Add TaskSeq.tail and tryTail functions (todo: tests)

src/FSharp.Control.TaskSeq/TaskSeq.fs

@@ -215,6 +215,13 @@ module TaskSeq =
         | None -> return Internal.raiseEmptySeq ()
     }
 
+    let tryTail source = Internal.tryTail source
+
+    let tail source = task {
+        match! Internal.tryTail source with
+        | Some result -> return result
+        | None -> return Internal.raiseEmptySeq ()
+    }
     let tryItem index source = Internal.tryItem index source
 
     let item index source = task {

src/FSharp.Control.TaskSeq/TaskSeq.fsi

@@ -232,21 +232,30 @@ module TaskSeq =
     val collectSeqAsync: binder: ('T -> #Task<'SeqU>) -> source: taskSeq<'T> -> taskSeq<'U> when 'SeqU :> seq<'U>
 
     /// <summary>
-    /// Returns the first element of the <see cref="taskSeq" />, or <see cref="None" /> if the sequence is empty.
+    /// Returns the first element of the task sequence from <paramref name="source" />, or <see cref="None" /> if the sequence is empty.
     /// </summary>
-    /// <exception cref="ArgumentException">Thrown when the sequence is empty.</exception>
     val tryHead: source: taskSeq<'T> -> Task<'T option>
 
     /// <summary>
-    /// Returns the first element of the <see cref="taskSeq" />.
+    /// Returns the first elementof the task sequence from <paramref name="source" />
     /// </summary>
     /// <exception cref="ArgumentException">Thrown when the sequence is empty.</exception>
     val head: source: taskSeq<'T> -> Task<'T>
 
     /// <summary>
-    /// Returns the last element of the <see cref="taskSeq" />, or <see cref="None" /> if the sequence is empty.
+    /// Returns the whole task sequence from <paramref name="source" />, minus its first element, or <see cref="None" /> if the sequence is empty.
+    /// </summary>
+    val tryTail: source: taskSeq<'T> -> Task<taskSeq<'T> option>
+
+    /// <summary>
+    /// Returns the whole task sequence from <paramref name="source" />, minus its first element.
     /// </summary>
     /// <exception cref="ArgumentException">Thrown when the sequence is empty.</exception>
+    val tail: source: taskSeq<'T> -> Task<taskSeq<'T>>
+
+    /// <summary>
+    /// Returns the last element of the task sequence from <paramref name="source" />, or <see cref="None" /> if the sequence is empty.
+    /// </summary>
     val tryLast: source: taskSeq<'T> -> Task<'T option>
 
     /// <summary>

src/FSharp.Control.TaskSeq/TaskSeqInternal.fs

@@ -338,11 +338,30 @@ module internal TaskSeqInternal =
 
     let tryHead (source: taskSeq<_>) = task {
         use e = source.GetAsyncEnumerator(CancellationToken())
-        let mutable go = true
-        let! step = e.MoveNextAsync()
-        go <- step
 
-        if go then return Some e.Current else return None
+        match! e.MoveNextAsync() with
+        | true -> return Some e.Current
+        | false -> return None
+    }
+
+    let tryTail (source: taskSeq<_>) = task {
+        use e = source.GetAsyncEnumerator(CancellationToken())
+
+        match! e.MoveNextAsync() with
+        | false -> return None
+        | true ->
+            return
+                taskSeq {
+                    let mutable go = true
+                    let! step = e.MoveNextAsync()
+                    go <- step
+
+                    while go do
+                        yield e.Current
+                        let! step = e.MoveNextAsync()
+                        go <- step
+                }
+                |> Some
     }
 
     let tryItem index (source: taskSeq<_>) = task {