1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
|
(*
* A reference that allows undo.
*
* -- Allen
*)
signature UNDOABLE_REF =
sig
eqtype 'a uref
val uref : 'a -> 'a uref
val ! : 'a uref -> 'a
val := : 'a uref * 'a -> unit
end
functor UndoableRef (Log : TRANSACTION_LOG) : UNDOABLE_REF =
struct
type 'a uref = 'a ref * Log.version ref
fun uref a = (ref a, ref(!Log.version))
fun !! (r,_) = !r
fun commit (x,v) = fn ver => v := ver
fun rollback (x,v) =
let val x' = !x
in fn ver => (x := x'; v := ver)
end
fun ::= (r as (x,v),y) =
let val ver = !Log.version
in if !v <> ver then (Log.add_object{rollback = rollback r,
commit = commit r
};
v := ver)
else ();
x := y
end
val ! = !!
val op := = ::=
end
|
