Solving the Zebra puzzle (aka. Einstein puzzle) using the clpfd Prolog library

Question

I have been given an exercise to solve the zebra puzzle using a constraint solver of my choice, and I tried it using the Prolog clpfd library.

I am aware that there are other more idiomatic ways to solve this problem in Prolog, but this question is specifically about the clpfd package!

So the specific variation of the puzzle (given that there are many of them) I'm trying to solve is this one:

There are five houses

1. The Englishman lives in the red house
2. The Swedish own a dog
3. The Danish likes to drink tea
4. The green house is left to the white house
5. The owner of the green house drinks coffee
6. The person that smokes Pall Mall owns a bird
7. Milk is drunk in the middle house
8. The owner of the yellow house smokes Dunhill
9. The norwegian lives in the first house
10. The marlboro smoker lives next to the cat owner
11. The horse owner lives next to the person who smokes dunhill
12. The winfield smoker likes to drink beer
13. The norwegian lives next to the blue house
14. The german smokes rothmanns
15. The marlboro smoker has a neighbor who drinks water

I tried to solve it with the following approach:

Each attribute a house can have is modeled as a variable, e.g. "British", "Dog", "Green", etc. The attributes can take values from 1 to 5, depending on the house in which they occur, e.g. if the variable "Dog" takes the value 3, the dog lives in the third house.

This approach makes it easy to model neighbor constraints like this:

def neighbor(X, Y) :-
    (X #= Y-1) #\/ (X #= Y+1).

But somehow, the clpfd package does not yield a solution, even though (IMO) the problem is modeled correctly (I used the exact same model with the Choco constraint solver and the result was correct).

Here is the complete code:

:- use_module(library(clpfd)).

neighbor(X, Y) :-
    (X #= (Y - 1)) #\/ (X #= (Y + 1)).

solve([British, Swedish, Danish, Norwegian, German], Fish) :-

    Nationalities = [British, Swedish, Danish, Norwegian, German],
    Colors = [Red, Green, Blue, White, Yellow],
    Beverages = [Tea, Coffee, Milk, Beer, Water],
    Cigarettes = [PallMall, Marlboro, Dunhill, Winfield, Rothmanns],
    Pets = [Dog, Bird, Cat, Horse, Fish],

    all_different(Nationalities),
    all_different(Colors),
    all_different(Beverages),
    all_different(Cigarettes),
    all_different(Pets),

    Nationalities ins 1..5,
    Colors ins 1..5,
    Beverages ins 1..5,
    Cigarettes ins 1..5,
    Pets ins 1..5,

    British #= Red, % Hint 1
    Swedish #= Dog, % Hint 2
    Danish #= Tea, % Hint 3

    Green #= White - 1 , % Hint 4
    Green #= Coffee, % Hint 5,
    PallMall #= Bird, % Hint 6

    Milk #= 3, % Hint 7
    Yellow #= Dunhill, % Hint 8,
    Norwegian #= 1, % Hint 9

    neighbor(Marlboro, Cat), % Hint 10
    neighbor(Horse, Dunhill), % Hint 11
    Winfield #= Beer, % Hint 12

    neighbor(Norwegian, Blue), % Hint 13
    German #= Rothmanns, % Hint 14,
    neighbor(Marlboro, Water). % Hint 15

Did I misunderstand a concept within clpfd, or am I simply missing something obvious here? In case it helps, here you can find the same approach implemented using Choco and Scala.

---

Edit: The reason why I believe that the solver isn't able to solve the problem ist that it never comes up with definite values for the variables, but only with ranges, e.g. "Fish 1..3\/5".

Answer 1

There are several misconceptions here: You state "the clpfd package does not yield a solution", but actually it does yield one:

?- solve(Ls, Fish), label(Ls).
Ls = [3, 5, 2, 1, 4],
Fish in 1\/4,
all_different([5, 3, _G3699, 2, Fish]),
_G3699 in 1\/4,
_G3699+1#=_G3727,
_G3741+1#=_G3699,
_G3727 in 2\/4..5,
2#=_G3727#<==>_G3766,
_G3766 in 0..1,
_G3792#\/_G3766#<==>1,
_G3792 in 0..1,
2#=_G3741#<==>_G3792,
_G3741 in 0\/2..3.

So we know that if there is a solution, then Fish is either 1 or 4. Let's try 1:

?- solve(Ls, Fish), label(Ls), Fish = 1.
false.

No. So let's try 4:

?- solve(Ls, Fish), label(Ls), Fish = 4.
Ls = [3, 5, 2, 1, 4],
Fish = 4.

This works and is a ground solution to the problem. You can get it in a different way for example by including Fish in the variables that are to be labeled:

?- solve(Ls, Fish), label([Fish|Ls]).
Ls = [3, 5, 2, 1, 4],
Fish = 4 ;
false.

The purpose of labeling is exactly to try concrete values for constrained variables, independent of whether there actually is a solution. By coincidence, all_distinct/1 is strong enough to yield a ground solution by itself in this case, but in general this is of course not the case and you must eventually use labeling to obtain an unconditional (i.e., no more pending constraints) answer. Of course you must then in general also label all variables that are of interest to you, not just a subset of them as you did initially. To label a single variable, you can use indomain/1, so appending indomain(Fish) to the first query above would also work. I could not reproduce the instantiation error you mentioned in a further comment, in fact as you see above the most general query solve(X, Y) works with the code you posted. Finally, check this out:

neighbor(X, Y) :- abs(X-Y) #= 1.