Merge pull request #200 from fsprojects/daily-perf-improver/optimize-iterasync-performance

Daily Perf Improver: Optimize iterAsync and iteriAsync for better performance

Author: dsyme

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -734,23 +734,64 @@ module AsyncSeq =
                                 dispose e
                             | _ -> () } }
 
+  // Optimized iterAsync implementation to reduce allocations
+  type internal OptimizedIterAsyncEnumerator<'T>(enumerator: IAsyncEnumerator<'T>, f: 'T -> Async<unit>) =
+    let mutable disposed = false
+    
+    member _.IterateAsync() =
+      let rec loop() = async {
+        let! next = enumerator.MoveNext()
+        match next with
+        | Some value ->
+            do! f value
+            return! loop()
+        | None -> return ()
+      }
+      loop()
+    
+    interface IDisposable with
+      member _.Dispose() =
+        if not disposed then
+          disposed <- true
+          enumerator.Dispose()
+
+  // Optimized iteriAsync implementation with direct tail recursion  
+  type internal OptimizedIteriAsyncEnumerator<'T>(enumerator: IAsyncEnumerator<'T>, f: int -> 'T -> Async<unit>) =
+    let mutable disposed = false
+    
+    member _.IterateAsync() =
+      let rec loop count = async {
+        let! next = enumerator.MoveNext()
+        match next with
+        | Some value ->
+            do! f count value
+            return! loop (count + 1)
+        | None -> return ()
+      }
+      loop 0
+    
+    interface IDisposable with
+      member _.Dispose() =
+        if not disposed then
+          disposed <- true
+          enumerator.Dispose()
+
   let iteriAsync f (source : AsyncSeq<_>) =
       async {
-          use ie = source.GetEnumerator()
-          let count = ref 0
-          let! move = ie.MoveNext()
-          let b = ref move
-          while b.Value.IsSome do
-              do! f !count b.Value.Value
-              let! moven = ie.MoveNext()
-              do incr count
-                 b := moven
+          let enum = source.GetEnumerator()
+          use optimizer = new OptimizedIteriAsyncEnumerator<_>(enum, f)
+          return! optimizer.IterateAsync()
       }
 
   let iterAsync (f: 'T -> Async<unit>) (source: AsyncSeq<'T>)  =
     match source with
     | :? AsyncSeqOp<'T> as source -> source.IterAsync f
-    | _ -> iteriAsync (fun i x -> f x) source
+    | _ -> 
+        async {
+          let enum = source.GetEnumerator()
+          use optimizer = new OptimizedIterAsyncEnumerator<_>(enum, f)
+          return! optimizer.IterateAsync()
+        }
 
   let iteri (f: int -> 'T -> unit) (inp: AsyncSeq<'T>)  = iteriAsync (fun i x -> async.Return (f i x)) inp