泛型中奇怪的嵌套结构类型 [英] Weird nested structural type in generics

问题描述

有人可以解释嵌套在泛型中的结构类型的奇怪构造吗:

Can someone explain weird construction of structural type nested in generics:

implicit def Function1Functor[R]: Functor[({type λ[α]=(R) => α})#λ] = 
  new Functor[({type λ[α]=(R) => α})#λ] ....

这个例子来自Scalaz库:Functor.scala

This example comes from Scalaz library: Functor.scala

为什么需要这种结构?写起来不会更简单:

Why this construction is needed there? Wouldn't be simpler to write:

 implicit def Function1Functor[R,A]: Functor[R =>A]

或

 implicit def Function1Functor[R,A]: Functor[Function1[R,A]]

推荐答案

Functor 类型构造函数的签名表明它被另一个一元类型构造函数 F:

The signature of the Functor type constructor shows that it is parameterised with another, unary, type constructor F:

trait Functor[F[_]] extends InvariantFunctor[F]

R =>A 和 Function1[R,A] 都是类型构造函数；它们不带任何参数.

Neither R => A nor Function1[R,A] are type constructors; they take no parameters.

然而在:

type λ[α] = (R) => α

λ 是一个带一个参数 α 的类型构造函数.(R 已在此上下文中定义.)

λ is a type constructor taking one parameter, α. (R is already defined in this context.)

语法 ({type λ[α]=(R) => α})#λ 被称为类型 lambda.这是一种语法技巧，允许内联创建类型别名并通过投影引用，因此可以在需要类型的地方使用整个表达式.

The syntax ({type λ[α]=(R) => α})#λ is known as a type lambda. It is a syntactic trick allowing a type alias to be created inline and referred to via a projection, so the whole expression can be used where a type is required.

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