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scalascalazstructural-typing

Weird nested structural type in generics

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]]
Solution

  • 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.