to use a cut.

Moving the last clause to the beginning like this fixed the issue:

:- use_module(library(si)).

list_not_chars([]).
list_not_chars(X) :- chars_si(X), !, fail.
list_not_chars(X) :- var(X), !, fail.
list_not_chars([_|Xs]) :- list_not_chars(Xs).

How can I get the same behavior without using cuts?

[UWN]

list_but_not_chars_si(Xs) :-
   list_si(Xs),
   ( Xs = [] -> true ; \+ chars_si(Xs) ).

[mvolkmann]

Thanks so much!