网上找的一篇关于 By-name-parameter 和 Function区别,虽然是2009年的文章了,但解释的非常好。 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:
Compare that to a by-name-parameter function:
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:
the alternative syntax is:
- 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}
- 2
参考:http://daily-scala.blogspot.sg/2009/12/by-name-parameter-to-function.html
相关内容参考:Scala“call by name“和 “call by value” 比较
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