Future.onSuccess versus Future.map

[Oliver Ruebenacker]

     Hello,

  Suppose I have:

  val fut: Future[A] = { ...}

  val f: A => B = { ... }

  Can I always replace this:

  fut.onSuccess { case a: A => f(a) }

  by this?

  fut.map(f(a))

  Thanks!

     Best, Oliver

[Joheinz]

No. unless you are not interested in B and f is full of side effects as on success returns unit and map a Future[B].

Markus

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[√iktor Ҡlang]

Hi Oliver,

`onSuccess` is just an alias for `foreach` to harmonize with `onFailure`. (Both onSuccess and onFailure is scheduled to be deprecated for 2.12 because they are redundant due to `foreach` and `failed.foreach`)

So the question then becomes: Can I always replace `foreach` with `map`?

Interesting!

Well there are a couple of different things to consider:

1) `map` returns the result of the application of the function passed in as a new `Future` and `foreach` returns `Unit`.

2) Since `foreach` does not return anything—any thrown exceptions will be "invisible" to the caller, since they are executed asynchronously, which means that they will be passed to the `ExecutionContext`s `reportfailure`-method whereas with `map` the returned `Future` will contain any non fatal exceptions thrown by the application of the function passed in.

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Cheers,

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