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I am surprised by a stack overflow in my async-based program. I suspect the main problem is with the following function, which is supposed to compose two async computations to execute in parallel and wait for both to finish: let ( <|> ) (a: Async<unit>) (b: Async<unit>) = async { let! x = Async.StartChild a let! y = Async.StartChild b do! x do! y } With this defined, I have the following mapReduce program that attempts to exploit parallelism in both the map and the reduce part. Informally, the idea is to spark N mappers and N-1 reducers using a shared channel, wait for them to finish, and read the result from the channel. I had my own Channel implementation, here replaced by a ConcurrentBag for shorter code (the problem affects both): let mapReduce (map : 'T1 -> Async<'T2>) (reduce : 'T2 -> 'T2 -> Async<'T2>) (input : seq<'T1>) : Async<'T2> = let bag = System.Collections.Concurrent.ConcurrentBag() let rec read () = async { match bag.TryTake() with | true, value -> return value | _ -> do! Async.Sleep 100 return! read () } let write x = bag.Add x async.Return () let reducer = async { let! x = read () let! y = read () let! r = reduce x y return bag.Add r } let work = input |> Seq.map (fun x -> async.Bind(map x, write)) |> Seq.reduce (fun m1 m2 -> m1 <|> m2 <|> reducer) async { do! work return! read () } Now the following basic test starts to throw StackOverflowException on n=10000: let test n = let m

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