按名称参数与匿名函数

问题描述：

仍然不清楚的是，在延迟评估和其他好处(如果有的话)方面，按名称参数相对于匿名函数的优势是什么:

What is still unclear for is what's the advantage by-name parameters over anonymous functions in terms of lazy evaluation and other benefits if any:

def func1(a: => Int)
def func2(a: () => Int)

什么时候用第一个，什么时候用第二个?

When should I use the first and when the second one?

这不是有什么区别=>, ()=>, and Unit=>

答

懒惰在这两种情况下是一样的，但也有细微的差别.考虑:

Laziness is the same in the both cases, but there are slight differences. Consider:

def generateInt(): Int = { ... }
def byFunc(a: () => Int) { ... }
def byName(a: => Int) { ... }

// you can pass method without
// generating additional anonymous function
byFunc(generateInt)

// but both of the below are the same
// i.e. additional anonymous function is generated
byName(generateInt)
byName(() => generateInt())

带有 call-by-name 的函数对于制作 DSL 很有用.例如:

Functions with call-by-name however is useful for making DSLs. For instance:

def measured(block: ⇒ Unit): Long = {
  val startTime = System.currentTimeMillis()
  block
  System.currentTimeMillis() - startTime
}

Long timeTaken = measured {
  // any code here you like to measure
  // written just as there were no "measured" around
}

按名称参数与匿名函数

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