Add multi-iteration side-effect tests for TaskSeq.isEmpty and TaskSeq.empty

Author: abelbraaksma

src/FSharp.Control.TaskSeq.Test/TaskSeq.Empty.Tests.fs

@@ -62,3 +62,29 @@ let ``TaskSeq-empty multiple times in a taskSeq context`` () = task {
 
     Array.isEmpty sq |> should be True
 }
+
+[<Fact>]
+let ``TaskSeq-empty multiple times with side effects`` () = task {
+    let mutable x = 0
+
+    let sq = taskSeq {
+        yield! TaskSeq.empty<string>
+        yield! TaskSeq.empty<string>
+        x <- x + 1
+        yield! TaskSeq.empty<string>
+        x <- x + 1
+        yield! TaskSeq.empty<string>
+        x <- x + 1
+        yield! TaskSeq.empty<string>
+        x <- x + 1
+        x <- x + 1
+    }
+
+    // executing side effects once
+    (TaskSeq.toArray >> Array.isEmpty) sq |> should be True
+    x |> should equal 5
+
+    // twice
+    (TaskSeq.toArray >> Array.isEmpty) sq |> should be True
+    x |> should equal 10
+}

src/FSharp.Control.TaskSeq.Test/TaskSeq.IsEmpty.fs

@@ -60,6 +60,21 @@ module SideEffects =
         |> Task.map (should be True)
         |> Task.map (fun () -> i |> should equal 2)
 
+    [<Fact>]
+    let ``TaskSeq-isEmpty executes side effects each time`` () =
+        let mutable i = 0
+
+        taskSeq {
+            i <- i + 1
+            i <- i + 1
+        }
+        |>> TaskSeq.isEmpty
+        |>> TaskSeq.isEmpty
+        |>> TaskSeq.isEmpty
+        |> TaskSeq.isEmpty // 4th time: 8
+        |> Task.map (should be True)
+        |> Task.map (fun () -> i |> should equal 8)
+
     [<Theory; ClassData(typeof<TestSideEffectTaskSeq>)>]
     let ``TaskSeq-isEmpty returns false for non-empty`` variant =
         Gen.getSeqWithSideEffect variant

src/FSharp.Control.TaskSeq/TaskSeq.fs

@@ -168,6 +168,7 @@ module TaskSeq =
         Internal.checkNonNull (nameof sources) sources
 
         for ts in sources do
+            // no null-check of inner taskseqs, similar to seq
             yield! (ts :> taskSeq<'T>)
     }