Question

scala By-name-parameter 和 Function type的区别

Answer 1

原来By-name-parameter和函数类型是不一样的概念，以前还以为By-name-paramete 就是函数类型的一个用法呢

By-name-parameter to Function
Today's topic is related to Defining Custom Control Structures. By name parameters are not function objects in the same way as other functions. Normally you need to invoke a function:
scala> def exec(func: () => Int) = func()
exec: (() => Int)Int
// exec invokes the function so 10 is the result
scala> exec (() => 10)
res1: Int = 10
scala> def exec(func: () => Int) = func
exec: (() => Int)() => Int
// here exec returns the function:
scala> exec (() => 10)
res2: () => Int = <function>

Compare that to a by-name-parameter function:

scala> def exec(func: => Int) = func
exec: (=> Int)Int
// by-name-parameters are executed when they are referenced
// so the result is 10
scala> exec {10}
res3: Int = 10
// This is not legal because by-name-parameters
// are not normal functions
scala> def exec(func: => Int) = func()
<console>:4: error: func of type Int does not take parameters
def exec(func: => Int) = func()

So the issue is how can you pass a by-name-parameter to a method that takes a function as a parameter without having to do:

scala> def takesFunc(func: () => Int) = println(func())
takesFunc: (() => Int)Unit
scala> def send(x: => Int) = takesFunc(() => x)
send: (=> Int)Unit
scala> send{2}
2

the alternative syntax is:

scala> def send(x: => Int) = takesFunc (x _)
send: (=> Int)Unit
scala> send {2}