说我有两组类,第一组继承自Foo,第二组继承自Bar.
class Foo class Baz extends Foo class Bar class Qux extends Bar
我想使一个通用的隐式转换函数将任何Foo转换为Bar,给定范围内有一个隐式的转换器类型.
trait Converter[A <: Foo,B <: Bar] { def convert(a : A) : B } implicit object BazToQuxConverter extends Converter[Baz,Qux] { def convert(a : Baz) : Qux = new Qux } implicit def FooToBar[A <: Foo,B <: Bar](a : A)(implicit converter : Converter[A,B]) : B = converter.convert(a)
不幸的是,这似乎不像我期望的那样工作.当我将以下行插入REPL时:
val a : Baz = new Baz val b : Qux = a
…我收到以下错误:
<console>:17: error: type mismatch; found : Baz required: Qux val b : Qux = a ^
有没有办法让这个工作?最近我能够来的是以下几点:
implicit def BadFooToBar[A <: Foo,_]) : B = converter.convert(a).asInstanceOf[B]
这对我以前的例子是有效的,但它不是很安全.
class Qax extends Bar val a : Baz = new Baz val b : Qax = a
这将会很好地编译,但是由于Qux(转换器转换(a)的结果))不能转换为Qax(asInstanceOf [Qax]),因此在运行时会产生爆炸.理想情况下,我想要它,因为在编译时将会捕获上面一行,因为在范围内没有任何转换器[Bax,Qax].
解决方法
这是一个在2.11中修复的错误.自
PR 2822以来,它已经被修复了,相关票是
SI-3346.
Welcome to Scala version 2.11.0-20131030-090728-c38235fd44 (OpenJDK 64-Bit Server VM,Java 1.7.0_45). Type in expressions to have them evaluated. Type :help for more information. scala> :paste // Entering paste mode (ctrl-D to finish) class Foo class Baz extends Foo class Bar class Qux extends Bar trait Converter[A <: Foo,B <: Bar] { def convert(a : A) : B } implicit object BazToQuxConverter extends Converter[Baz,Qux] { def convert(a : Baz) : Qux = new Qux } import scala.language.implicitConversions implicit def FooToBar[A <: Foo,B]) : B = converter.convert(a) val a : Baz = new Baz val b : Qux = a // Exiting paste mode,Now interpreting. defined class Foo defined class Baz defined class Bar defined class Qux defined trait Converter defined object BazToQuxConverter import scala.language.implicitConversions FooToBar: [A <: Foo,B <: Bar](a: A)(implicit converter: Converter[A,B])B a: Baz = Baz@4f4db2ac b: Qux = Qux@760d62e0