Scala中類型不相等的證據

Question

有沒有什麼辦法來約束一個方法，只有在兩種類型都被證明是而不是是相等時纔有意義？

trait Something[A, B] { 
    // I can only be called if type A is the same as type B 
    def ifEqual(implicit ev: A =:= B) 

    // Now I cannot be called if type A is proven to be the same as type B 
    def ifNotEqual(implicit ev: A ??? B) 
} 

Answer 1

是的。來自shapeless，

// Type inequalities 
trait =:!=[A, B] 

implicit def neq[A, B] : A =:!= B = new =:!=[A, B] {} 
implicit def neqAmbig1[A] : A =:!= A = ??? 
implicit def neqAmbig2[A] : A =:!= A = ???