AsyncSeq.concatSeq

Author: eulerfx

src/FSharpx.Async/AsyncSeq.fs

@@ -496,6 +496,16 @@ module AsyncSeq =
       yield! unfoldAsync f s
     | None -> () }
 
+  /// Flattens an AsyncSeq of sequences.
+  let rec concatSeq (input:AsyncSeq<#seq<'T>>) : AsyncSeq<'T> = asyncSeq {
+    let! v = input
+    match v with
+    | Nil -> ()
+    | Cons (hd, tl) ->
+      for item in hd do 
+        yield item
+      yield! concatSeq tl }
+
 [<AutoOpen>]
 module AsyncSeqExtensions = 
   /// Builds an asynchronou sequence using the computation builder syntax

tests/FSharpx.Async.Tests/AsyncSeqTests.fs

@@ -43,6 +43,24 @@ let ``toList should collect the results into an array``() =
   Assert.True(([1;2;3] = a))
 
 
+[<Test>]
+let ``concatSeq should flatten a sequence``() =
+  
+  let s = asyncSeq {
+    yield [1;2]
+    yield [3;4]
+  }
+  
+  let s = 
+    s
+    |> AsyncSeq.concatSeq
+    |> AsyncSeq.toList
+    |> Async.RunSynchronously
+
+  Assert.True(([1;2;3;4] = s))
+
+
+
 [<Test>]
 let ``unfoldAsync should generate a sequence``() =