Inference of traits in scala

Question

I'm having trouble with type inference in scala. I'm using classes with higher-kinded types, which extend some traits; but the scala compiler can't resolve the types to the traits they extend. The minimal example is shown here:

trait traitA[X] trait traitB[X] class A[X] extends traitA[X] {} class B extends traitB[C] {} class C {} val a = Seq[A[B]]() val b: Seq[traitA[traitB[C]]] = a Error: type mismatch; found : Seq[A[B]] required: Seq[traitA[traitB[C]]] val b: Seq[traitA[traitB[C]]] = a

I can get a Seq[traitA[B]] but not a Seq[traitA[traitB[C]]].

What am I missing here?

Thanks for the help

Works with Seq[A[traitB[C]]](), no?

user2864740
– user2864740

2018-02-23 16:49:54 +00:00
Commented Feb 23, 2018 at 16:49 — user2864740
– user2864740, Commented Feb 23, 2018 at 16:49

nicodp · Accepted Answer · 2018-02-23 17:10:00Z

You have to ask for covariance in traitA as follows:

trait traitA[+X]

And you'll get:

scala> val b: Seq[traitA[traitB[C]]] = a res0: Seq[traitA[traitB[C]]] = List()

Why? because A[X] is a subtype of traitA[X] and you want to enforce that Seq[A[X]] is also a subtype of Seq[traitA[X]].

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