问题
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] ....
This example comes from Scalaz library: Functor.scala
Why this construction is needed there? Wouldn't be simpler to write:
implicit def Function1Functor[R,A]: Functor[R =>A]
or
implicit def Function1Functor[R,A]: Functor[Function1[R,A]]
回答1:
The signature of the Functor type constructor shows that it is parameterised with another, unary, type constructor F:
trait Functor[F[_]] extends InvariantFunctor[F]
Neither R => A nor Function1[R,A] are type constructors; they take no parameters.
However in:
type λ[α] = (R) => α
λ is a type constructor taking one parameter, α. (R is already defined in this context.)
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.
来源：https://stackoverflow.com/questions/8243680/weird-nested-structural-type-in-generics