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(*
* This is a simple cache datatype.
*
* -- Allen
*)
signature CACHE_REF =
sig
type 'a cache
val cache : ('a -> 'b) -> 'a -> 'b cache
val flush : 'a cache -> unit
val ! : 'a cache -> 'a
val := : 'a cache * 'a -> unit
end
structure CacheRef :> CACHE_REF =
struct
type 'a cache = 'a option ref * (unit -> 'a)
fun cache f x = (ref NONE, fn _ => f x)
fun flush (x,_) = x := NONE
fun ! (r as ref NONE,f) = let val x = f() in r := SOME x; x end
| ! (r as ref(SOME x),f) = x
val op := = fn((r, _),x) => r := SOME x
end
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