F#问题与异步工作流和try/with

Question

我正在努力整理一个简单的功能.

考虑以下定义:

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>

这Task<T>是因为它们是用其他.NET语言开发的库.

(我为了这个例子创建了这段代码)

基本上,我想在存储库服务中保存数据,然后将数据保存在通知服务中.但是如果第二个操作失败,并且包含异常,我想回滚存储库中的操作.然后有两种情况我想要调用回滚操作,第一种if notification.SaveAsync返回false,第二种if抛出异常.当然,我想编写一次调用回滚,但我找不到方法.

这是我尝试过的:

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

但不幸的是我得到了一个编译错误let! nr = ...:这个结构只能在计算表达式中使用

这是正确的方法吗？

Answer 1

问题是,当您let v = e在计算表达式中使用时,表达式e是一个普通表达式,不能包含其他异步结构.这正是这里发生的事情:

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

您可以将其转换为嵌套async块:

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