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(*
Title: References that allow a single assignment
Author: David Matthews
Copyright David Matthews 2010, 2016
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License version 2.1 as published by the Free Software Foundation.
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) =
(
saVar := SOME newValue;
RunCall.clearMutableBit saVar
)
| saset _ = raise Locked
end;
|
