Type mismatch when using higher-kinded types

compiler-errors, higher-kinded-types, scala, types

Solution

Type labmdas should help:

  class Bar[T[_]] {
    def bar[A]: Option[T[A]] = None
  }

  def foo[A] = {
    new Bar[({type M[B] = Map[A, B]})#M]
  }

  val f: Option[Map[String, Int]] = foo[String].bar[Int]

However I can't answer why type T doesn't work in this case.

Problem

In a library, there is a class with a higher-kinded type taking one type parameter. I want to give it a type that takes two type parameters, so I use a `type` expression to fix the other parameter. But it doesn't turn out like I expect. The code reduces to this: ``` object Main { class Bar[T[_]] { def bar[A]: Option[T[A]] = None } def foo[A] = { type T[B] = Map[A, B] new Bar[T] } val f: Option[Map[String, Int]] = foo[String].bar[Int] } ``` I get an error when compiling (Scala 2.11.4): ``` test.scala:12: error: type mismatch; found : Option[T[Int]] (which expands to) Option[scala.collection.immutable.Map[A,Int]] required: Option[Map[String,Int]] val f: Option[Map[String, Int]] = foo[String].bar[Int] ^ one error found ``` Why is there a type error?

Original source