绕过状态中的固定结果类型

Question

我想定义一个State，它构建了一个特征的具体子类型，如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
}

这不会编译，因为 cats 中State定义为 type State[S, A] 编译器响应：

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

我可以通过将decodeBar和decodeBaz的定义扩大到State[Seq[Byte], Foo]型来解决此问题。这是最好的出路吗？或者我可以采取不同的方法来避免扩大这些类型？

Answer 1

Functor.widen

Functor.widen应该可以做到这一点。完全可编译的示例（使用投影仪）：

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)
  }
}

在您的情况下，它将是这样的：

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)
}

---

其他可能的解决方法

（不是真的必要，只是为了演示可能鲜为人知的语法）：

• 显式类型归属：

  val decodeFoo: State[Seq[Byte], Foo] = int.flatMap {
    case 0 => decodeBar.map(x => (x: Foo))
    case 1 => decodeBaz.map(x => (x: Foo))
  }
  

• 使用<:<作为方法（这些东西实际上确实有一个有意义的apply）：

  val decodeFoo: State[Seq[Byte], Foo] = int.flatMap {
    case 0 => decodeBar.map(implicitly: Bar <:< Foo)
    case 1 => decodeBaz.map(implicitly: Baz <:< Foo)
  }