[英]Mutually recursive F# functions, typing issue
遇到此代碼的麻煩:
let subscribe (nm : NamespaceManager) (subName : string) (desc : TopicDescription) : Async<SubscriptionDescription> =
let rec first_create () =
async {
let! exists = desc |> exists nm
if exists then return! (then_create_subscription () : Async<SubscriptionDescription>)
try
let beginCreate = nm.BeginCreateTopic : string * AsyncCallback * obj -> IAsyncResult
logger.DebugFormat("creating topic '{0}'", desc)
let! tdesc = Async.FromBeginEnd(desc.Path, beginCreate, nm.EndCreateTopic)
return! first_create ()
with | :? MessagingEntityAlreadyExistsException -> return! then_create_subscription () }
and then_create_subscription () : Async<SubscriptionDescription> =
async {
let beginCreate = nm.BeginCreateSubscription : string * string * AsyncCallback * obj -> IAsyncResult
return! Async.FromBeginEnd(desc.Path, subName, beginCreate, nm.EndCreateSubscription) }
first_create ()
在第5行,它着重then_create_subscription () : Async<SubscriptionDescription>
指出:
類型不匹配。 期望使用
Async<unit>
但給出了Async<SubscriptionDescription>
類型'unit'與類型'SubscriptionDescription
'不匹配
存在看起來像這樣:
let exists (nm : NamespaceManager ) (desc : PathBasedEntity) =
async { return! Async.FromBeginEnd(desc.Path, nm.BeginTopicExists, nm.EndTopicExists) }
我希望它創建主題,然后繼續為其創建訂閱。
有任何想法嗎?
else
之后必須有一個else
分支,否則if是一個語句,而不是一個表達式。 其他不是隱式的。
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