[英]Currying in Scala: Multiple parameters in a function including an anonymous function of type ( => A)
这两个代码块有什么区别?
def measure[A](histogram: Histogram)(thunk: ⇒ A): A = {
val start = RelativeNanoTimestamp.now
try thunk finally {
val latency = NanoInterval.since(start).nanos
histogram.record(latency)
}
def measure[A](histogram: Histogram, thunk: ⇒ A): A = {
val start = RelativeNanoTimestamp.now
try thunk finally {
val latency = NanoInterval.since(start).nanos
histogram.record(latency)
}
=> A
是一个惰性参数。 它将在函数中引用时进行评估。 它可以是产生值的函数,也可以只是一个值。
单例和多参数列表之间的主要区别如下所示:
def measure[A](histogram: Histogram)(thunk: ⇒ A)
def measure[A](histogram: Histogram, thunk: ⇒ A)
(不考虑implicits和类型推断)是你如何应用函数:
scala> def f[A](i: Int)(p: => A): A = { p }
f: [A](i: Int)(p: => A)A
scala> f(1)(2)
res0: Int = 2
scala> f(1){ println("something") }
something
scala> f(1){
| println("test")
| }
test
scala> def f2[A](i: Int, p: => A): A = { p }
f2: [A](i: Int, p: => A)A
scala> f2(1, 2)
res4: Int = 2
scala> f2(1, println("test"))
test
scala> f2(1, { println("test") })
test
看到带有多个参数列表的f
允许我们用这种风格写: f(...){...}
,而如果你有多行代码块作为第二个参数, f2
就不那么优雅了: f(..., {...})
。
此外,如果你做了很多currying /部分应用,那么f2
比f
更容易处理:
scala> val f_withFirstArg = f(1) _
f_withFirstArg: (=> Nothing) => Nothing = <function1>
scala> val f2_withFirstArg = f2(1, _)
<console>:8: error: missing parameter type for expanded function ((x$1) => f2(1, x$1))
val f2_withFirstArg = f2(1, _)
^
我们必须明确指定参数类型,类型推断失败简短:
scala> val f2_withFirstArg = f2(1, _: String)
f2_withFirstArg: String => String = <function1>
如果你想获得技术,那么值得指出它们实际上是不同的类型:
scala> :type f _
Int => ((=> Nothing) => Nothing)
scala> :type f2 _
(Int, => Nothing) => Nothing
f
是一个函数,它接受一个Int
并返回另一个接受类型A
并将产生类型A
函数。 f2
是一个带2个参数的函数: Int
和A
并返回A
这真的取决于你的代码。 如果由于类型推断缺点需要进行大量部分应用或需要较少的注释,则使用多个参数列表。 否则,不需要过度复杂化并使用常规的单个参数列表函数。
最后,只要有意义,你总是可以从一种类型的函数转换为另一种函数:
scala> f2 _
res13: (Int, => Nothing) => Nothing = <function2>
scala> f2 _ curried
warning: there were 1 feature warning(s); re-run with -feature for details
res14: Int => ((=> Nothing) => Nothing) = <function1>
scala> f _ curried
<console>:9: error: value curried is not a member of Int => ((=> Nothing) => Nothing)
f _ curried
^
scala> f _ tupled
<console>:9: error: value tupled is not a member of Int => ((=> Nothing) => Nothing)
f _ tupled
^
scala> f2 _ tupled
warning: there were 1 feature warning(s); re-run with -feature for details
res17: ((Int, => Nothing)) => Nothing = <function1>
请注意,我们不能让f
令行禁止,因为它已经是了。 我们不能让f
tupled因为它不会改变任何事情。 但是,我们可以使用curried
将f2
转换为f
:
scala> :type f _
Int => ((=> Nothing) => Nothing)
scala> :type f2 _ curried _
Int => ((=> Nothing) => Nothing)
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