refactor(Batching)!: Relax arbitrary arrays constraint

Author: bartelink

src/Equinox.Core/Batching.fs

@@ -18,16 +18,16 @@ type internal AsyncBatch<'Req, 'Res>() =
             // sadly there's no way to detect without a try/catch
             try queue.TryAdd(item)
             with :? InvalidOperationException -> false
-    let mutable attempt = Unchecked.defaultof<Lazy<Task<'Res[]>>>
+    let mutable attempt = Unchecked.defaultof<Lazy<Task<'Res>>>
 
     /// Attempt to add a request to the flight
     /// Succeeds during linger interval (which commences when the first caller triggers the workflow via AwaitResult)
     /// Fails if this flight has closed (caller should initialize a fresh Batch, potentially holding off until the current attempt completes)
-    member _.TryAdd(req, dispatch: Func<'Req[], CancellationToken, Task<'Res[]>>, lingerMs: int, limiter: System.Threading.SemaphoreSlim voption, ct) =
+    member _.TryAdd(req, dispatch: Func<'Req[], CancellationToken, Task<'Res>>, lingerMs: int, limiter: System.Threading.SemaphoreSlim voption, ct) =
         if not (tryEnqueue req) then false else
 
-        // Prepare a new instance, with cancellation under our control (it won't start until the Force triggers it though)
-        let newInstance: Lazy<Task<'Res[]>> = lazy task {
+        // Prepare a new instance, with cancellation under our control (it won't start until the .Value triggers it though)
+        let newInstance: Lazy<Task<'Res>> = lazy task {
             do! Task.Delay(lingerMs, ct)
             match limiter with ValueNone -> () | ValueSome s -> do! s.WaitAsync(ct)
             try queue.CompleteAdding()
@@ -45,12 +45,12 @@ type internal AsyncBatch<'Req, 'Res>() =
 /// Requests are added to pending batch during the wait period, which consists of two phases:
 /// 1. a defined linger period (min 1ms)
 /// 2. (optionally) a wait to acquire capacity on a limiter semaphore (e.g. one might have a limit on concurrent dispatches across a pool)
-type Batcher<'Req, 'Res> private (tryInclude: Func<AsyncBatch<_, _>, 'Req, CancellationToken, bool>) =
+type Batcher<'Req, 'Res> private (tryInclude: Func<AsyncBatch<'Req, 'Res>, 'Req, CancellationToken, bool>) =
     let mutable cell = AsyncBatch<'Req, 'Res>()
-    new(dispatch: Func<'Req[], CancellationToken, Task<'Res[]>>, lingerMs, limiter) =
+    new(dispatch: Func<'Req[], CancellationToken, Task<'Res>>, lingerMs, limiter) =
         if lingerMs < 1 then invalidArg (nameof(lingerMs)) "Minimum linger period is 1ms" // concurrent waiters need to add work to the batch across their threads
         Batcher(fun cell req ct -> cell.TryAdd(req, dispatch, lingerMs, limiter, ct = ct))
-    new(dispatch: 'Req[] -> Async<'Res[]>, ?linger : TimeSpan,
+    new(dispatch: 'Req[] -> Async<'Res>, ?linger: TimeSpan,
         // Extends the linger phase to include a period during which we await capacity on an externally managed Semaphore
         // The Batcher doesn't care, but a typical use is to enable limiting the number of concurrent in-flight dispatches
         ?limiter) =
@@ -99,7 +99,7 @@ type BatcherCache<'Id, 'Entry>(cache: Cache<'Entry>, toKey: Func<'Id, string>, c
         let mapKey = Func<'Id, string>(fun id -> "$Batcher-" + string id)
         BatcherCache(Cache cache, mapKey, createEntry, ?cacheWindow = cacheWindow)
 
-    member _.GetOrAdd(id : 'Id) : 'Entry =
+    member _.GetOrAdd(id: 'Id) : 'Entry =
         // Optimise for low allocations on happy path
         let key = toKey.Invoke(id)
         match cache.TryGet key with

tests/Equinox.Core.Tests/BatchingTests.fs

@@ -12,7 +12,7 @@ open Xunit
 let ``Batcher correctness`` () = async {
     let mutable batches = 0
     let mutable active = 0
-    let dispatch (reqs : int[]) = async {
+    let dispatch (reqs: int[]) = async {
         let concurrency = Interlocked.Increment &active
         1 =! concurrency
         Interlocked.Increment &batches |> ignore
@@ -23,15 +23,15 @@ let ``Batcher correctness`` () = async {
     }
     let cell = Batcher(dispatch, linger = TimeSpan.FromMilliseconds 40)
     let! results = [1 .. 100] |> Seq.map cell.Execute |> Async.Parallel
-    test <@ set (Seq.collect id results) = set [1 .. 100] @>
+    test <@ set (Seq.concat results) = set [1 .. 100] @>
     // Linger of 40ms makes this tend strongly to only be 1 batch, but no guarantees
     test <@ 1 <= batches && batches < 3 @>
 }
 
 [<Property>]
 let ``Batcher error handling`` shouldFail = async {
     let fails = ConcurrentBag() // Could be a ResizeArray per spec, but this removes all doubt
-    let dispatch (reqs : int[]) = async {
+    let dispatch (reqs: int[]) = async {
         if shouldFail () then
             reqs |> Seq.iter fails.Add
             failwith $"failing %A{reqs}"
@@ -43,7 +43,7 @@ let ``Batcher error handling`` shouldFail = async {
     let oks = results |> Array.choose (function Choice1Of2 r -> Some r | _ -> None)
     // Note extraneous exceptions we encounter (e.g. if we remove the catch in TryAdd)
     let cancels = results |> Array.choose (function Choice2Of2 e when not (e.Message.Contains "failing") -> Some e | _ -> None)
-    let inputs, outputs = set input, set (Seq.collect id oks |> Seq.append fails)
+    let inputs, outputs = set input, set (Seq.concat oks |> Seq.append fails)
     test <@ inputs.Count = outputs.Count
             && Array.isEmpty cancels
             && inputs = outputs @>