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(*
* This implements a transaction log. This is used
* for undoable data structures.
*
* -- Allen
*)
structure TransactionLog : TRANSACTION_LOG =
struct
exception TransactionLog
type version = int
val version = ref 0
val log = ref [] : (version * { rollback : version -> unit,
commit : version -> unit
} list ref) list ref
fun add_object f =
case !log of
(ver,trail)::_ => trail := f :: !trail
| [] => raise TransactionLog
fun init() = (version := 0; log := [])
fun begin () =
let val new_ver = !version+1
in version := new_ver;
log := (new_ver,ref []) :: !log
end
fun abort () =
let val old_ver = !version - 1
in case !log of
(_,ref trail)::rest =>
(app (fn {rollback,...} => rollback old_ver) trail;
version := old_ver;
log := rest)
| [] => raise TransactionLog
end
fun commit () =
let val old_ver = !version - 1
in case !log of
(_,ref trail)::rest =>
(app (fn {commit,...} => commit old_ver) trail;
version := old_ver;
log := rest)
| [] => raise TransactionLog
end
end
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