向类型构造函数传递下划线：Future[_]返回类型含义问询

Great question! Let's break down exactly what this return type means in Scala:

• First off, yes—def foo(): Future[_] absolutely means this method returns an instance of Future. The _ here is a wildcard type, and its core meaning matches your initial guess: we don't care (or don't need to use) the specific type of the value that the Future will eventually compute.

• To put it concretely: when you call foo(), you can still interact with the Future as you normally would—wait for it to complete, attach success/failure callbacks, chain it with other async operations, etc. The only catch is that if you do access the Future's result, the compiler will treat that result as Any (since it has no info about the actual underlying type).

• This pattern is commonly used in scenarios where you only care about the asynchronous execution flow, not the operation's output. For example:

  • Triggering a background database write where you just need to confirm it finished (successfully or not) but don't need a return value from the write.
  • Starting an async job to send a notification, where the notification itself is the end goal and you don't need to process a result from it.

• It's also useful to distinguish this from Future[Unit]: Future[Unit] explicitly states the Future will complete with a Unit value (no meaningful result), whereas Future[_] means the result could be any type—we just don't intend to use it.

内容的提问来源于stack exchange，提问作者user3248346