Seven Languages in Seven Weeks - Prolog

This blog post is a next article from series related with books "Seven Languages in Seven Weeks" and its sequel. Each post will describe a single language chosen by this book and its most interesting and influencing features, from my point of view and previous experiences. I hope that you will find this series interesting. Do not hesitate with sharing your feedback and comments below!


In this blog post I would like to spent some time with one of the most interesting programming languages ever - with Prolog. In Bruce Tate’s book this language is compared to the Raymond from the Rain Man movie. For most people Prolog is mostly known as a language that answers no to everything. :wink: It has very nice characteristics and often it is used in various domains and applications when other languages miserably failed.

Logo on the right is representing the most popular Prolog distribution called SWI Prolog, but in this blog post we will take a slightly different approach - we will use Erlog, which is a Prolog implementation on top of Erlang VM. We will use Elixir as a glue for everything.

Facts, Relations, Rules and Queries

The main advantage of Prolog is its declarative approach. You are building knowledge base by declaring facts and relations that connect all of them together. Then, you can define rules with which you can query this knowledge base and retrieve information, like in the example below:

1 loves(vincent, mia). 
2 loves(marsellus, mia).
3 loves(pumpkin, honey_bunny).
4 loves(honey_bunny, pumpkin). 
6 jealous(X, Y):- loves(X, Z), loves(Y, Z).

In this example loves defines a fact between two entities (in our case represented by atoms, a unique symbols) - so we defined relations there. At the 6th line we defined rule, which uses previous relations between two facts, represented as variables X and Z (as in the Erlang variables should be started with a capital letter).

Then if we query such database basing on rule jealous we will receive:

1 ?-  jealous(marsellus, W).
2 W = vincent.

Which obviously true - fans of Pulp Fiction will already know why. :wink:


If we use unification together with the aforementioned elements, we will receive a place where Prolog shines the most. Building on top of a pile of provided facts, relations and rules, it can effectively deduce missing parts. How? We already used it in previous example, but let’s look at the example:

1 ?- append([1], [2], L).
2 L = [1, 2].
4 ?- append(W, [2], [1, 2]).
5 W = [1].
7 ?- append(W, [3], [1, 2]).
8 false.

First query is a pretty much obvious array concatenation (in Prolog you have to return value by the parameter). But, in the second case something strange happened - interpreter responded how it should look first argument of a function call, if we want to receive [1,2] as a result. Third example is a similar case - it is not possible to substitute any value under W in order to satisfy these conditions.

Sudoku Solver

So, let’s try to use our knowledge in practice. Inside my small pet project - afronski/erlog_sudoku_solver - I have used Erlog interpreter on top of Erlang VM. Unfortunately there is no module similar to the clpfd which is available in the SWI Prolog distribution, so the Sudoku solver example will be a little more complicated and it will solve smaller boards - only 4x4.

 1 sudoku(Cells) :-
 2   Cells =
 3   [
 4       [A1,A2,A3,A4],
 5       [B1,B2,B3,B4],
 6       [C1,C2,C3,C4],
 7       [D1,D2,D3,D4]
 8   ],
10   Possible = [1,2,3,4],
12   pick_value(Possible, Possible, A1, RowA_234, Col1_BCD),
13   pick_value(RowA_234, Possible, A2, RowA__34, Col2_BCD),
14   pick_value(RowA__34, Possible, A3, RowA___4, Col3_BCD),
15   pick_value(RowA___4, Possible, A4, _RowA___, Col4_BCD),
17   pick_value(Possible, Col1_BCD, B1, RowB_234, Col1__CD), A2 \= B1,
18   pick_value(RowB_234, Col2_BCD, B2, RowB__34, Col2__CD), A1 \= B2,
19   pick_value(RowB__34, Col3_BCD, B3, RowB___4, Col3__CD), A4 \= B3,
20   pick_value(RowB___4, Col4_BCD, B4, _RowB___, Col4__CD), A3 \= B4,
22   pick_value(Possible, Col1__CD, C1, RowC_234, Col1___D),
23   pick_value(RowC_234, Col2__CD, C2, RowC__34, Col2___D),
24   pick_value(RowC__34, Col3__CD, C3, RowC___4, Col3___D),
25   pick_value(RowC___4, Col4__CD, C4, _RowC___, Col4___D),
27   pick_value(Possible, Col1___D, D1, RowD_234, _), C2 \= D1,
28   pick_value(RowD_234, Col2___D, D2, RowD__34, _), C1 \= D2,
29   pick_value(RowD__34, Col3___D, D3, RowD___4, _), C4 \= D3,
30   pick_value(RowD___4, Col4___D, D4, _RowD___, _), C3 \= D4,
32   true.
34 pick_value(RowVals, ColVals, Value, RowValRest, ColValRest) :-
35    pickValue(RowVals, Value, RowValRest),
36    pickValue(ColVals, Value, ColValRest).
38 pickValue([H|Rest], H, Rest).
39 pickValue([H|Tail], Picked, [H|Rest]) :- pickValue(Tail, Picked, Rest).
41 solve(L) :-
42     L = [ [  _, 2, _, 4  ], [  _, 3, _, _  ], [  _, _, _, 1  ], [  _, _, 2, _  ] ],
43     sudoku(L).

And that is it. As you may noticed - we defined only rules for the game, rest is done thanks to the backtracking algorithms implemented in the interpreter, which are searching and pruning all paths build on top of knowledge base made from facts, relations and rules. This example can be even more concise and clear, if you have module like clpfd, as I mentioned before - you can find an example which uses aforementioned module here.


In the next blog post we will talk about relatively new (compared to the other languages described in the book), but very popular hybrid programming language called Scala. It was my starting point with world of functional programming languages, and it is also often recommended as a starting point - especially if you have background as an object oriented programmer. See you soon! :wink: