It has several meanings in Scala, all related to its mathematical meaning as implication.
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In a value, it introduces a function literal（通译为匿名函数，有时候也叫函数显式声明，函数字面量）, or lambda（参考lambda表达式的文章，其实也是匿名函数）.e.g. the bit inside the curly braces in：
List(1,2,3).map { (x: Int) => x * 2 }
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In a type, with symbols on both sides of the arrow (e.g.
A => T,(A,B) => T,(A,B,C) => T, etc.) it's sugar（syntactic sugar语法糖） forFunction<n>[A[,B,...],T], that is, a function that takes parameters of typeA[,B...], and returns a value of typeT.（语法糖通过更简洁的语法达到目的，直接把所需要的参数、类型、函数最简化，然后把解析的工作交给编译器来完成，这步称为去糖化。例如，(A,B)=>T，包含了函数，参数以及类型，实际上是一个匿名函数，func(A,B,T)或者func(A T,B T)）。
=> is syntactic sugar for creating instances of functions. Recall that every function in scala is an instance of a class.
For example, the type Int => String, is equivalent to the type Function1[Int,String] i.e. a function that takes an argument of type Int and returns a String.
scala>
val f: Function1[Int,String] = myInt => "my int: "+myInt.toString
f: (Int) => String = <function1>
scala> f(0)
res0: String = my int: 0
scala>
val f2: Int => String = myInt => "my int v2: "+myInt.toString
f2: (Int) => String = <function1>
scala> f2(1)
res1: String = my int v2: 1
scala>
val f2: Function2[Int,Int,String] = (myInt1,myInt2) => "This is my function to transfer " + myInt1 + " and " + myInt2 + " as a string component."
f2: (Int, Int) => String = <function2>
scala> f2(1,2)
res6: String = This is my function to transfer 1 and 2 as a string component.
scala>
val f22:(Int,Int)=>String = (myInt1,myInt2) => "This is my function to transfer " + myInt1 + " and " + myInt2 + " as a string component."
f22: (Int, Int) => String = <function2>
scala> f22(2,4)
res7: String = This is my function to transfer 2 and 4 as a string component.
Here myInt is binded to the argument value passed to f and f2.
() => T is the type of a function that takes no arguments and returns a T. It is equivalent to Function0[T]. () is called a zero parameter list I believe.
scala>
val f: () => Unit = () => { println("x")}
f: () => Unit = <function0>
scala> f()
x
scala> val f2: Function0[Unit] = () => println("x2")
f: () => Unit = <function0>
scala> f2()
x2
f: () => Unit = <function0>
scala> f()
x
scala> val f2: Function0[Unit] = () => println("x2")
f: () => Unit = <function0>
scala> f2()
x2
As the most simplified answer, you can substitute whatever is on the left-hand side of => with the word "LEFT" and whatever is on the right-hand side with the word "RIGHT".
Then, the meaning of "LEFT => RIGHT" becomes:
Take LEFT then do RIGHT.
This means that if you have a "()=>" that you can take nothing (that is, no parameters) and then do whatever is on the right-hand side.
This is the most common meaning.
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Empty parens on the left hand side (e.g.
() => T) indicate that the function takes no parameters (also sometimes called a "thunk");
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定时器的示例程序还可以做一些改进。首先，timeFlies 函数只被用过一次，也就是当做回调函数传给 oncePerSecond 的时候，对于这种函数，在用到的时候即时构造更合理，因为可以省去定义和命名的麻烦，在 Scala 中，这样的函数称为匿名函数（anonymousfunctions），也就是没有名字的函数。使用匿名函数代替 timeFlies 函数后的程序代码如下：
object TimerAnonymous {
def oncePerSecond(callback: () => Unit) {
<span style="white-space:pre"> </span>while (true) { callback(); Thread sleep 1000 }
}
def main(args: Array[String]) {
<span style="white-space:pre"> </span>oncePerSecond(() =>
<span style="white-space:pre"> </span>println("time flies like an arrow..."))
}
}
代码中的右箭头‘=>’表明程序中存在一个匿名函数，箭头左边是匿名函数的参数列表，右边是函数体。在本例中，参数列表为空(箭头左边是一对空括号)，而函数体和改造前定义的 timeFlies 函数保持一致。
函数求值有两种策略，一种是传值调用，一种是传名调用，其中编译器的"传名调用"实现，往往是将参数放到一个临时函数之中，再将这个临时函数传入函数体。这个临时函数就叫做 Thunk 函数，也称为柯里化。
function f(m){
return m * 2;
}
f(x + 5);
// 等同于
var thunk = function () {
return x + 5;
};
function f(thunk){
return thunk() * 2;
}
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Empty parens on the right hand side denote that it returns
()— the sole value of typeUnit, 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.
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Scala对于不返回值的函数有特殊的表示法。如果函数体包含在花括号当中但没有前面的=号，那么返回值类型就是Unit。这样的函数被称做过程（procedure）。过程不返回值，我们调用它仅仅是为了它的副作用。例如：
scala> def box(s: String) {
| var border = "-" * s.length + "--\n"
| println(border + "|" + s + "|\n" + border)
| }
box: (s: String)Unit
scala> box("Hello")
-------
|Hello|
-------
| var border = "-" * s.length + "--\n"
| println(border + "|" + s + "|\n" + border)
| }
box: (s: String)Unit
scala> box("Hello")
-------
|Hello|
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关于side effect的翻译
点击了一个按钮，用户所期待的行为不叫“side effects”，用户没有期待的行为才叫“side effects”。
函数一般认为是没有状态的，每次用相同的输入调用时应当给出相同的输出，且不会影响程序的其它部分。如果函数对程序的状态有影响，这种影响就叫作“side effects”。
总之，“side effects”指的就是那些本不应有或者用户意料之外的作用，翻译成“副作用”我觉得挺恰当的。它可以看成医学上“副作用”一词的合理引申。
举个简单的例子好了：
int counter = 0;
// with side effectint incCounter() {
counter += 1;
return counter;}
// without side effectint incNumber(int m) {
return m + 1;}
// with side effectint incCounter() {
counter += 1;
return counter;}
// without side effectint incNumber(int m) {
return m + 1;}
incCounter 修改了它 scope 以外的东西，它跑过一遍之后会修改外部环境，我们讲这叫 side effect。intNumber 跑完之后本来什么样还是什么样，这就叫没有 side effects。翻成「副作用」没有问题。
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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"（传名参数，或者翻译为叫名参数）, meaning that is evaluated every time it's used within the body of the function, and not before. Ordinary "by-value" parameters （传值参数）are evaluated before entry into the function/method.
By-Value参数，我们都比较熟悉，参数在进入函数前就进行计算，最后传入的最终的计算结果。
By-Name参数，表示参数在进入函数后，每次在函数体内调用的时候才会计算。
def myByName1(param: () => Boolean): Unit = {
println("start")
println(param) //println(param.apply())也可以
println("end")
}
//要实现传名函数，要定义的类型开始于“=>”，而不是“() =>”。
//myByName1{7>9}不会有效，因为缺少“() =>”
myByName1{() =>
println("enter")
7 > 9
}
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In a
caseclause（case子句）, they separate the pattern (and optional guard) from the result expression, e.g.case x => y.
switch
val friend =
firstArg match {
case "salt" => "pepper"
case "chips" => "salsa"
case "eggs" => "bacon"
case _ => "huh?"
}
exception
try {
val file = new FileReader("input.txt")
} catch {
case ex: FileNotFoundException => ex.printStackTrace()
case ex: IOException => //ex.printStackTrace()可以省略
} finally {
file.close()
}
这个try-catch表达式的行为与其它语言中的异常处理一致。程序体被执行，如果抛出异常，每个catch子句依次被尝试。本例中，如果异常是FileNotFoundException，那么第一个子句将被执行。如果是IOException类型，第二个子句将被执行。如果都不是，那么try-catch将终结并把异常上升出去。
注意
你将很快发现与Java的一个差别是Scala里不需要你捕获检查异常：checked exception，或把它们声明在throws子句中。如果你愿意，可以用ATthrows标注声明一个throws子句，但这不是必需的。