Tour of Scala

# 高阶函数

Language

``````val salaries = Seq(20000, 70000, 40000)
val doubleSalary = (x: Int) => x * 2
val newSalaries = salaries.map(doubleSalary) // List(40000, 140000, 80000)
``````

``````val salaries = Seq(20000, 70000, 40000)
val newSalaries = salaries.map(x => x * 2) // List(40000, 140000, 80000)
``````

``````val salaries = Seq(20000, 70000, 40000)
val newSalaries = salaries.map(_ * 2)
``````

## 强制转换方法为函数

``````case class WeeklyWeatherForecast(temperatures: Seq[Double]) {

private def convertCtoF(temp: Double) = temp * 1.8 + 32

def forecastInFahrenheit: Seq[Double] = temperatures.map(convertCtoF) // <-- passing the method convertCtoF
}
``````

## 接收函数作为参数的函数

``````object SalaryRaiser {

def smallPromotion(salaries: List[Double]): List[Double] =
salaries.map(salary => salary * 1.1)

def greatPromotion(salaries: List[Double]): List[Double] =
salaries.map(salary => salary * math.log(salary))

def hugePromotion(salaries: List[Double]): List[Double] =
salaries.map(salary => salary * salary)
}
``````

``````object SalaryRaiser {

private def promotion(salaries: List[Double], promotionFunction: Double => Double): List[Double] =
salaries.map(promotionFunction)

def smallPromotion(salaries: List[Double]): List[Double] =
promotion(salaries, salary => salary * 1.1)

def bigPromotion(salaries: List[Double]): List[Double] =
promotion(salaries, salary => salary * math.log(salary))

def hugePromotion(salaries: List[Double]): List[Double] =
promotion(salaries, salary => salary * salary)
}
``````

## 返回函数的函数

``````def urlBuilder(ssl: Boolean, domainName: String): (String, String) => String = {
val schema = if (ssl) "https://" else "http://"
(endpoint: String, query: String) => s"\$schema\$domainName/\$endpoint?\$query"
}

val domainName = "www.example.com"
def getURL = urlBuilder(ssl=true, domainName)
val endpoint = "users"
val query = "id=1"
val url = getURL(endpoint, query) // "https://www.example.com/users?id=1": String
``````