F# issue with async workflow and try/with

Question

I am having a headache trying to put together a simple functionality.

Consider the following definitions:

type Entity = {Id:int;Data:string}
type IRepository =
 abstract member SaveAsync: array<Entity> -> Task<bool>
 abstract member RollBackAsync: array<Entity> -> Task<bool>
type INotification =
 abstract member SaveAsync: array<Entity> -> Task<bool>

The use Task<T> because they are libraries developed in other .NET languages.

(I have created this code for the sake of the example)

Basically, I want to save data in the repository service, and then save the data in the notification service. But if this second operation fails, and that includes exceptions, I want to rollback the operation in the repository. Then there are two situations where I would want to call the rollback operation, the first if notification.SaveAsync returns false, and the second if it throws an exception. And of course, I would like to code that call to rollback once, but I cannot find the way.

This is what I have tried:

type Controller(repository:IRepository, notification:INotification) =
 let saveEntities entities:Async<bool> = async{
 let! repoResult = Async.AwaitTask <| repository.SaveAsync(entities)
 if(not repoResult) then
 return false
 else 
 let notifResult =
 try
 let! nr = Async.AwaitTask <| notification.SaveAsync(entities)
 nr
 with
 | _-> false
 if(not notifResult) then
 let forget = Async.AwaitTask <| repository.RollBackAsync(entities)
 return false
 else
 return true
 }
 member self.SaveEntitiesAsync(entities:array<Entity>) =
 Async.StartAsTask <| saveEntities entities

But unfortunately I get a compiler error on the let! nr = ... saying: This construct may only be used within computation expressions

Which would be the right way of doing this?

Answer 1

The problem is that when you use let v = e in computation expressions, the expression e is an ordinary expression that cannot contain further asynchronous constructs. That's exactly what happens here:

let notifResult =
 try
 let! nr = Async.AwaitTask <| notification.SaveAsync(entities)
 nr
 with _-> false

You can turn this into a nested async block:

let! notifResult = async {
 try
 let! nr = Async.AwaitTask <| notification.SaveAsync(entities) 
 return nr
 with _-> return false }

• vtortola: That is interesting, I didn't notice you can nest them. Thanks!