signature MLTON_FINALIZABLE =
sig
type 'a t
val addFinalizer: 'a t * ('a > unit) > unit
val finalizeBefore: 'a t * 'b t > unit
val new: 'a > 'a t
val touch: 'a t > unit
val withValue: 'a t * ('a > 'b) > 'b
end
A finalizable value is a container to which finalizers can be attached. A container holds a value, which is reachable as long as the container itself is reachable. A finalizer is a function that runs at some point after garbage collection determines that the container to which it is attached has become unreachable. A finalizer is treated like a signal handler, in that it runs asynchronously in a separate thread, with signals blocked, and will not interrupt a critical section (see MLtonThread).

addFinalizer (v, f)
adds f as a finalizer to v. This means that sometime after the last call to withValue on v completes and v becomes unreachable, f will be called with the value of v.

finalizeBefore (v1, v2)
ensures that v1 will be finalized before v2. A cycle of values v = v1, …, vn = v with finalizeBefore (vi, vi+1) will result in none of the vi being finalized.

new x
creates a new finalizable value, v, with value x. The finalizers of v will run sometime after the last call to withValue on v when the garbage collector determines that v is unreachable.

touch v
ensures that v's finalizers will not run before the call to touch.

withValue (v, f)
returns the result of applying f to the value of v and ensures that v's finalizers will not run before f completes. The call to f is a nontail call.
Example
Suppose that finalizable.sml contains the following:
signature CLIST =
sig
type t
val cons: int * t > t
val sing: int > t
val sum: t > int
end
functor CList (structure F: MLTON_FINALIZABLE
structure P: MLTON_POINTER
structure Prim:
sig
val cons: int * P.t > P.t
val free: P.t > unit
val sing: int > P.t
val sum: P.t > int
end): CLIST =
struct
type t = P.t F.t
fun cons (n: int, l: t) =
F.withValue
(l, fn w' =>
let
val c = F.new (Prim.cons (n, w'))
val _ = F.addFinalizer (c, Prim.free)
val _ = F.finalizeBefore (c, l)
in
c
end)
fun sing n =
let
val c = F.new (Prim.sing n)
val _ = F.addFinalizer (c, Prim.free)
in
c
end
fun sum c = F.withValue (c, Prim.sum)
end
functor Test (structure CList: CLIST
structure MLton: sig
structure GC:
sig
val collect: unit > unit
end
end) =
struct
fun f n =
if n = 1
then ()
else
let
val a = Array.tabulate (n, fn i => i)
val _ = Array.sub (a, 0) + Array.sub (a, 1)
in
f (n  1)
end
val l = CList.sing 2
val l = CList.cons (2,l)
val l = CList.cons (2,l)
val l = CList.cons (2,l)
val l = CList.cons (2,l)
val l = CList.cons (2,l)
val l = CList.cons (2,l)
val _ = MLton.GC.collect ()
val _ = f 100
val _ = print (concat ["listSum(l) = ",
Int.toString (CList.sum l),
"\n"])
val _ = MLton.GC.collect ()
val _ = f 100
end
structure CList =
CList (structure F = MLton.Finalizable
structure P = MLton.Pointer
structure Prim =
struct
val cons = _import "listCons": int * P.t > P.t;
val free = _import "listFree": P.t > unit;
val sing = _import "listSing": int > P.t;
val sum = _import "listSum": P.t > int;
end)
structure S = Test (structure CList = CList
structure MLton = MLton)
Suppose that cons.c contains the following.
#include <stdio.h>
typedef unsigned int uint;
typedef struct Cons {
struct Cons *next;
int value;
} *Cons;
Cons listCons (int n, Cons c) {
Cons res;
res = (Cons) malloc (sizeof(*res));
fprintf (stderr, "0x%08x = listCons (%d)\n", (uint)res, n);
res>next = c;
res>value = n;
return res;
}
Cons listSing (int n) {
Cons res;
res = (Cons) malloc (sizeof(*res));
fprintf (stderr, "0x%08x = listSing (%d)\n", (uint)res, n);
res>next = NULL;
res>value = n;
return res;
}
void listFree (Cons p) {
fprintf (stderr, "listFree (0x%08x)\n", (uint)p);
free (p);
}
int listSum (Cons c) {
int res;
fprintf (stderr, "listSum\n");
res = 0;
for (; c != NULL; c = c>next)
res += c>value;
return res;
}
We can compile these to create an executable with
% mlton defaultann 'allowFFI true' finalizable.sml cons.c
Running this executable will create output like the following.
% finalizable 0x08072890 = listSing (2) 0x080728a0 = listCons (2) 0x080728b0 = listCons (2) 0x080728c0 = listCons (2) 0x080728d0 = listCons (2) 0x080728e0 = listCons (2) 0x080728f0 = listCons (2) listSum listSum(l) = 14 listFree (0x080728f0) listFree (0x080728e0) listFree (0x080728d0) listFree (0x080728c0) listFree (0x080728b0) listFree (0x080728a0) listFree (0x08072890)
Synchronous Finalizers
Finalizers in MLton are asynchronous. That is, they run at an unspecified time, interrupting the user program. It is also possible, and sometimes useful, to have synchronous finalizers, where the user program explicitly decides when to run enabled finalizers. We have considered this in MLton, and it seems possible, but there are some unresolved design issues. See the thread at