erasure.worksheet.scala

case class Column[A](name: String)

object Column {
  def string(name: String): Column[String] = Column[String](name)
  def int(name: String): Column[Int]       = Column[Int](name)
}

// Type arguments are unchecked because they are eliminated by erasure at runtime
def select[A](c: Column[A]): A = c match {
  case _: Column[String] => "String"
  case _: Column[Int]    => 42
}

// select(Column.int("col1")) == 42 // this will throw a ClassCastException at runtime

/// How to fix this?

case class ColumnFix[A: ColumnType](name: String) { // ***
  def columnType: ColumnType[A] = implicitly[ColumnType[A]]
}

// 1. Create a new trait to represent a column types
sealed trait ColumnType[A]

// 2. Create a companion object for each column type as implicit values
object ColumnType {
  implicit case object StringColumnType extends ColumnType[String]
  implicit case object IntColumnType    extends ColumnType[Int]
}

object ColumnFix {
  def string(name: String): ColumnFix[String] = ColumnFix[String](name)
  def int(name: String): ColumnFix[Int]       = ColumnFix[Int](name)
}

// 3. Create a function in the `ColumnFix` class that returns the type of the column and add a context bounded to the column type ***

// 4. Create a function to select a column with context bound to the column type.
// This will tell the compiler that we can call the function as long as there is an implicit instance for the column type defined for A.

def selectFix[A: ColumnType](c: ColumnFix[A]): A = c.columnType match {
  case ColumnType.IntColumnType    => 42
  case ColumnType.StringColumnType => "String"
}

selectFix(ColumnFix.int("col1")) == 42
// res0: Boolean = true