sagoez · April 6, 2022 19:55 · Apr 6, 2022
diff --git a/erasure.worksheet.scala b/erasure.worksheet.scala
@@ -0,0 +1,48 @@
+
+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
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