type MaybeBuilder() = member this.Bind(m, f) = match m with | None -> None | Some x -> printfn "Got `%A`" x f x member this.Return(x) = printfn "Returning `Some(%A)`" x Some x let maybe_worker = new MaybeBuilder() // MaybeBuilder was defined so that in this context, `let!` // unwraps the value immediately. let result1 = maybe_worker { // x is 3 let! x = Some 3 // y is 4 let! y = Some 4 // z is 5 let! z = Some 5 // returns the computation rewrapped return x * y + z } // This one is different. This entire construct desugars to a // functional construct from Option -> Option -> Option -> return // and so the None will be passed to the end result let result2 = maybe_worker { let! x = None let! y = Some 4 let! z = Some 5 return x * y + z } printfn "%A" result1 // Results in Some 17 printfn "%A" result2 // <null> is None