Getting around invariant result type in State - scala

Question

I would like to define a State that builds a concrete subtype of a trait, as per decodeFoo:
sealed trait Foo
case class Bar(s: String) extends Foo
case class Baz(i: Int) extends Foo
val int: State[Seq[Byte], Int] = State[Seq[Byte], Int] {
case bs if bs.length >= 4 =>
bs.drop(4) -> ByteBuffer.wrap(bs.take(4).toArray).getInt
case _ => sys.error(s"Insufficient data remains to parse int")
}
def bytes(len: Int): State[Seq[Byte], Seq[Byte]] = State[Seq[Byte], Seq[Byte]] {
case bs if bs.length >= len => bs.drop(len) -> bs.take(len)
case _ => sys.error(s"Insufficient data remains to parse $len bytes")
}
val bytes: State[Seq[Byte], Seq[Byte]] = for {
len <- int
bs <- bytes(len)
} yield bs
val string: State[Seq[Byte], String] = bytes.map(_.toArray).map(new String(_, Charset.forName("UTF-8")))
val decodeBar: State[Seq[Byte], Bar] = string.map(Bar)
val decodeBaz: State[Seq[Byte], Baz] = int.map(Baz)
val decodeFoo: State[Seq[Byte], Foo] = int.flatMap {
case 0 => decodeBar
case 1 => decodeBaz
}
This will not compile as State is defined in cats as type State[S, A] and the compiler responds:
Error:(36, 15) type mismatch;
found : cats.data.State[Seq[Byte],FooBarBaz.this.Bar]
(which expands to) cats.data.IndexedStateT[cats.Eval,Seq[Byte],Seq[Byte],FooBarBaz.this.Bar]
required: cats.data.IndexedStateT[cats.Eval,Seq[Byte],Seq[Byte],FooBarBaz.this.Foo]
Note: FooBarBaz.this.Bar <: FooBarBaz.this.Foo, but class IndexedStateT is invariant in type A.
You may wish to define A as +A instead. (SLS 4.5)
case 0 => decodeBar
I can work around this by widening the definitions of decodeBar & decodeBaz to be of type State[Seq[Byte], Foo]. Is that the best way forward? Or can I take a different approach that avoids widening these types?

Answer 1

Functor.widen
Functor.widen should do the trick. Full compilable example (with kind-projector):
import cats.data.State
import cats.Functor
object FunctorWidenExample {
locally {
sealed trait A
case class B() extends A
val s: State[Unit, B] = State.pure(new B())
val t: State[Unit, A] = Functor[State[Unit, ?]].widen[B, A](s)
}
}
in your case, it would be something like:
val decodeFoo: State[Seq[Byte], Foo] = int.flatMap {
case 0 => Functor[State[Seq[Byte], ?]].widen[Bar, Foo](decodeBar)
case 1 => Functor[State[Seq[Byte], ?]].widen[Bar, Foo](decodeBaz)
}
Other possible work-arounds
(not really necessary, just to demonstrate the syntax that might be less known):
Explicit type ascriptions:
val decodeFoo: State[Seq[Byte], Foo] = int.flatMap {
case 0 => decodeBar.map(x => (x: Foo))
case 1 => decodeBaz.map(x => (x: Foo))
}
Using <:< as method (those things actually do have a meaningful apply):
val decodeFoo: State[Seq[Byte], Foo] = int.flatMap {
case 0 => decodeBar.map(implicitly: Bar <:< Foo)
case 1 => decodeBaz.map(implicitly: Baz <:< Foo)
}