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|
structure Promise
:> sig
type 'fulfillment promise
val delay: (unit -> 'fulfillment) -> 'fulfillment promise
val force: 'fulfillment promise -> 'fulfillment
end = struct
local
datatype 'expectation lazy =
Delayed of unit -> 'expectation
| Forced of 'expectation
in
type 'expectation promise = 'expectation lazy ref
fun delay fulfill = ref (Delayed fulfill)
fun force promise = case !promise of
Delayed fulfill =>
let
val expectation = fulfill ()
in
promise := Forced expectation
; expectation
end
| Forced expectation => expectation
end
end
|
