For is too eager

Author: dsyme

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -234,10 +234,11 @@ module AsyncSeq =
     /// For loop that iterates over a synchronous sequence (and generates
     /// all elements generated by the asynchronous body)
     member x.For(seq:seq<'T>, action:'T -> AsyncSeq<'TResult>) = 
-      let enum = seq.GetEnumerator()
-      x.TryFinally(x.While((fun () -> enum.MoveNext()), x.Delay(fun () -> 
-        action enum.Current)), (fun () -> 
-          if enum <> null then enum.Dispose() ))
+      x.Delay(fun () -> 
+          let enum = seq.GetEnumerator()
+          x.TryFinally(x.While((fun () -> enum.MoveNext()), x.Delay(fun () -> 
+            action enum.Current)), (fun () -> 
+              if enum <> null then enum.Dispose() )))
 
     /// Asynchronous for loop - for all elements from the input sequence,
     /// generate all elements produced by the body (asynchronously). See

tests/FSharp.Control.AsyncSeq.Tests/AsyncSeqTests.fs

@@ -284,4 +284,15 @@ let ``AsyncSeq.skipUntil should not skip with completed signal``() =
 [<Test>]
 let ``AsyncSeq.skipUntil should skip everything with never signal``() =
   let actual = [1;2;3;4] |> AsyncSeq.ofSeq |> AsyncSeq.skipUntil AsyncOps.never
-  Assert.True(EQ AsyncSeq.empty actual)
+  Assert.True(EQ AsyncSeq.empty actual)
+
+
+  
+[<Test>]
+let ``asyncSeq.For should delay``() =
+  let (s:seq<int>) = 
+     { new System.Collections.Generic.IEnumerable<int> with 
+           member x.GetEnumerator() = failwith "fail" 
+       interface System.Collections.IEnumerable with 
+           member x.GetEnumerator() = failwith "fail"  }
+  Assert.DoesNotThrow(fun _ -> asyncSeq.For(s, (fun v -> AsyncSeq.empty)) |> ignore)