浅谈 Scala 中下划线的用途

Scala 作为一门函数式编程语言，对习惯了指令式编程语言的同学来说，会不大习惯，这里除了思维方式之外，还有语法层面的，比如 underscore（下划线）就会出现在多种场合，令初学者相当疑惑，今天就来总结下 Scala 中下划线的用法。

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这里需要注意的是，以下两种写法实现的是完全不一样的功能：

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

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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中的下划线到底有多少种应用场景？

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简明教程

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