Sicstus prolog: labeling a list in a list

Question

it's quite a long time since I'm dealing with a problem of labeling in Sicstus prolog. I want to create an array of length 7, where each item is again an array of 4 integers. I tried following code, but it doesn't work and gives an instantiation error.

:- use_module(library(clpfd)).

schedule(Ss) :-    
    length(Ss, 7),
    Ss = [[A, B, C, D]|T],
    solve_days(Ss),
    labeling([], Ss).

solve_days([]).
solve_days([[A, B, C, D]|T]):-
    A in 1..3,
    B in 4..7,
    C in 7..9,
    D in 6..10,
   solve_days(T).        

Can anyone give me an advice how to solve it? Thanks a lot!

Answer 1

An approach often seen to solve this kind of annoyance, is to use the predicate term_variables/2 before calling labeling.

The predcate term_variables/2 is an ISO core predicate since Corr.2 and determines the list of variables in a term.

In the present case one would replace

labeling([], Ss) 

by

term_variables(Ss, Vars), labeling([], Vars)

Answer 2

The argument for labeling must be a plain list. One way to fix this is use use append/3 to flatten A..D and T, e.g. append([A,B,C,D],T,Vars).