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(*
Title: References that allow a single assignment
Author: David Matthews
Copyright David Matthews 2010
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*)
structure SingleAssignment:>
sig
type 'a saref (* Equality not allowed *)
exception Locked
val saref: unit -> 'a saref
val savalue: 'a saref -> 'a option
val saset: 'a saref * 'a -> unit
end
=
struct
exception Locked
type 'a saref = 'a option ref
fun saref () = ref NONE
val savalue = !
fun saset(saVar as ref NONE, newValue) =
let
val lock = RunCall.run_call1 RuntimeCalls.POLY_SYS_lockseg
in
saVar := SOME newValue;
lock saVar
end
| saset _ = raise Locked
end;
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