问题
This is a followup to this question.
I'm trying to implement vectors in scala with a generic super class using self-types:
trait Vec[V] { self:V =>
def /(d:Double):Vec[V]
def dot(v:V):Double
def norm:Double = math.sqrt(this dot this)
def normalize = self / norm
}
Here's an implementation of a 3D vector:
class Vec3(val x:Double, val y:Double, val z:Double) extends Vec[Vec3]
{
def /(d:Double) = new Vec3(x / d, y / d, z / d)
def dot(v:Vec3) = x * v.x + y * v.y + z * v.z
def cross(v:Vec3):Vec3 =
{
val (a, b, c) = (v.x, v.y, v.z)
new Vec3(c * y - b * z, a * z - c * x, b * x - a * y)
}
def perpTo(v:Vec3) = (this.normalize).cross(v.normalize)
}
Unfortunately this doesn't compile:
Vec3.scala:10: error: value cross is not a member of Vec[Vec3]
def perpTo(v:Vec3) = (this.normalize).cross(v.normalize)
^
What's going wrong, and how do I fix it?
Additionally, any references on self-types would be appreciated because I think these errors are cropping up from my lack of understanding.
回答1:
To get rid of all the nastiness, you have to specify that the type parameter V
is a subclass of Vec
.
Now you can just use V
everywhere, because your trait knows that V
inherits all Vec[V]
methods.
trait Vec[V <: Vec[V]] { self: V =>
def -(v:V): V
def /(d:Double): V
def dot(v:V): Double
def norm:Double = math.sqrt(this dot this)
def normalize: V = self / norm
def dist(v: V) = (self - v).norm
def nasty(v: V) = (self / norm).norm
}
Note the method nasty
which won’t compile with Easy Angel’s approach.
回答2:
I think, that method /
in Vec
should return V
instead of Vec[V]
:
trait Vec[V] { self:V =>
def /(d:Double): V
def dot(v:V):Double
def norm:Double = math.sqrt(this dot this)
def normalize = self / norm
}
method cross exists in Vec3
(or in other words in V
) but not in Vec[V]
来源:https://stackoverflow.com/questions/4776864/scala-self-type-value-is-not-a-member-error