Rewrite empty without CE

src/FSharp.Control.TaskSeq/FSharp.Control.TaskSeq.fsproj

@@ -24,6 +24,7 @@ Generates optimized IL code through the new resumable state machines, and comes
     <PackageReleaseNotes>
       Release notes:
       0.2.3
+       - improve TaskSeq.empty by not relying on resumable state, #89
        - do not throw exception for unequal lengths in TaskSeq.zip, fixes #32
       0.2.2
        - removes TaskSeq.toSeqCachedAsync, which was incorrectly named. Use toSeq or toListAsync instead.

src/FSharp.Control.TaskSeq/TaskSeq.fs

@@ -11,10 +11,15 @@ module TaskSeq =
     // Just for convenience
     module Internal = TaskSeqInternal
 
-    let empty<'T> = taskSeq {
-        for c: 'T in [] do
-            yield c
-    }
+    let empty<'T> =
+        { new IAsyncEnumerable<'T> with
+            member _.GetAsyncEnumerator(_) =
+                { new IAsyncEnumerator<'T> with
+                    member _.MoveNextAsync() = ValueTask.False
+                    member _.Current = Unchecked.defaultof<'T>
+                    member _.DisposeAsync() = ValueTask.CompletedTask
+                }
+        }
 
     let isEmpty source = Internal.isEmpty source
 

src/FSharp.Control.TaskSeq/TaskSeqBuilder.fs

@@ -13,6 +13,7 @@ open System.Threading.Tasks.Sources
 
 open FSharp.Core.CompilerServices
 open FSharp.Core.CompilerServices.StateMachineHelpers
+open FSharp.Control
 
 
 [<AutoOpen>]
@@ -279,14 +280,14 @@ and [<NoComparison; NoEquality>] TaskSeq<'Machine, 'T
             if this._machine.ResumptionPoint = -1 then // can't use as IAsyncEnumerator before IAsyncEnumerable
                 logInfo "at MoveNextAsync: Resumption point = -1"
 
-                ValueTask<bool>()
+                ValueTask.False
 
             elif this._machine.Data.completed then
                 logInfo "at MoveNextAsync: completed = true"
 
                 // return False when beyond the last item
                 this._machine.Data.promiseOfValueOrEnd.Reset()
-                ValueTask<bool>()
+                ValueTask.False
 
             else
                 logInfo "at MoveNextAsync: normal resumption scenario"
@@ -343,18 +344,18 @@ and [<NoComparison; NoEquality>] TaskSeq<'Machine, 'T
                 // the Current value
                 data.current <- ValueNone
 
-            ValueTask<bool>(result)
+            ValueTask.FromResult result
 
         | ValueTaskSourceStatus.Faulted
         | ValueTaskSourceStatus.Canceled
         | ValueTaskSourceStatus.Pending as state ->
             logInfo ("at MoveNextAsyncResult: case ", state)
 
-            ValueTask<bool>(this, version) // uses IValueTaskSource<'T>
+            ValueTask.ofIValueTaskSource this version
         | _ ->
             logInfo "at MoveNextAsyncResult: Unexpected state"
             // assume it's a possibly new, not yet supported case, treat as default
-            ValueTask<bool>(this, version) // uses IValueTaskSource<'T>
+            ValueTask.ofIValueTaskSource this version
 
 and TaskSeqCode<'T> = ResumableCode<TaskSeqStateMachineData<'T>, unit>
 and TaskSeqStateMachine<'T> = ResumableStateMachine<TaskSeqStateMachineData<'T>>

src/FSharp.Control.TaskSeq/Utils.fs

@@ -5,12 +5,26 @@ open System.Threading.Tasks
 [<AutoOpen>]
 module ValueTaskExtensions =
     /// Extensions for ValueTask that are not available in NetStandard 2.1, but are
-    /// available in .NET 5+.
+    /// available in .NET 5+. We put them in Extension space to mimic the behavior of NetStandard 2.1
     type ValueTask with
 
         /// (Extension member) Gets a task that has already completed successfully.
         static member inline CompletedTask = Unchecked.defaultof<ValueTask>
 
+
+module ValueTask =
+    /// A successfully completed ValueTask of boolean that has the value false.
+    let False = ValueTask<bool>()
+
+    /// A successfully completed ValueTask of boolean that has the value true.
+    let True = ValueTask<bool> true
+
+    /// Creates a ValueTask with the supplied result of the successful operation.
+    let inline FromResult (x: 'T) = ValueTask<'T> x
+
+    /// Creates a ValueTask with an IValueTaskSource representing the operation
+    let inline ofIValueTaskSource taskSource version = ValueTask<bool>(taskSource, version)
+
 module Task =
     /// Convert an Async<'T> into a Task<'T>
     let inline ofAsync (async: Async<'T>) = task { return! async }