From 7ccd866be37f9485ac6368539404a6fc6a342950 Mon Sep 17 00:00:00 2001
From: Abel Braaksma <abel.online@xs4all.nl>
Date: Sun, 6 Nov 2022 19:34:16 +0100
Subject: [PATCH] Add implementation for TaskSeq.delay

---
 README.md                                     |  3 +-
 .../FSharp.Control.TaskSeq.Test.fsproj        |  1 +
 .../TaskSeq.Delay.Tests.fs                    | 54 +++++++++++++++++++
 src/FSharp.Control.TaskSeq/TaskSeq.fs         |  5 ++
 src/FSharp.Control.TaskSeq/TaskSeq.fsi        |  8 +++
 5 files changed, 70 insertions(+), 1 deletion(-)
 create mode 100644 src/FSharp.Control.TaskSeq.Test/TaskSeq.Delay.Tests.fs

diff --git a/README.md b/README.md
index a102b443..10c1b46f 100644
--- a/README.md
+++ b/README.md
@@ -83,7 +83,7 @@ The following is the progress report:
 |                  | `compareWith`      | `compareWith`   | `compareWithAsync`   | |
 | &#x2705; [#69][] | `concat`           | `concat`        |                      | |
 | &#x2705; [#70][] | `contains`         | `contains`      |                      | |
-|                  | `delay`            | `delay`         |                      | |
+| &#x2705; [#82][] | `delay`            | `delay`         |                      | |
 |                  | `distinct`         | `distinct`      |                      | |
 |                  | `distinctBy`       | `dictinctBy`    | `distinctByAsync`    | |
 | &#x2705; [#2][]  | `empty`            | `empty`         |                      | |
@@ -610,4 +610,5 @@ module TaskSeq =
 [#70]: https://github.com/fsprojects/FSharp.Control.TaskSeq/pull/70
 [#76]: https://github.com/fsprojects/FSharp.Control.TaskSeq/pull/76
 [#81]: https://github.com/fsprojects/FSharp.Control.TaskSeq/pull/81
+[#82]: https://github.com/fsprojects/FSharp.Control.TaskSeq/pull/82
 
diff --git a/src/FSharp.Control.TaskSeq.Test/FSharp.Control.TaskSeq.Test.fsproj b/src/FSharp.Control.TaskSeq.Test/FSharp.Control.TaskSeq.Test.fsproj
index 4ab3edd3..d030c085 100644
--- a/src/FSharp.Control.TaskSeq.Test/FSharp.Control.TaskSeq.Test.fsproj
+++ b/src/FSharp.Control.TaskSeq.Test/FSharp.Control.TaskSeq.Test.fsproj
@@ -18,6 +18,7 @@
     <Compile Include="TaskSeq.Collect.Tests.fs" />
     <Compile Include="TaskSeq.Concat.Tests.fs" />
     <Compile Include="TaskSeq.Contains.Tests.fs" />
+    <Compile Include="TaskSeq.Delay.Tests.fs" />
     <Compile Include="TaskSeq.Empty.Tests.fs" />
     <Compile Include="TaskSeq.ExactlyOne.Tests.fs" />
     <Compile Include="TaskSeq.Exists.Tests.fs" />
diff --git a/src/FSharp.Control.TaskSeq.Test/TaskSeq.Delay.Tests.fs b/src/FSharp.Control.TaskSeq.Test/TaskSeq.Delay.Tests.fs
new file mode 100644
index 00000000..20026528
--- /dev/null
+++ b/src/FSharp.Control.TaskSeq.Test/TaskSeq.Delay.Tests.fs
@@ -0,0 +1,54 @@
+module TaskSeq.Tests.Delay
+
+open System
+
+open Xunit
+open FsUnit.Xunit
+open FsToolkit.ErrorHandling
+
+open FSharp.Control
+open System.Collections.Generic
+
+//
+// TaskSeq.delay
+//
+
+let validateSequence ts =
+    ts
+    |> TaskSeq.toSeqCachedAsync
+    |> Task.map (Seq.map string)
+    |> Task.map (String.concat "")
+    |> Task.map (should equal "12345678910")
+
+module EmptySeq =
+    [<Theory; ClassData(typeof<TestEmptyVariants>)>]
+    let ``TaskSeq-delay with empty sequences`` variant =
+        fun () -> Gen.getEmptyVariant variant
+        |> TaskSeq.delay
+        |> verifyEmpty
+
+module Immutable =
+    [<Theory; ClassData(typeof<TestImmTaskSeq>)>]
+    let ``TaskSeq-delay`` variant =
+        fun () -> Gen.getSeqImmutable variant
+        |> TaskSeq.delay
+        |> validateSequence
+
+module SideEffect =
+    [<Fact>]
+    let ``TaskSeq-delay executes side effects`` () = task {
+        let mutable i = 0
+
+        let ts =
+            fun () -> taskSeq {
+                yield! [ 1..10 ]
+                i <- i + 1
+            }
+            |> TaskSeq.delay
+
+        do! ts |> validateSequence
+        i |> should equal 1
+        let! len = TaskSeq.length ts
+        i |> should equal 2 // re-eval of the sequence executes side effect again
+        len |> should equal 10
+    }
diff --git a/src/FSharp.Control.TaskSeq/TaskSeq.fs b/src/FSharp.Control.TaskSeq/TaskSeq.fs
index a9ac3bd9..e26ea68c 100644
--- a/src/FSharp.Control.TaskSeq/TaskSeq.fs
+++ b/src/FSharp.Control.TaskSeq/TaskSeq.fs
@@ -164,6 +164,11 @@ module TaskSeq =
     let initAsync count initializer = Internal.init (Some count) (InitActionAsync initializer)
     let initInfiniteAsync initializer = Internal.init None (InitActionAsync initializer)
 
+    let delay (generator: unit -> taskSeq<'T>) =
+        { new IAsyncEnumerable<'T> with
+            member _.GetAsyncEnumerator(ct) = generator().GetAsyncEnumerator(ct)
+        }
+
     let concat (sources: taskSeq<#taskSeq<'T>>) = taskSeq {
         for ts in sources do
             yield! (ts :> taskSeq<'T>)
diff --git a/src/FSharp.Control.TaskSeq/TaskSeq.fsi b/src/FSharp.Control.TaskSeq/TaskSeq.fsi
index 8787e63e..6c367526 100644
--- a/src/FSharp.Control.TaskSeq/TaskSeq.fsi
+++ b/src/FSharp.Control.TaskSeq/TaskSeq.fsi
@@ -39,6 +39,14 @@ module TaskSeq =
     /// </summary>
     val lengthByAsync: predicate: ('T -> #Task<bool>) -> source: taskSeq<'T> -> Task<int>
 
+    /// <summary>
+    /// Returns a task sequence that is given by the delayed specification of a task sequence.
+    /// </summary>
+    ///
+    /// <param name="generator">The generating function for the task sequence.</param>
+    /// <returns>The generated task sequence.</returns>
+    val delay: generator: (unit -> taskSeq<'T>) -> taskSeq<'T>
+
     /// <summary>
     /// Generates a new task sequence which, when iterated, will return successive elements by calling the given function
     /// with the current index, up to the given count. Each element is saved after its initialization for successive access to