Scala 作為一門函數(shù)式編程語(yǔ)言，對(duì)習(xí)慣了指令式編程語(yǔ)言的同學(xué)來(lái)說(shuō)，會(huì)不大習(xí)慣，這里除了思維方式之外，還有語(yǔ)法層面的，比如?underscore（下劃線）就會(huì)出現(xiàn)在多種場(chǎng)合，令初學(xué)者相當(dāng)疑惑，今天就來(lái)總結(jié)下 Scala 中下劃線的用法。

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1、存在性類型：Existential?types def?foo(l:?List[Option[_]])?=?... 2、高階類型參數(shù)：Higher?kinded?type?parameters case?class?A[K[_],T](a:?K[T]) 3、臨時(shí)變量：Ignored?variables val?_?=?5 4、臨時(shí)參數(shù)：Ignored?parameters List(1,?2,?3)?foreach?{?_?=>?println("Hi")?} 5、通配模式：Wildcard?patterns Some(5)?match?{?case?Some(_)?=>?println("Yes")?} val?(a,?_)?=?(1,?2) for?(_?<-?1?to?10) 6、通配導(dǎo)入：Wildcard?imports import?java.util._ 7、隱藏導(dǎo)入：Hiding?imports import?java.util.{ArrayList?=>?_,?_} 8、連接字母和標(biāo)點(diǎn)符號(hào)：Joining?letters?to?punctuation def?bang_!(x:?Int)?=?5 9、占位符語(yǔ)法：Placeholder?syntax List(1,?2,?3)?map?(_?+?2) _?+?_??? 10、偏應(yīng)用函數(shù)：Partially?applied?functions List(1,?2,?3)?foreach?println?_ 11、初始化默認(rèn)值：default?value var?i:?Int?=?_ 12、訪問(wèn)元組：tuple?getters t._2? 13、參數(shù)序列：parameters?Sequence? _*作為一個(gè)整體，告訴編譯器你希望將某個(gè)參數(shù)當(dāng)作參數(shù)序列處理！例如val?s?=?sum(1?to?5:_*)就是將1?to?5當(dāng)作參數(shù)序列處理。

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這里需要注意的是，以下兩種寫(xiě)法實(shí)現(xiàn)的是完全不一樣的功能：

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foo?_???????????????//?Eta?expansion?of?method?into?method?value foo(_)??????????????//?Partial?function?application

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Example showing why foo(_) and foo _ are different:

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trait?PlaceholderExample?{ ??def?process[A](f:?A?=>?Unit) ??val?set:?Set[_?=>?Unit] ??set.foreach(process?_)?//?Error? ??set.foreach(process(_))?//?No?Error }

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In the first case, process _ represents a method; Scala takes the polymorphic method and attempts to make it monomorphic by filling in the type parameter, but realizes that there is no type that can be filled in for A that will give the type (_ => Unit) => ? (Existential _ is not a type).

In the second case, process(_) is a lambda; when writing a lambda with no explicit argument type, Scala infers the type from the argument that foreach expects, and _ => Unit is a type (whereas just plain _ isn't), so it can be substituted and inferred.

This may well be the trickiest gotcha in Scala I have ever encountered.

Refer：

[1]?What are all the uses of an underscore in Scala?

http://stackoverflow.com/questions/8000903/what-are-all-the-uses-of-an-underscore-in-scala

[2]?Scala punctuation (AKA symbols and operators)

http://stackoverflow.com/questions/7888944/scala-punctuation-aka-symbols-and-operators/7890032#7890032

[3]?Scala中的下劃線到底有多少種應(yīng)用場(chǎng)景？

http://www.zhihu.com/question/21622725

[4]?Strange type mismatch when using member access instead of extractor

http://stackoverflow.com/questions/9610736/strange-type-mismatch-when-using-member-access-instead-of-extractor/9610961

[5]?Scala簡(jiǎn)明教程

http://colobu.com/2015/01/14/Scala-Quick-Start-for-Java-Programmers/

轉(zhuǎn)載于:https://www.cnblogs.com/tonychai/p/4546264.html

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