在scala中混合类型参数和抽象类型 [英] Mixing type parameters and abstract types in scala
问题描述
我尝试使用前面的问题的答案来实施一个小图库。我们的想法是将图形看作colections，顶点包装集合元素。
我想使用抽象类型来表示Vertex和Edge类型（因为类型安全）和我想使用类型参数来表示集合元素的类型（因为我想在实例化时很容易地定义它们）。然而，当尝试最基本的示例I可以想一想，我遇到了编译错误。下面是例子：
包图
抽象类GraphKind [T] {
type V <：Vertex [T]
type G <：Graph [T]
def newGraph（）：G
抽象类图表[T]扩展集合[T] {
self：G =>
def vertices（）：List [V]
def add（t：T）：单位
def size（）：Int
def elements（）：Iterator [T]
}
trait Vertex [T] {
self：V =>
def graph（）：G
def value（）：T
}
}
以下是基本实现：
class SimpleGraphKind [T] extends GraphKind [T] {
type G = GraphImpl [T]
type V = VertexImpl [T]
def newGraph（）= new GraphImpl [T]
class GraphImpl [T] extends Graph [T] {
private var vertices_ = List [V]（）
def vertices = vertices_
def add（t：T ）{vertices_ :: = new VertexImpl [T]（t，this）}
def size（）= vertices_.size
def elements（）= vertices.map（_.value）.elements
class VertexImpl [T]（val value：T，val graph：GraphImpl [T]）extends Vertex [T] {
override lazy val toString =Vertex（+ value.toString +）
}
}
当试图编译时，我得到：
/prg/ScalaGraph/study/Graph.scala:10：error：illegal inheritance;
自我类型GraphKind.this.G不符合Collection [T]的自定义集合[T]
抽象类图形[T]扩展集合[T] {
^
/prg/ScalaGraph/study/Graph.scala:33：错误：非法继承;
self-type SimpleGraphKind.this.GraphImpl [T]不符合SimpleGraphKind.this.Graph [T]的自我类型SimpleGraphKind.this.G
class GraphImpl [T] extends Graph [T] {
^
/prg/ScalaGraph/study/Graph.scala:36：error：type mismatch; type mismatch;
found：SimpleGraphKind.this.VertexImpl [T]
required：SimpleGraphKind.this.V
def add（t：T）{vertices_ :: = new VertexImpl [T]（t，this ）}
^
/prg/ScalaGraph/study/Graph.scala:38：error：type mismatch;
found：Iterator [T（in class SimpleGraphKind）]
required：Iterator [T（in class GraphImpl）]
def elements（）= vertices.map（_.value）.elements
^
/prg/ScalaGraph/study/Graph.scala:41：错误：非法继承;
自我类型SimpleGraphKind.this.VertexImpl [T]不符合SimpleGraphKind.this.Vertex [T]的自我类型SimpleGraphKind.this.V
类VertexImpl [T]（val值：T， val graph：GraphImpl [T]）extends Vertex [T] {
^
找到5个错误
我完全不知道这些错误的含义......但是，如果我在实现中专门化类型T（ class SimpleGraphKind extends GraphKind [Int] 我只得到第一个错误。
你有什么想法吗？
使用 -explaintypes 编译此结果：
< console>：11：error：非法继承;
自我类型GraphKind.this.G不符合Collection [T]的自定义集合[T]
抽象类Graph [T] extends Collection [ T] {
^
图表Kind.this.G<：收集[T]？
Iterable [T] <：Iterable [T]？
T <：T？
T <：什么都没有？
< notype> <：什么都没有？
false
任何<：无？
< notype> <：什么都没有？
false
false
false
任何<：T？
任何<：没有？
< notype> <：什么都没有？
false
false
false
false
false
GraphKind.this.Graph [T]<：Iterable [T]？
Iterable [T] <：Iterable [T]？
T <：T？
T <：什么都没有？
< notype> <：什么都没有？
false
任何<：无？
< notype> <：什么都没有？
false
false
false
任何<：T？
任何<：没有？
< notype> <：什么都没有？
false
false
false
false
false
false
false
现在，我正要写我不明白 T <：T 可以如何假 - 它几乎就像 T 被定义了两次，当然这是整个问题。这里：
抽象类GraphKind [T] {
type V <：Vertex [T]
type G <：Graph [T]
def newGraph（）：G
抽象类Graph [T]扩展Collection [T] {
好的，类 GraphKind 被参数化为 T 和类型 G 必须是图形[T] 。现在，类 Graph 也被参数化，其参数也被称为 T 。为避免混淆，我们重写它：
抽象类图[T2]扩展了Collection [T2] {
self： G =>
def vertices（）：List [V]
def add（t：T2）：单位
def size（）：Int
def elements（）：Iterator [T2]
}
请注意，这与您所写的完全相同。我只是为类型参数使用了一个不同的名称，所以它不会与参数化 GraphKind T 混淆。
所以，这里是逻辑： > G <：图表[T]
图表[T2] <：集合[T2]
图表[T2] <：G //自我类型
这意味着
pre $ 图[T2] <：Graph [T]
而且，因为图扩展集合：
集合[T2]<：集合[T]
但不能保证这是真的。我不明白为什么在继承不存在时不显示问题。 Fix：
抽象类GraphKind [T] {
类型V <：顶点
类型G <：图形
def newGraph（）：G
抽象类Graph扩展Collection [T] {
self：G =>
def vertices（）：List [V]
def add（t：T）：单位
def size（）：Int
def elements（）：Iterator [T]
}
特征顶点{
self：V =>
def graph（）：G
def value（）：T
}
}
class SimpleGraphKind [T]扩展GraphKind [ T] {
type G = GraphImpl
type V = VertexImpl
def newGraph（）= new GraphImpl
类GraphImpl extends Graph {
private var vertices_ = List [V]（）
def vertices = vertices_
def add（t：T）{vertices_ :: = new VertexImpl（t，this）}
覆盖def size（）= vertices_.size
覆盖def elements（）= vertices.map（_.value）.elements
}
class VertexImpl（val value：T ，val graph：GraphImpl）extends Vertex {
override lazy val toString =Vertex（+ value.toString +）
}
}
由于顶点和图会绑定到 GraphKind 的一个实例，然后 T 将被固定为针对该实例定义的任何实例。例如：
scala> new SimpleGraphKind [Int]
res0：SimpleGraphKind [Int] = SimpleGraphKind @ 1dd0fe7
scala>新的res0.GraphImpl
res1：res0.GraphImpl = line10（）
scala> res1.add（10）
scala> res1.add（abc）
< console>：9：error：type mismatch;
found：java.lang.String（abc）
required：Int
res1.add（abc）
^
I am trying to use the answer of a preceding question to implement a small graph library. The idea is to consider graphs as colections, where vertices wrap collection elements.
I would like to use abstract types to represent Vertex and Edge types (because of type safety) and I want to use type parameters to represent the type of the collection elements (because I want to define them at instantiation easily).
However, when trying the most basic example I can think about, I am stuck with compile errors. Here is the example:
package graph
abstract class GraphKind[T] {
type V <: Vertex[T]
type G <: Graph[T]
def newGraph(): G
abstract class Graph[T] extends Collection[T]{
self: G =>
def vertices(): List[V]
def add(t: T): Unit
def size(): Int
def elements(): Iterator[T]
}
trait Vertex[T] {
self: V =>
def graph(): G
def value(): T
}
}
And here is the basic implementations:
class SimpleGraphKind[T] extends GraphKind[T] {
type G = GraphImpl[T]
type V = VertexImpl[T]
def newGraph() = new GraphImpl[T]
class GraphImpl[T] extends Graph[T] {
private var vertices_ = List[V]()
def vertices = vertices_
def add( t: T ) { vertices_ ::= new VertexImpl[T](t,this) }
def size() = vertices_.size
def elements() = vertices.map( _.value ).elements
}
class VertexImpl[T](val value: T, val graph: GraphImpl[T]) extends Vertex[T] {
override lazy val toString = "Vertex(" + value.toString + ")"
}
}
When trying to compile, I get:
/prg/ScalaGraph/study/Graph.scala:10: error: illegal inheritance;
self-type GraphKind.this.G does not conform to Collection[T]'s selftype Collection[T]
abstract class Graph[T] extends Collection[T]{
^
/prg/ScalaGraph/study/Graph.scala:33: error: illegal inheritance;
self-type SimpleGraphKind.this.GraphImpl[T] does not conform to SimpleGraphKind.this.Graph[T]'s selftype SimpleGraphKind.this.G
class GraphImpl[T] extends Graph[T] {
^
/prg/ScalaGraph/study/Graph.scala:36: error: type mismatch;
found : SimpleGraphKind.this.VertexImpl[T]
required: SimpleGraphKind.this.V
def add( t: T ) { vertices_ ::= new VertexImpl[T](t,this) }
^
/prg/ScalaGraph/study/Graph.scala:38: error: type mismatch;
found : Iterator[T(in class SimpleGraphKind)]
required: Iterator[T(in class GraphImpl)]
def elements() = vertices.map( _.value ).elements
^
/prg/ScalaGraph/study/Graph.scala:41: error: illegal inheritance;
self-type SimpleGraphKind.this.VertexImpl[T] does not conform to SimpleGraphKind.this.Vertex[T]'s selftype SimpleGraphKind.this.V
class VertexImpl[T](val value: T, val graph: GraphImpl[T]) extends Vertex[T] {
^
5 errors found
I have absolutely no idea of the meaning of these errors... However, if I specialize the type T in the implementation (class SimpleGraphKind extends GraphKind[Int] I get only the first error.
Do you have some ideas ?
Compiling this with -explaintypes yields:
<console>:11: error: illegal inheritance;
self-type GraphKind.this.G does not conform to Collection[T]'s selftype Collection[T]
abstract class Graph[T] extends Collection[T]{
^
GraphKind.this.G <: Collection[T]?
Iterable[T] <: Iterable[T]?
T <: T?
T <: Nothing?
<notype> <: Nothing?
false
Any <: Nothing?
<notype> <: Nothing?
false
false
false
Any <: T?
Any <: Nothing?
<notype> <: Nothing?
false
false
false
false
false
GraphKind.this.Graph[T] <: Iterable[T]?
Iterable[T] <: Iterable[T]?
T <: T?
T <: Nothing?
<notype> <: Nothing?
false
Any <: Nothing?
<notype> <: Nothing?
false
false
false
Any <: T?
Any <: Nothing?
<notype> <: Nothing?
false
false
false
false
false
false
false
Now, I was about to write I don't understand how T <: T can be false -- it is almost like T was defined twice, which, of course, is the whole problem. Here:
abstract class GraphKind[T] {
type V <: Vertex[T]
type G <: Graph[T]
def newGraph(): G
abstract class Graph[T] extends Collection[T]{
Ok, class GraphKind is parameterized with T and type G must be a Graph[T]. Now, class Graph is also parameterized, and its parameter is also called T. To prevent confusing, let's rewrite it:
abstract class Graph[T2] extends Collection[T2]{
self: G =>
def vertices(): List[V]
def add(t: T2): Unit
def size(): Int
def elements(): Iterator[T2]
}
Note that this is EXACTLY EQUAL to what you wrote. I'm just using a different name for the type parameter, so that it doesn't get confused with the T that is parameterizing GraphKind.
So, here is the logic:
G <: Graph[T]
Graph[T2] <: Collection[T2]
Graph[T2] <: G // self type
which implies that
Graph[T2] <: Graph[T]
And, because Graph extends Collection:
Collection[T2] <: Collection[T]
But there is no guarantee that this is true. I do not understand why the problem does not show up when the inheritance is not present. Fix:
abstract class GraphKind[T] {
type V <: Vertex
type G <: Graph
def newGraph(): G
abstract class Graph extends Collection[T]{
self: G =>
def vertices(): List[V]
def add(t: T): Unit
def size(): Int
def elements(): Iterator[T]
}
trait Vertex {
self: V =>
def graph(): G
def value(): T
}
}
class SimpleGraphKind[T] extends GraphKind[T] {
type G = GraphImpl
type V = VertexImpl
def newGraph() = new GraphImpl
class GraphImpl extends Graph {
private var vertices_ = List[V]()
def vertices = vertices_
def add( t: T ) { vertices_ ::= new VertexImpl(t,this) }
override def size() = vertices_.size
override def elements() = vertices.map( _.value ).elements
}
class VertexImpl(val value: T, val graph: GraphImpl) extends Vertex {
override lazy val toString = "Vertex(" + value.toString + ")"
}
}
Since Vertex and Graph will be tied to one instance of GraphKind, then T will be fixed to whatever it was defined for that instance. For example:
scala> new SimpleGraphKind[Int]
res0: SimpleGraphKind[Int] = SimpleGraphKind@1dd0fe7
scala> new res0.GraphImpl
res1: res0.GraphImpl = line10()
scala> res1.add(10)
scala> res1.add("abc")
<console>:9: error: type mismatch;
found : java.lang.String("abc")
required: Int
res1.add("abc")
^
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