Move implementation of singleton to 'internals' file, update doc comment and remove exception

Author: abelbraaksma

src/FSharp.Control.TaskSeq/TaskSeq.fs

@@ -21,19 +21,7 @@ module TaskSeq =
                 }
         }
 
-    let singleton (source: 'T) =
-        { new IAsyncEnumerable<'T> with
-            member _.GetAsyncEnumerator(_) =
-                let mutable started = false
-                { new IAsyncEnumerator<'T> with
-                    member _.MoveNextAsync () =
-                        let r = ValueTask.FromResult (not started)
-                        started <- true
-                        r
-                    member _.get_Current () : 'T = if started then source else invalidOp "Enumeration has not started. Call MoveNextAsync."
-                    member _.DisposeAsync () = ValueTask.CompletedTask
-                }
-        }
+    let singleton (source: 'T) = Internal.singleton source
 
     let isEmpty source = Internal.isEmpty source
 

src/FSharp.Control.TaskSeq/TaskSeq.fsi

@@ -7,9 +7,11 @@ module TaskSeq =
 
     /// Initialize an empty taskSeq.
     val empty<'T> : taskSeq<'T>
-    
-    /// Creates a taskSeq sequence that generates a single element and then ends.
-    val singleton : source: 'T -> taskSeq<'T>
+
+    /// <summary>
+    /// Creates a <see cref="taskSeq" /> sequence from <paramref name="source" /> that generates a single element and then ends.
+    /// </summary>
+    val singleton: source: 'T -> taskSeq<'T>
 
     /// <summary>
     /// Returns <see cref="true" /> if the task sequence contains no elements, <see cref="false" /> otherwise.

src/FSharp.Control.TaskSeq/TaskSeqInternal.fs

@@ -61,6 +61,22 @@ module internal TaskSeqInternal =
         return not step
     }
 
+    let singleton (source: 'T) =
+        { new IAsyncEnumerable<'T> with
+            member _.GetAsyncEnumerator(_) =
+                let mutable ended = false
+
+                { new IAsyncEnumerator<'T> with
+                    member _.MoveNextAsync() =
+                        let vt = ValueTask.FromResult(not ended)
+                        ended <- true
+                        vt
+
+                    member _.Current: 'T = if ended then Unchecked.defaultof<'T> else source
+                    member _.DisposeAsync() = ValueTask.CompletedTask
+                }
+        }
+
     /// Returns length unconditionally, or based on a predicate
     let lengthBy predicate (source: taskSeq<_>) = task {
         use e = source.GetAsyncEnumerator(CancellationToken())