Bi-directional conversion between hex strings and numbers in Prolog

hex.pl

:- use_module(library(clpfd)).

hex_number(Hex, Num) :-
    ground(Hex),
    !,
    string_concat("0x", Hex, HexStr),
    number_string(Num, HexStr).

hex_number(Hex, Num) :-
    var(Hex),
    !,
    Num in 0..0xfffffff,
    label([Num]),
    format(string(Hex), "~16r", Num).

Example outputs:

?- Hex = 'fffff', hex_number(Hex, Num).
Hex = fffff,
Num = 1048575.

?- Num = 98765, hex_number(Hex, Num).
Num = 98765,
Hex = "181cd".

?- hex_number(Hex, Num).
Hex = "0",
Num = 0 ;
Hex = "1",
Num = 1 ;
Hex = "2",
Num = 2 ;
Hex = "3",
Num = 3 ; % etc etc

Code is released in the public domain.