scala 将参数传递给特征
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Passing parameters to a trait
提问by octavian
I want to model the game of chess.
For that, I want to make an abstract class, Piecewhich takes a player and a position as arguments. From that, I want to extend to other classes such as Pawn:
我想模拟国际象棋游戏。为此，我想创建一个抽象类，Piece它将玩家和位置作为参数。从那以后，我想扩展到其他类，例如Pawn：
trait Piece(player: Int, pos: Pos) = {
def spaces(destination: Pos): List[Pos]
}
case class Pawn extends Piece = {
//some other code
}
However, I think I'm not allowed to pass parameters to a trait, like this trait Piece(player: Int, pos: Pos).
但是，我认为我不允许将参数传递给特征，例如 this trait Piece(player: Int, pos: Pos)。
So how can I have an abstract class Piecethat has fields?
那么我怎样才能拥有一个Piece具有字段的抽象类呢？
回答by 0__
You could use an abstract class
你可以使用抽象类
abstract class Piece(player: Int, pos: Pos) {
...
}
case class Pawn(player: Int, pos: Pos) extends Piece(player, pos)
Or (probably better) you define those members abstractly in a trait
或者（可能更好）您在特征中抽象地定义这些成员
trait Piece {
def player: Int
def pos: Pos
...
}
case class Pawn(player: Int, pos: Pos) extends Piece
回答by AZ_
Dotty allows traits to have parameters, just like classes have parameters.
trait Greeting(val name: String) {
def msg = s"How are you, $name"
}
class C extends Greeting("Bob") {
println(msg)
}
回答by SkyWalker
I wasn't happy with the accepted answer for my use-case so I did the following. Note that you have two possibilities here for this same approach:
我对我的用例接受的答案不满意，所以我做了以下事情。请注意，对于这种相同的方法，您有两种可能性：
trait Piece {
// these can be referred to within this trait to implement reusable code
val player: Int
val pos: Pos
def spaces(destination: Pos): List[Pos] = {
// use player and pos at will e.g.
List(pos, destination)
}
}
case class Pawn(playerArg: Int, posArg: Pos) extends Piece = {
// now override those with whatever you like
override val player: Int = playerArg
override val pos: Pos = posArg
//some other code
}
The second alternative is to use and override methods instead e.g. def getPlayer: Int.
第二种选择是使用和覆盖方法，例如def getPlayer: Int.
Yet another possibility is to use impliciton the trait methods that would require accessing those attributes but I'm not a big fan of this approach.
另一种可能性是implicit在需要访问这些属性的特征方法上使用，但我不是这种方法的忠实粉丝。
That said, apparently they have been thinking about it SIP-25 Trait Parameters.
也就是说，显然他们一直在考虑SIP-25 Trait Parameters。
