Functor Instance for Type Constructor with Two Parameters in Scala

Question

I have a class Foo with two parameters, and I am trying to write a Functor instance for Foo with the first parameter fixed, as follows:

object S         


        

Answer 1

but inspection of their source code reveals an odd ?, which doesn't compile for me

? comes from the kind-projector project, which is a Scala compiler plugin you need to add to your build.sbt:

resolvers += Resolver.sonatypeRepo("releases")

addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.9.4")

Which prettifies the creation of type lambdas for you:

implicit def fooInstances[X]: Functor[Foo[X, ?]] =
  new Functor[Foo[X, ?]] {
    def fmap[A, B](f: A => B): Foo[X, A] => Foo[X, B] =
      foo => Foo[X, B](f(foo.value))
}

Remember that we can also use partial type application with type aliases:

implicit def fooInstances[X] = {
  type PartiallyAppliedFoo[A] = Foo[X, A]
  new Functor[PartiallyAppliedFoo] {
    override def fmap[A, B](f: (A) => B): (PartiallyAppliedFoo[A]) => PartiallyAppliedFoo[B] = foo => Foo[X, B](f(foo.value))
  }
}

Edit (05/04/2020)

Note the syntax for kind project has changed from ? to * for partially applied types, thus the above example should be:

sbt:

resolvers += Resolver.sonatypeRepo("releases")

addCompilerPlugin("org.spire-math" %% "kind-projector" % "0.11.0")

Code:

implicit def fooInstances[X]: Functor[Foo[X, *]] =
  new Functor[Foo[X, *]] {
    def fmap[A, B](f: A => B): Foo[X, A] => Foo[X, B] =
      foo => Foo[X, B](f(foo.value))
}