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Asynchronous computation with callback
f#.netasynchronous

Hi,

I know there is a high-level Async module in F#. But suppose I still want to program with the .Net BCL, what is the supposed way to register an asynchronous computation with callback?

For an example, I came up with the following code (forgive the stupidness in the code, I'm a .net newbie): the simple logic is just start a asynchronous computation "processor", and register with it a callback "tardeGenerated" just to print the result and cleanup the invocation.

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open System.Runtime.Remoting.Messaging

type Order() = class end;;
type Trade() = class end;;

let newOrder = Order();;
let processor order = [|Trade()|]

let processOrder = new System.Converter<_,_>(processor);;

let tradeGenerated (result:System.IAsyncResult) =
  let r = result :?> AsyncResult in
  let processOrder = r.AsyncDelegate :?> System.Converter<_,_> in
  let trades : Trade [] = processOrder.EndInvoke result in
  printfn "Total Trade Generated : %d" trades.Length;;

processOrder.BeginInvoke(newOrder, new System.AsyncCallback(tradeGenerated), processOrder);;
read_line();;

It compiled, then got segment falt when executed. Could any one helps? Thanks!

.

In the definition of tradeGenerated, r.AsyncDelegate will have the type Converter<Order,Trade[]> and you cast it to Converter<_,_>, which can be fine. The underscores mean that F# should infer the actual values of the type arguments used to instantiate Converter. From the next line F# can work out that the second argument must be Trade[]. However processOrder is never used in such a way that forces the first argument to be anything in particular. So from lack of evidence F# defaults the value of this type argument to be Object.

Hence the code effectively has the line

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let processOrder = r.AsyncDelegate :?> System.Converter<Object,Trade[]>

This cast is invalid at run-time which is why you get the invalid cast class exception.

To fix the problem you must specify the value of the first type argument explicitly because it is not possible for the compiler to infer it. So the line will become

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let processOrder = r.AsyncDelegate :?> System.Converter<Order,_>

As a rule of thumb, I would avoid using underscores in types involved with casting. Type casting is an messy OO operation and it often doesn't play well with functional-style type inference.

By on 11/8/2007 9:42 PM ()

Hi,

Thanks you! You're totally right!

Hence the code effectively has the line

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let processOrder = r.AsyncDelegate :?> System.Converter<Object,Trade[]>

This cast is invalid at run-time which is why you get the invalid cast class exception.

So you're saying that the actual behavior of a casting depends on the types? This is really unexpected for a OCaml refugee, :(

By on 11/9/2007 1:25 AM ()

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