"=>" 在scala 中的用法

例一：《Spark大数据处理技术》程序2-25

val compareElement:((String,Int),(String,Int)) => (String,Int) = (val1,val2) => {

if (val1._2 >= val2._2){

?val1

} else {

?val2

}

}

注释：

1、用val定义函数

2、compareElement函数是匿名函数

3、((String,Int)

compareElement函数的函数类型声明，左边是输入参数类型，右边是方法返回值类型

4、(val1,val2) => {......}

左边是参数，右边是函数实现体

例二：

网址：http://zhouchaofei2010.iteye.com/blog/2251731

目前知道的基本和匿名函数定义和函数类型声明有关，以及在case语句中的使用

?

不过stackoverflow?上关于这方面的讨论挺多的，故在此记录一下

比如来自?http://stackoverflow.com/questions/6951895/what-does-and-mean-in-scala?的一位的回复：

?

=>?has several meanings in Scala,all related to its mathematical meaning as implication.

• In a value,it introduces a function literal,or?lambda. e.g. the bit inside the curly braces in?List(1,2,3).map { (x: Int) => x * 2 }（匿名函数定义）

• In a type,with symbols on both sides of the arrow (e.g.?A => T,?(A,B) => T,B,C) => T,etc.) it's sugar for?Function<n>[A[,...],T],that is,a function that takes parameters of type?A[,B...],and returns a value of type?T.（函数类型定义）

  • Empty parens on the left hand side (e.g.?() => T) indicate that the function takes no parameters (also sometimes called a "thunk");

  • Empty parens on the right hand side denote that it returns?()—the sole value of type?Unit,whose name can also be written?()—confused yet? :)

    A function that returns Unit is also known as a?procedure,normally a method that's called only for its side effect.

• In the type declaration for a method or function parameter,with no symbol on the left hand side (e.g.?def f(param: => T)) it's a "by-name parameter",meaning that is evaluated every time it's used within the body of the function,and not before. Ordinary "by-value" parameters are evaluated before entry into the function/method.（函数类型定义）

• In a?case?clause,they separate the pattern (and optional guard) from the result expression,e.g.?case x => y.（case 语句）

http://stackoverflow.com/questions/6951895/what-does-and-mean-in-scala

http://stackoverflow.com/questions/7888944/scala-punct