Merge pull request #13 from fsprojects/fix-6

For is too eager

Author: dsyme

src/FSharp.Control.AsyncSeq/AsyncSeq.fs

@@ -207,10 +207,11 @@ module AsyncSeq =
         if box resource <> null then resource.Dispose())
 
     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() )))
 
     member x.For (seq:AsyncSeq<'T>, action:'T -> AsyncSeq<'TResult>) = 
       collect action seq

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

@@ -432,3 +432,12 @@ let ``AsyncSeq.getIterator should work``() =
         | Some _ -> Assert.Fail("expected None")
 
 
+  
+[<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)