Find path between 2 points from given connections - prolog

Question

I want to solve a typical Prolog problem from here in Ocaml
Connections are represented by following data:
arc(1,2).
arc(2,3).
arc(3,4).
arc(3,5).
arc(2,5).
arc(5,6).
arc(2,6).
One has to find a path given start and end points (solution to be stored in P), e.g.:
?- path(1,6,P).
Prolog code for this is as follows:
path(X,Y,[arc(X,Y)]) :- arc(X,Y).
path(X,Y,[arc(X,Z)|P]) :- arc(X,Z),path(Z,Y,P).
Output will be as:
P = [arc(1, 2), arc(2, 6)] .
I see that that there are many pattern matching capabilities in Ocaml. How can above problem be solved in Ocaml?
Edit: I am not insisting on unification method.

Answer 1

OCaml is not a logic programming language. It is a conventional multi-paradigm, functional, imperative, and object-oriented language. So there are no builtin constructs for solving logical problems (beyond type checking) or anything like this.
However, your problem is a typical graph problem, so you can use any graph library available in OCaml, to solve it, here is the solution implemented using Graphlib's shortest path algorithm, which implements the classical Dijkstra's algorithm
open Core_kernel
open Graphlib.Std
module G = Graphlib.Make(Int)(Unit)
let graph = Graphlib.create (module G) () ~edges:[
1,2,();
2,3,();
3,4,();
3,5,();
2,5,();
5,6,();
2,6,();
]
let () =
match Graphlib.shortest_path (module G) graph 1 6 with
| None -> printf "No path\n"
| Some p ->
Path.edges p |> Sequence.iter ~f:(fun e ->
printf "Arc(%d, %d)\n" (G.Edge.src e) (G.Edge.dst e))
You can install graphlib using opam
opam install graphlib
then, if you put your program in a separate folder and name, let's say, check_path.ml, you can built and run it using ocamlbuild, e.g.
$ ocamlbuild -package graphlib check_path.native --
Arc(1, 2)
Arc(2, 6)