Named singleton variables

General
1

Hi!

I’d like to know if there’s some way to detect Named singleton variables (i.e. variables starting with a double underscore (__) or a single underscore followed by an uppercase letter, e.g., __var or _Var, Section 2.15.1.10 of the manual).

I’ve seen read_term can inform the list of singleton variables but that’s not exactly what I’m looking for. For example in:

append([1,2,3],[a,b,c],_U), append(_U,[[b,1],[s,6]],A).

_U is a named singleton variable but it is not a singleton variable.

Thanks for the help.

Cheers!

2

Yeah the underscore is quite handy. Take this example (*):

/* What are the last 8 digits of fib(1000000) */

?- fib(1000000, _X), Y is _X mod 10^8.
Y = 42546875.

_X would be huge, but as a marked variable it gets suppressed.

If you read such a query, you see all variables in variable_names/1:

?- read_term(T, [variable_names(L)]).
fib(1000000, _X), Y is _X mod 10^8.
T = (fib(1000000, _A), _B is _A mod 10^8),
L = ['_X'=_A, 'Y'=_B].

(*) Can you do it? Grok and DeepSeek failed, ChatGPT could do it.

3

That’s exactly why I want access to those variables. I can write a piece of code based on variable_names but I thought there would some standard predicate doing the job.

4

In SWI-Prolog the thing is controlled by toplevel_print_anon:

?- append([1,2],[3,4],_X), append(_X,[5,6],Y).
Y = [1, 2, 3, 4, 5, 6].

?- set_prolog_flag(toplevel_print_anon, true).
true.

?- append([1,2],[3,4],_X), append(_X,[5,6],Y).
_X = [1, 2, 3, 4],
Y = [1, 2, 3, 4, 5, 6].

I was just currious which piece of code uses this flag:

hide_names([Name|T0], Skel, Subst, T) :-
    (   sub_atom(Name, 0, _, _, '_'),
        current_prolog_flag(toplevel_print_anon, false),
        sub_atom(Name, 1, 1, _, Next),
        char_type(Next, prolog_var_start)
    ->  true
    ;   Subst == [],
        Skel == '$VAR'(Name)
    ),
    !,
    hide_names(T0, Skel, Subst, T).

In the SWI-Prolog file toplevel.pl. So it uses sub_atom/4, and hard
coded ‘_’ to detect marked variables. Instead you could also use a certain
Unicode class. Don’t know how common this is, it would extend the

notion of underscore, like using Pc from Unicode.

1 Like
5

Good to know. Thanks!

7

Thanks. Fixed (commit 8d9b23a2ee5858af086dd70601dc452a7f85bf19)

9

That is a nice opportunity to create your first PR :slight_smile: And yes, I agree the text is now a bit awkward, so I’ll accept an improved version …