On type error of `arg(1, a, _)`

General
,
1

I have found an uxpected type error.

?- arg(1, a, X).
ERROR: Type error: `compound' expected, found `a' (an atom)
ERROR: In:
ERROR:   [10] arg(1,a,_102)
ERROR:    [9] toplevel_call('<garbage_collected>') at /Users/cantor/lib/swipl/boot/toplevel.pl:1173

I expected it should be simply false because related queries on arg/3, functor/3, and =../2 suggested it should simply fail.

% ?- arg(2, a(1), X).
%@ false.

?- functor(T, a, 0).
T = a.

?- functor(T, a, 1).
T = a(_).

?- a =.. U.
U = [a].

?- a(1)=.. U.
U = [a, 1].

Perhaps I prefer to view atoms as a special case in which terms of arity is 0 as suggested by functor/3, and =../2. I hope the query arg(1, a, _) simply fails in the future version.

2

The syntax of SWI-Prolog is rather more complex that ISO Prolog. Did you considered

?- arg(1,a(),X).
false.

Basically, I interpret this feature as an attempt to better separate atomic terms - that get their semantic by metalanguage (i.e. builtins) - from compounds (except unification).

I don’t think there are many chances Jan will change the arg/3 behaviour… but you could wrap your own preferred behaviour by means of term rewriting, no ?

3

I am not sure I fully understand the meaning of f(). But it was useful for converting S-expressions in lisp and prolog terms. In particular, S-expression (a) to the swi-prolog term a(). Without the form a(), it was difficult to convert lisp atom a in to prolog term. That is, my understanding is that a() was the call form of predicate a without argument, though I am far from sure for the semantics of a().

As your suggestion, there is an easy workaoound for that type error. Thanks for comment.