CODEWITHSUNDEEP
scala
Reza Sameei
Reza Sameei

Reputation: 838

How to fix Type Mismatch Error on path dependent types in Scala (2.11)?

I have a code like this

trait Toy

trait Child {
    type T <: Toy
    def toys : Seq[T]
    def play(toys: Seq[T]): Unit
}

trait Parent {
    type C <: Child
    def firstChild: C
}

trait Home {
    def parent: Parent
    def toys: Seq[Parent#C#T]
    def apply() = {
        val ts = toys
        parent.firstChild.play(toys)
    }
}

but I can't compile it:

[error] .../module/common/src/main/scala/test/Debug.scala:21:32: type mismatch;
[error]  found   : Seq[test.Parent#C#T]
[error]  required: Seq[_12.T] where val _12: test.Parent#C
[error]         parent.firstChild.play(toys)

Is there any way to fix this error without converting all abstract-types to param-types?

Upvotes: 0

Views: 136

Answers (1)

ghik
ghik

Reputation: 10764

One way to make it compile is:

trait Toy

trait Child {
    type T <: Toy
    def toys : Seq[T]
    def play(toys: Seq[T]): Unit
}

trait Parent { parent =>
    type C <: Child
    val firstChild: C
}

trait Home {
    val parent: Parent
    def toys: Seq[parent.firstChild.T]
    def apply() = {
        val ts = toys
        parent.firstChild.play(toys)
    }
}

Remember that type members are bound to their outer instances. If you give a toy to a child, it must be a toy meant specifically for that particular child of that particular parent (parent.firstChild.T), not any potential child of any potential parent (Parent#C#T).

Upvotes: 2

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