为什么Scala中单元素元组没有Tuple1 Literal？

implicit def value2tuple[T](x: T): Tuple1[T] = Tuple1(x)

隐式转换仅适用于参数的静态类型不符合方法参数的类型.假设你的方法需要一个Product参数

def m(v: Product) = // ...

转换将适用于非产品价值,但不适用于例如Tuple2.警告：所有案例类都会扩展“产品特征”,因此转换也不适用于它们.相反,product元素将是case类的构造函数参数.

产品是TupleX类的最小上限,但如果要将隐式Tuple1转换应用于所有非元组,则可以使用类型类：

// given a Tupleable[T],you can call apply to convert T to a Product
sealed abstract class Tupleable[T] extends (T => Product)
sealed class ValueTupler[T] extends Tupleable[T] { 
   def apply(x: T) = Tuple1(x) 
}
sealed class TupleTupler[T <: Product] extends Tupleable[T] { 
   def apply(x: T) = x 
}

// implicit conversions
trait LowPriorityTuple {
   // this provides a Tupleable[T] for any type T,but is the 
   // lowest priority conversion
   implicit def anyIsTupleable[T]: Tupleable[T] = new ValueTupler
}
object Tupleable extends LowPriorityTuple {
   implicit def tuple2isTuple[T1,T2]: Tupleable[Tuple2[T1,T2]] = new TupleTupler
   implicit def tuple3isTuple[T1,T2,T3]: Tupleable[Tuple3[T1,T3]] = new TupleTupler
   // ... etc ...
}

您可以在API中使用此类型的类,如下所示：

def m[T: Tupleable](v: T) = { 
   val p = implicitly[Tupleable[T]](v) 
   // ... do something with p
}

如果您的方法返回产品,您可以看到如何应用转化：

scala> def m[T: Tupleable](v: T) = implicitly[Tupleable[T]](v)
m: [T](v: T)(implicit evidence$1: Tupleable[T])Product

scala> m("asdf") // as Tuple1
res12: Product = (asdf,)

scala> m(Person("a","n")) // also as Tuple1,*not* as (String,String)
res13: Product = (Person(a,n),)

scala> m((1,2)) // as Tuple2
res14: Product = (1,2)