(* Title: Pure/RAW/single_assignment_polyml.ML
Author: Makarius
References with single assignment. Unsynchronized! Emulates
structure SingleAssignment from Poly/ML 5.4.
*)
signature SINGLE_ASSIGNMENT =
sig
type 'a saref
exception Locked
val saref: unit -> 'a saref
val savalue: 'a saref -> 'a option
val saset: 'a saref * 'a -> unit
end;
structure SingleAssignment: SINGLE_ASSIGNMENT =
struct
exception Locked;
abstype 'a saref = SARef of 'a option ref
with
fun saref () = SARef (ref NONE);
fun savalue (SARef r) = ! r;
fun saset (SARef (r as ref NONE), x) =
(r := SOME x; RunCall.run_call1 RuntimeCalls.POLY_SYS_lockseg r)
| saset _ = raise Locked;
end;
end;