nbc added

1 files changed, 24 insertions, 0 deletions

diff --git a/src/nbc/promise.sml b/src/nbc/promise.sml
new file mode 100644
index 0000000..6bb2655
--- /dev/null
+++ b/src/nbc/promise.sml
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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