我正在向远程服务器发出请求,有时由于网络不可靠而请求失败。如果失败,我希望请求重复,但n
次数最多。如果我使用命令式语言,我会将请求发送代码放在一个 while 循环中,但我想以一种功能性的方式来做。
我为此目的编写了助手:
/** Repeatedly executes function `f`
* while predicate `p` holds
* but no more than `nTries` times.
*/
def repeatWhile[A](f: => A)(p: A => Boolean)(nTries: Int): Option[A] =
if (nTries == 0) {
None
} else {
f match {
case a if p(a) => repeatWhile(f)(p)(nTries - 1)
case a => Some(a)
}
}
并像这样使用它:
// Emulating unreliable connection
var n = 0
def receive(): Option[String] =
if (n < 4) {
n += 1
println("No result...")
None
} else {
println("Result!")
Some("Result")
}
// Repeated call
val result = repeatWhile(receive)(!_.isDefined)(10)
wherereceive
是用于测试目的的愚蠢功能。receive
此代码在最终成功之前进行了 4 次调用Some(Result)
:
No result...
No result...
No result...
No result...
Result!
我的repeatWhile
作品很好,但我想重新发明轮子。我正在学习函数式编程,想知道我的问题是否有简单/标准的解决方案。
Ps我已经定义了更多的助手,也许他们已经在语言/标准库中了?
/** Repeatedly executes function `f`
* while predicated `p` not holds
* but no more than `nTries` times.
*/
def repeatWhileNot[A](f: => A)(p: A => Boolean)(nTries:Int): Option[A] =
repeatWhile(f)(!p(_))(nTries)
/** Repeatedly executes function `f`
* while it returns None
* but no more than `nTries` times.
*/
def repeatWhileNone[A](f: => Option[A])(nTries:Int): Option[A] =
repeatWhileNot(f)(_.isDefined)(nTries).getOrElse(None)