I know that apply method is syntactic sugar when used in companion object.
However, what is apply method for when it is used in type definition just like below?
type Applyn = {
def apply[A](f: A=>A, n: Int, x: A): A
}
Is there a deference between this sentence? As I guess, this sentence is used for assigning generic function value to Applyn.
For example, by using above sentence, we can make
(Int=>Int, Int, Int)=>Int
(String=>String, Int, String)=>String
etc., into only a single type Applyn.
Is this correct?
What you are looking is a structural type. It doesn't refer to any specific type but any type that has a method apply[A](f: A=>A, n: Int, x: A): A. Functions have an apply method, so a function with the correct signature applies, but it's also possible to create types that have that apply method that are not functions. Imagine the following.
object NotAFunction {
def apply[A](f: A=>A, n: Int, x: A): A = {...}
}
This object doesn't extend Function, so it is not a function but it still satisfies the structural type requirement.
Structural types provide a nice syntax but they are very slow, so they should be avoided where possible, especially in performance critical code.
来源：https://stackoverflow.com/questions/43388872/what-is-apply-method-in-scala-especially-used-in-type-definition