我正在通过“Scala for the Impatient”一书中的练习来学习Scala。一个问题是：
"abc".map(_.toUpper)的结果是String，但结果是"abc".map(_.toInt)Vector是CanBuildFrom。找出原因。
阅读“The Architecture of Scala Collections”后，我认为这是因为选择了不同的 struct card {
int value;
char suit;
}
隐式实现。我怎样才能找出为上述表达式选择哪些？
答案 0 :(得分：8)
您可以使用scala> import scala.reflect.runtime.{universe => ru}
import scala.reflect.runtime.{universe=>ru}
scala> ru.show{ ru.reify{ "abc".map(_.toUpper) }.tree }
res0: String = Predef.augmentString("abc").map(((x$1) => Predef.charWrapper(x$1).toUpper))(Predef.StringCanBuildFrom)
scala> ru.show{ ru.reify{ "123".map(_.toInt) }.tree }
res1: String = Predef.augmentString("123").map(((x$1) => x$1.toInt))(Predef.fallbackStringCanBuildFrom)
来获取所有隐含的内容：
Predef.StringCanBuildFrom所以CanBuildFrom[String,Char,String] _.toUpper用于Predef.fallbackStringCanBuildFrom，而CanBuildFrom[String,Int,IndexedSeq[Int]] _.toInt用于scala> Predef.StringCanBuildFrom
res2: scala.collection.generic.CanBuildFrom[String,Char,String] = scala.Predef$$anon$3@43b7a6fa
scala> Predef.fallbackStringCanBuildFrom[Int]
res3: scala.collection.generic.CanBuildFrom[String,Int,scala.collection.immutable.IndexedSeq[Int]] = scala.LowPriorityImplicits$$anon$4@61af1aa0
。
Vector请注意，类型不是IndexedSeq，而是Vector。 @MappedSuperclass
public abstract class BaseEntity {
@Id
@GeneratedValue(strategy=GenerationType.SEQUENCE)
private Long id;
private boolean deleted;
@Convert(converter = LocalDateTimePersistenceConverter.class)
private LocalDateTime createdAt;
@Convert(converter = LocalDateTimePersistenceConverter.class)
private LocalDateTime updatedAt;
}
是具体实施。