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Date: Sat, 25 Feb 2006 22:15:51 +1300
From: "Jonathan Roewen" <jonathan.roewen@gmail.com>
To: OCaml <caml-list@yquem.inria.fr>
Subject: [Caml-list] Weird ref behaviour...
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	version=3.0.3

Excuse my denseness, but can someone explain the following:

source:


let fold_string l s f =3D List.fold_left (fun a b -> if a =3D "" then f b
else a ^ s ^ (f b)) "" l;;

module Expr =3D struct
   type t =3D Entity of string
          | Unknown of string
          | Relation of (string * t list)
   let rec to_string =3D function
   | Entity x -> x
   | Unknown x -> x
   | Relation (_, []) -> ""
   | Relation (n, l) -> Printf.sprintf "%s(%s)" n (fold_string l "," to_str=
ing)
  end;;

open Expr;;

let relation n l =3D Relation (n,l);;
let unknown n =3D Unknown n;;
let entity n =3D Entity n;;

let test (a,b) =3D
    let bindings =3D ref [] in
    let rec test left right =3D match left, right with
    | Entity a, Entity b -> a =3D b
    | Relation (a,x), Relation (b,y) -> a =3D b && List.fold_left2 (fun
a b c -> a && (test b c)) true x y
    | Entity _ as a, Unknown b
    | (Relation _ as a), Unknown b -> begin try
            test a (List.assoc b !bindings)
        with Not_found -> bindings :=3D (b, a) :: !bindings;
Printf.printf "Unified %s to %s\n" b (to_string a); true
      end
    | _ -> false
    in test a b, !bindings

let test_one =3D relation "Loves" [relation "Dog" [entity "Fred"];
entity "Fred"], relation "Loves" [unknown "x"; unknown "y"];;
let test_two =3D relation "Loves" [relation "Dog" [entity "Fred"];
entity "Fred"], relation "Loves" [relation "Dog" [unknown "y"];
unknown "y"];;
let test_three =3D relation "Loves" [relation "Dog" [entity "Fred"];
entity "Fred"], relation "Loves" [unknown "y"; unknown "y"];;

Toplevel session:

# test test_one;;
Unified x to Dog(Fred)
Unified y to Fred
- : bool * (string * Expr.t) list =3D (true, [])
# test test_three;;
Unified y to Dog(Fred)
- : bool * (string * Expr.t) list =3D (false, [])

How can the return value have an empty list???? In fact, for
test_three to correctly return false, the list -has to be- non-empty!

Jonathan