scala:二进制操作的类型界限
问题很简单,我有一个对象,它使用两个参数执行二进制操作。我只想添加相同类型的燃油,如下所示:scala:二进制操作的类型界限,scala,monoids,type-bounds,Scala,Monoids,Type Bounds,问题很简单,我有一个对象,它使用两个参数执行二进制操作。我只想添加相同类型的燃油,如下所示: object Fuels { case class Fuel[F <: FuelType](amount: Double, fuelType: F = Petrol) { def +(that : Fuel[F]) = { copy(amount = this.amount + that.amount) } } def add[F <: FuelT
object Fuels {
case class Fuel[F <: FuelType](amount: Double, fuelType: F = Petrol) {
def +(that : Fuel[F]) = {
copy(amount = this.amount + that.amount)
}
}
def add[F <: FuelType](x: Fuel[F], y: Fuel[F]): Fuel[F] = x + y
sealed trait FuelType {
val name : String
}
case object Petrol extends FuelType{
override val name = "Petrol"
}
case object Diesel extends FuelType{
override val name = "Diesel"
}
case object Hydrogen extends FuelType{
override val name = "Hydrogen"
}
implicit def fuelMonoid[F <:FuelType](implicit fuelType: F) = new Monoid[Fuel]{
override def zero: Fuel[F] = Fuel(0, fuelType)
override def append(m1: Fuel[F], m2: Fuel[F]) : Fuel[F] = m1 + m2
}
}
编译错误:
Expression of Type Fuels.Fuel[Nothing] does not conform to Fuels.Fuel[F]
主要问题是add和fuelMonoid都无法确定我们处理的是相同类型的项。编译器无法解析类型约束,无法推断任何内容
为了完成,这里是幺半群,没有什么特别的:
trait Monoid[A] {
def zero: A
def append(a1: A, a2: A): A
}
我将类型更改为
Monoid[Fuel[F]]
,指定返回类型为implicit def
和Fuel++
,它可以工作:
object Fuels {
trait Monoid[F] {
def zero: F
def append(f1: F, f2: F): F
}
object Monoid {
def fold[F](as: Seq[F], m: Monoid[F]): F = as.foldLeft(m.zero)(m.append)
}
case class Fuel[F <: FuelType](amount: Double, fuelType: F = Petrol) {
def +(that : Fuel[F]): Fuel[F] = {
copy(amount = this.amount + that.amount)
}
}
def add[F <: FuelType](x: Fuel[F], y: Fuel[F]): Fuel[F] = x + y
sealed trait FuelType {
val name : String
}
case object Petrol extends FuelType{
override val name = "Petrol"
}
case object Diesel extends FuelType{
override val name = "Diesel"
}
case object Hydrogen extends FuelType{
override val name = "Hydrogen"
}
implicit def fuelMonoid[F <:FuelType](implicit fuelType: F): Monoid[Fuel[F]] = new Monoid[Fuel[F]] {
override def zero: Fuel[F] = Fuel(0, fuelType)
override def append(m1: Fuel[F], m2: Fuel[F]): Fuel[F] = m1 + m2
}
}
object Main {
def main (args: Array[String] ) {
import Fuels._
println(Fuel(10, Petrol) + Fuel(20, Petrol))
println(add(Fuel(10, Petrol), Fuel(20, Petrol)))
println(Monoid.fold(Seq(Fuel(10, Petrol), Fuel(20, Petrol), Fuel(30, Petrol)), fuelMonoid(Petrol)))
}
}
对象燃料{
特征幺半群[F]{
def零:F
def附加(f1:F,f2:F):F
}
对象幺半群{
def fold[F](as:Seq[F],m:Monoid[F]):F=as.foldLeft(m.zero)(m.append)
}
case class Fuel[FMonoid
的定义是什么?如果我将其定义为trait Monoid[M]{def zero:M;def append(m1:M,m2:M):M}
,那么如果不将fuelMonoid
的类型更改为隐式def fuelMonoid,我就无法编译代码[F幺半群特征确实是这样定义的,我将在主要问题中添加代码。添加新的幺半群[Fuel[F]]而不是新的幺半群[Fuel]没有什么区别,原因与添加(x,y)相同不起作用。有什么建议吗?发生了一些非常奇怪的事情。Intellij Idea报告了各种错误。使用Scalaz,我能够利用第二个参数,即按名称调用,其中我的版本需要一个严格的参数。我将继续对其进行修补,以使其正常工作,但很高兴知道解决方案应该有效问题是Intellij IDEA。识别类型的荧光笔肯定有一些错误。顺便问一下,你知道如何使用你建议的附加来进行折叠操作吗:trait Monoid[a]{def append(a1:a,a2:=>a):def zero:a}object Monoid{def fold[a](as:Seq[a],m:Monoid[a]):a=as.foldLeft(m.zero)(m.append)}。我不想为了这个而使用完整的Scalaz。谢谢!@fracca,对不起,我没有注意到你使用了自己的Monoid
。我和你一样,在非Scalaz特性上也会遇到同样的奇怪错误。我会尝试再修改一下。@fracca,看起来我已经找到了你两个问题的答案。我编辑了答案(添加了简单的Monoid
trait和Monoid
对象和fold
)。
object Fuels {
trait Monoid[F] {
def zero: F
def append(f1: F, f2: F): F
}
object Monoid {
def fold[F](as: Seq[F], m: Monoid[F]): F = as.foldLeft(m.zero)(m.append)
}
case class Fuel[F <: FuelType](amount: Double, fuelType: F = Petrol) {
def +(that : Fuel[F]): Fuel[F] = {
copy(amount = this.amount + that.amount)
}
}
def add[F <: FuelType](x: Fuel[F], y: Fuel[F]): Fuel[F] = x + y
sealed trait FuelType {
val name : String
}
case object Petrol extends FuelType{
override val name = "Petrol"
}
case object Diesel extends FuelType{
override val name = "Diesel"
}
case object Hydrogen extends FuelType{
override val name = "Hydrogen"
}
implicit def fuelMonoid[F <:FuelType](implicit fuelType: F): Monoid[Fuel[F]] = new Monoid[Fuel[F]] {
override def zero: Fuel[F] = Fuel(0, fuelType)
override def append(m1: Fuel[F], m2: Fuel[F]): Fuel[F] = m1 + m2
}
}
object Main {
def main (args: Array[String] ) {
import Fuels._
println(Fuel(10, Petrol) + Fuel(20, Petrol))
println(add(Fuel(10, Petrol), Fuel(20, Petrol)))
println(Monoid.fold(Seq(Fuel(10, Petrol), Fuel(20, Petrol), Fuel(30, Petrol)), fuelMonoid(Petrol)))
}
}