类型构造是抽象类型或高阶类型实例化的过程；
类型构造器是任意输入一个或几个已有类型，能够生成新类型的类型；
https://www.cnblogs.com/feng9exe/p/9925027.html
Type constructors
Essentially what HKT gives us is the ability to generalize across type constructors – where a type constructor is anything that has a type parameter. For instance List[_]* is not a type, the underscore is a hole into which another type may be plugged, constructing a complete type. List[String] and List[Int] being examples of complete (or distinct) types.
https://www.cnblogs.com/feng9exe/p/9156277.html
a type constructor is a feature of a typed formal language that builds new types from old ones.
https://en.wikipedia.org/wiki/Type_constructor
trait Functor[F[_]] {
def map[A, B](a: F[A])(f: A => B): F[B]
}
class Functor f where
fmap :: (a -> b) -> f a -> f b
由 f a 和 f b 我们可知，f 不是类型，而是类型构造器（type constructor），即 f 应接受另一类型作为参数并返回一个具体的类型（更精准的表达则是 f 的 kind 必须是 * -> *）。
函数作为参数或作为返回值的函数称为 高阶函数。在Scala的immutable.List.的方法中存在大量的高阶函数，我们看看其中一个map方法
class List[+A] ... {
def map[B](f: A => B) : List[B]
}
https://my.oschina.net/Barudisshu/blog/472564