在scala 2.13中,为什么有时无法显式调用类型类?- 第2部分
在scala 2.13中,为什么有时无法显式调用类型类?- 第2部分
这是对In scala 2.13, why is it sometimes impossible to summon type class explicitly?的后续问题:
以下代码可以正确编译:
import shapeless._ import record._ import syntax.singleton._ val book = ("author" ->> "Benjamin Pierce") :: ("title" ->> "Types and Programming Languages") :: ("id" ->> 262162091) :: ("price" ->> 44.11) :: HNil val v1 = book.values assert(v1.head == "Benjamin Pierce") case class HasValues[T <: HList: TypeTag](v: T) { // def vs(implicit v: Values[T]) = v // doesn't work def vs(implicit v: Values[T]): Values.Aux[T, v.Out] = v // works } val _vs = HasValues(book).vs val v2 = book.values(_vs) assert(v2.head == "Benjamin Pierce")
而以下代码,在编译时和运行时都是语法上相同的,却编译失败了:
import shapeless._ import record._ import syntax.singleton._ val book = ("author" ->> "Benjamin Pierce") :: ("title" ->> "Types and Programming Languages") :: ("id" ->> 262162091) :: ("price" ->> 44.11) :: HNil val v1 = book.values assert(v1.head == "Benjamin Pierce") case class HasValues[T <: HList: TypeTag](v: T) { type TT = T } val hv = HasValues(book) val _vs = implicitly[Values[hv.TT]] val _vs2: Values.Aux[hv.TT, _vs.Out] = _vs val v2 = book.values(_vs2) assert(v2.head == "Benjamin Pierce")
结果:
[Error] /home/peng/git-spike/scalaspike/common/src/test/scala/com/tribbloids/spike/shapeless_spike/RecordProblem2.scala:41: could not find implicit value for parameter c: shapeless.ops.hlist.IsHCons[com.tribbloids.spike.shapeless_spike.RecordProblem2._vs2.Out] one error found
此外,以下这行代码似乎非常繁琐。理想情况下,我认为编译器应该能够自动推导出这个细化类型,考虑到这完全是一个向上转型:
val _vs = implicitly[Values[hv.TT]] val _vs2: Values.Aux[hv.TT, _vs.Out] = _vs
为什么第二个代码编译失败,更进一步,如何摆脱这种样板式的鸭子类型声明?(当然,不能修改HasValues类的签名)
在Scala 2.13中,有时无法显式调用类型类的原因是implicitly
可以破坏类型细化。解决方法是使用shapeless.the
代替。
以下是解决问题的代码示例:
val hv = HasValues(book) val _vs = the[Values[hv.TT]] val _vs2: Values.Aux[hv.TT, _vs.Out] = _vs val v2 = book.values(_vs2) assert(v2.head == "Benjamin Pierce")
另外,以下代码也可以编译通过:
val hv = HasValues(book) val _vs2 = the[Values[hv.TT]] val v2 = book.values(_vs2) assert(v2.head == "Benjamin Pierce")
val hv = HasValues(book) val v2 = book.values assert(v2.head == "Benjamin Pierce")
此外,可以将以下代码:
def vs(implicit v: Values[T]): Values.Aux[T, v.Out] = v
替换为:
def vs(implicit v: Values[T]): v.type = v