課程內(nèi)容:
讓我們創(chuàng)建兩個(gè)函數(shù):
scala> def f(s: String) = "f(" + s + ")"
f: (String)java.lang.String
scala> def g(s: String) = "g(" + s + ")"
g: (String)java.lang.String
compose
?組合其他函數(shù)形成一個(gè)新的函數(shù)?f(g(x))
scala> val fComposeG = f _ compose g _
fComposeG: (String) => java.lang.String = <function>
scala> fComposeG("yay")
res0: java.lang.String = f(g(yay))
andThen
?和?compose
很像,但是調(diào)用順序是先調(diào)用第一個(gè)函數(shù),然后調(diào)用第二個(gè),即g(f(x))
scala> val fAndThenG = f _ andThen g _
fAndThenG: (String) => java.lang.String = <function>
scala> fAndThenG("yay")
res1: java.lang.String = g(f(yay))
這是一個(gè)名為PartialFunction的函數(shù)的子類。
他們是共同組合在一起的多個(gè)PartialFunction。
對(duì)給定的輸入?yún)?shù)類型,函數(shù)可接受該類型的任何值。換句話說(shuō),一個(gè)(Int) => String
?的函數(shù)可以接收任意Int值,并返回一個(gè)字符串。
對(duì)給定的輸入?yún)?shù)類型,偏函數(shù)只能接受該類型的某些特定的值。一個(gè)定義為(Int) => String
?的偏函數(shù)可能不能接受所有Int值為輸入。
isDefinedAt
?是PartialFunction的一個(gè)方法,用來(lái)確定PartialFunction是否能接受一個(gè)給定的參數(shù)。
注意?偏函數(shù)PartialFunction
?和我們前面提到的部分應(yīng)用函數(shù)是無(wú)關(guān)的。
參考?Effective Scala 對(duì)PartialFunction的意見。
scala> val one: PartialFunction[Int, String] = { case 1 => "one" }
one: PartialFunction[Int,String] = <function1>
scala> one.isDefinedAt(1)
res0: Boolean = true
scala> one.isDefinedAt(2)
res1: Boolean = false
您可以調(diào)用一個(gè)偏函數(shù)。
scala> one(1)
res2: String = one
PartialFunctions可以使用orElse
組成新的函數(shù),得到的PartialFunction反映了是否對(duì)給定參數(shù)進(jìn)行了定義。
scala> val two: PartialFunction[Int, String] = { case 2 => "two" }
two: PartialFunction[Int,String] = <function1>
scala> val three: PartialFunction[Int, String] = { case 3 => "three" }
three: PartialFunction[Int,String] = <function1>
scala> val wildcard: PartialFunction[Int, String] = { case _ => "something else" }
wildcard: PartialFunction[Int,String] = <function1>
scala> val partial = one orElse two orElse three orElse wildcard
partial: PartialFunction[Int,String] = <function1>
scala> partial(5)
res24: String = something else
scala> partial(3)
res25: String = three
scala> partial(2)
res26: String = two
scala> partial(1)
res27: String = one
scala> partial(0)
res28: String = something else
上周我們看到一些新奇的東西。我們?cè)谕ǔ?yīng)該使用函數(shù)的地方看到了一個(gè)case語(yǔ)句。
scala> case class PhoneExt(name: String, ext: Int)
defined class PhoneExt
scala> val extensions = List(PhoneExt("steve", 100), PhoneExt("robey", 200))
extensions: List[PhoneExt] = List(PhoneExt(steve,100), PhoneExt(robey,200))
scala> extensions.filter { case PhoneExt(name, extension) => extension < 200 }
res0: List[PhoneExt] = List(PhoneExt(steve,100))
為什么這段代碼可以工作?
filter使用一個(gè)函數(shù)。在這個(gè)例子中是一個(gè)謂詞函數(shù)(PhoneExt) => Boolean。
PartialFunction是Function的子類型,所以filter也可以使用PartialFunction!
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