[英]Multiple type parameters in type class
I want to use type class to design convert interface and codes are as below: 我想使用类型类来设计转换接口,代码如下:
case class Kilograms(value: Double)
case class Pounds(value: Double)
trait Convert[T, U] {
def convert(input: T): U
}
object Convert {
def apply[T:Convert, U:Convert] = implicitly[Convert[T,U]]
def covert[T, U](input: T)(implicit c: Convert[T, U]): U = c.convert(input)
implicit object kilogramsToPounds extends Convert[Kilograms, Pounds] {
override def convert(input: Kilograms): Pounds = Pounds(input.value * 2.20462)
}
implicit object poundsToKilograms extends Convert[Pounds, Kilograms] {
override def convert(input: Pounds): Kilograms = Kilograms(input.value / 2.20462)
}
}
But compile error: 但编译错误:
Error: wrong number of type arguments for A$A95.this.Convert, should be 2
def apply[T:Convert, U:Convert] = implicitly[Convert[T,U]]
Error: could not find implicit value for parameter e: A$A95.this.Convert[T,U]
def apply[T:Convert, U:Convert] = implicitly[Convert[T,U]]
Error: not enough arguments for method implicitly: (implicit e: A$A95.this.Convert[T,U])A$A95.this.Convert[T,U].
Unspecified value parameter e.
def apply[T:Convert, U:Convert] = implicitly[Convert[T,U]]
If I change def apply[T:Convert, U:Convert] = implicitly[Convert[T,U]]
into def apply[T, U](implicit c: Convert[T, U]): Convert[T, U] = c
, no compile error!!! 如果我改变
def apply[T:Convert, U:Convert] = implicitly[Convert[T,U]]
为def apply[T, U](implicit c: Convert[T, U]): Convert[T, U] = c
,没有编译错误!
I would like to know what's going on? 我想知道发生了什么事? Also, I look up some information, context bound is restricted with single type parameter(?) If I want to implement multiple type parameter type class, how should I do?
另外,我查了一些信息,上下文绑定是用单一类型参数限制的(?)如果我想实现多个类型参数类型类,我该怎么办?
The context-bound syntax T : U
only works for types U
that have only one type parameter S
(which conforms to T
). 上下文绑定语法
T : U
仅适用于只有一个类型参数 S
(符合T
)的类型U
This is valid because you are manually declaring the implicit for Convert[T, U]
: 这是有效的,因为您手动声明了
Convert[T, U]
的隐式:
def covert[T, U](input: T)(implicit c: Convert[T, U]): U = c.convert(input)
The following is not valid, because the compiler de-sugars the context bounds to Convert[T]
and Convert[U]
respectively, which doesn't make sense. 以下内容无效,因为编译器分别对
Convert[T]
和Convert[U]
的上下文边界进行去糖,这没有意义。
def apply[T:Convert, U:Convert] = implicitly[Convert[T,U]]
(attempts to de-sugar to) (尝试脱糖)
def apply[T, U](implicit ev1: Convert[T], ev2: Convert[U]) = ...
See the SLS 7.4 - Context and View Bounds : 请参阅SLS 7.4 - 上下文和视图边界 :
A type parameter
A
of a method or non-trait class may also have one or more context boundsA : T
.方法或非特征类的类型参数
A
也可以具有一个或多个上下文边界A : T
。 In this case the type parameter may be instantiated to any typeS
for which evidence exists at the instantiation point thatS
satisfies the boundT
.在这种情况下,类型参数可以被实例化为任何类型
S
对于该类型S
,证据存在于S
满足绑定T
的实例化点处。 Such evidence consists of an implicit value with typeT[S]
.这些证据由类型为
T[S]
的隐含值组成。
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