两者之间的区别在于,第一个是通过模式匹配创建的定义,第二个是函数文字内的值。看Scala 编程,第 23.1 节关于表达式 http://www.artima.com/pins1ed/for-expressions-revisited.html:
for {
p <- persons // a generator
n = p.name // a definition
if (n startsWith "To") // a filter
} yield n
当您使用以下命令编译源代码时,您会看到真正的区别scalac -Xprint:typer <filename>.scala
:
object X {
val x1 = for (i <- (1 to 5); x = i*2) yield x
val x2 = for (i <- (1 to 5)) yield { val x = i*2; x }
}
经过编译器转换代码后,您将得到如下内容:
private[this] val x1: scala.collection.immutable.IndexedSeq[Int] =
scala.this.Predef.intWrapper(1).to(5).map[(Int, Int), scala.collection.immutable.IndexedSeq[(Int, Int)]](((i: Int) => {
val x: Int = i.*(2);
scala.Tuple2.apply[Int, Int](i, x)
}))(immutable.this.IndexedSeq.canBuildFrom[(Int, Int)]).map[Int, scala.collection.immutable.IndexedSeq[Int]]((
(x$1: (Int, Int)) => (x$1: (Int, Int) @unchecked) match {
case (_1: Int, _2: Int)(Int, Int)((i @ _), (x @ _)) => x
}))(immutable.this.IndexedSeq.canBuildFrom[Int]);
private[this] val x2: scala.collection.immutable.IndexedSeq[Int] =
scala.this.Predef.intWrapper(1).to(5).map[Int, scala.collection.immutable.IndexedSeq[Int]](((i: Int) => {
val x: Int = i.*(2);
x
}))(immutable.this.IndexedSeq.canBuildFrom[Int]);
这可以简化为:
val x1 = (1 to 5).map {i =>
val x: Int = i * 2
(i, x)
}.map {
case (i, x) => x
}
val x2 = (1 to 5).map {i =>
val x = i * 2
x
}