reactive programming 1.4 Functional Random Generators
只有collection是可以用for表示式吗？ 不是的，任何实现了map, flatMap, filter的类都可以
1.整数生成器
package week2
trait Generator[+T] {
def generate: T
}
object Test extends App {
val integers = new Generator[Int] {
val random = new java.util.Random()
def generate = random.nextInt()
}
print (integers.generate)
}
2.布尔值生成器
val bools = new Generator[Boolean] {
def generate = integers.generate > 0
}
for (i <- 1 to 10) println (bools.generate)
3. Pair生成器
val pairs = new Generator[(Int, Int)] {
def generate = (integers.generate, integers.generate)
}
for (i <- 1 to 10) println (pairs.generate)
4. 能不能更简洁些，每次都用 new Generator很麻烦
如果能这样岂不很好
val booleans = for (x <- integers) yield x > 0
def pairs[T, U](t: Generator[T], u: Generator[U]) = for {
x <- t
y <- u
} yield (x, y)
通过上节我们知道for循环没翻译成map,flatMat,filter来执行，要想在for循环中用integers就得定义map方法
package week1
trait Generator[+T] {
self =>
def generate:T
def map[S](f: T => S): Generator[S] = new Generator[S] {
def generate = f(self.generate)
}
}
测试
package week1
object Test {
println("Welcome to the Scala worksheet") //> Welcome to the Scala worksheet
val Integers = new Generator[Int] {
val rand = new java.util.Random
def generate = rand.nextInt()
} //> Integers : week1.Generator[Int]{val rand: java.util.Random} = week1.Test$$a
//| nonfun$main$1$$anon$1@693ba66d
val Bools = for {x <- Integers} yield x > 0 //> Bools : week1.Generator[Boolean] = week1.Generator$$anon$1@498665a0
Bools.generate //> res0: Boolean = true
}
实现flatMap方法，这个有点抽象，我们先回忆一下for语句的展开规则
for (x <- e1) yield e2 // 被展开成 e1.map(x => e2) for (x <- e1; y <- e2 ) yield e3 // 被展开成 e1.flatMap(x => for (y <- e2) yield e3) // 进一步被展开成 e1.flatMap(x => e2.map(y => e3))
我们的目标是：
def pairs[T, U](t: Generator[T], u: Generator[U]) = for {
x <- t
y <- u
} yield (x, y)
// 会被展开成
t.flatMap(x => u.map(y => (x, y)))
//进而
t.flatMap(x => new Generator[U] { def generate = (x, self.generate(y))})
那么flatMap应该做些什么呢？
将x替换成 T.this.generate(x), 交给参数
package week1
trait Generator[+T] {
self =>
def generate:T
def map[S](f: T => S): Generator[S] = new Generator[S] {
def generate = f(self.generate)
}
def flatMap[S](f: T => Generator[S]): Generator[S] = new Generator[S] {
def generate = f(self.generate).generate
}
}
测试
package week1
object Test {
println("Welcome to the Scala worksheet") //> Welcome to the Scala worksheet
val Integers = new Generator[Int] {
val rand = new java.util.Random
def generate = rand.nextInt()
} //> Integers : week1.Generator[Int]{val rand: java.util.Random} = week1.Test$$a
//| nonfun$main$1$$anon$1@33b93f89
val Bools = for {x <- Integers} yield x > 0 //> Bools : week1.Generator[Boolean] = week1.Generator$$anon$1@6f55137
Bools.generate //> res0: Boolean = true
val Pairs = for (x <- Integers; y <- Bools) yield (x, y)
//> Pairs : week1.Generator[(Int, Boolean)] = week1.Generator$$anon$2@bcb4598
Pairs.generate //> res1: (Int, Boolean) = (-918487409,true)
}