Scala for-comprehension返回有序地图

Question

如何使用for-comprehension返回可以分配给有序Map的内容？这是我所拥有的代码的简化:

class Bar
class Foo(val name: String, val bar: Bar)
val myList: java.util.List[Foo] = ...
val result: ListMap[String, Bar] =
    for {
        foo <- myList
    } yield (foo.name, foo.bar)

我需要确保我的结果是一个有序的Map,顺序是从for-comprehension返回的元组.

有了上述,我得到错误:

error: type mismatch;
found   : scala.collection.mutable.Buffer[(String,Bar)]
required: scala.collection.immutable.ListMap[String,Bar]
foo <- myList

这编译:

class Bar
class Foo(val name: String, val bar: Bar)
val myList: java.util.List[Foo] = ...
val result: Predef.Map[String, Bar] =
    {
        for {
            foo <- myList
        } yield (foo.name, foo.bar)
    } toMap

但后来我假设地图不会被排序,我需要一个明确的toMap调用.

我怎样才能做到这一点？

Answer 1

该collection.breakOut是在这样的情况下,你的好朋友,

val result: collection.immutable.ListMap[String, Bar] = 
  myList.map{ foo => (foo.name, foo.bar) }(collection.breakOut)

如果使用for-comprehension表达很重要,它将按如下方式完成,

val result: collection.immutable.ListMap[String, Bar] = {
  for { foo <- myList } yield (foo.name, foo.bar)
}.map(identity)(collection.breakOut)

Scala 2.8 breakOut已经解释了collection.breakOut非常好.