rewrite with option

src/Compiler/Utilities/illib.fs

@@ -16,39 +16,32 @@ type InterruptibleLazy<'T> private (value, valueFactory: unit -> 'T) =
     let syncObj = obj ()
 
     [<VolatileField>]
-    let mutable valueFactory = valueFactory
+    let mutable lazyValue = value
 
-    let mutable value = value
+    new(valueFactory: unit -> 'T) = InterruptibleLazy(ValueNone, valueFactory)
 
-    new(valueFactory: unit -> 'T) = InterruptibleLazy(Unchecked.defaultof<_>, valueFactory)
-
-    member this.IsValueCreated =
-        match box valueFactory with
-        | null -> true
-        | _ -> false
+    member this.IsValueCreated = lazyValue.IsSome
 
     member this.Value =
-        match box valueFactory with
-        | null -> value
+        match lazyValue with
+        | ValueSome value -> value
         | _ ->
             Monitor.Enter(syncObj)
 
             try
-                match box valueFactory with
-                | null -> ()
+                match lazyValue with
+                | ValueSome value -> value
                 | _ ->
-
-                    value <- valueFactory ()
-                    valueFactory <- Unchecked.defaultof<_>
+                    let value = valueFactory ()
+                    lazyValue <- ValueSome(value)
+                    value
             finally
                 Monitor.Exit(syncObj)
 
-            value
-
     member this.Force() = this.Value
 
     static member FromValue(value) =
-        InterruptibleLazy(value, Unchecked.defaultof<_>)
+        InterruptibleLazy(ValueSome(value), Unchecked.defaultof<_>)
 
 module InterruptibleLazy =
     let force (x: InterruptibleLazy<'T>) = x.Value