gadts-session1.scala

package net.degoes.zio

trait Sql {
  type ColumnName
  type TableName

  sealed trait Table[+A]

  /**
   * (SELECT *, "foo", table.a + table.b AS sum... FROM table WHERE cond) UNION (SELECT ... FROM table)
   * UPDATE table SET ...
   * INSERT ... INTO table
   * DELETE ... FROM table
   */
  sealed trait Sql[-A, +B]

  /**
   * A `Read[A]` models a selection of a set of values of type `A`.
   */
  sealed trait Read[+A]
  object Read {
    final case class Select[A, B](
      selection: Selection[A, B], table: Table[A], where: Where[A]) extends Read[B]
    
    final case class Union[B](left: Read[B], right: Read[B], distinct: Boolean) extends Read[B]

    final case class Literal[B](values: Iterable[B]) extends Read[B]
  }

  /**
   * A columnar selection of `B` from a source `A`, modeled as `A => B`.
   */
  sealed trait Selection[-A, +B]
  object Selection {
    final case class Identity[A]() extends Selection[A, A]
    final case class Constant[A](value: A) extends Selection[Any, A]
    final case class Concat[A, L, R](left: Selection[A, L], right: Selection[A, R]) 
      extends Selection[A, (L, R)]
    final case class Computed[A, B](expr: Expr[A, B], name: Option[ColumnName]) extends 
      Selection[A, B]
  }

  /**
   * Models a function `A => B`.
   */
  sealed trait Expr[-A, +B]

  /**
   * Models a function `A => Boolean` that decides whether to retain elements
   * in a source structure `A`.
   * 
   * {{{
   * WHERE (age * 2) >= 32 && name IS NOT NULL
   * }}}
   */
  sealed trait Where[-A] { self =>
    def && [A1 <: A](that: Where[A1]): Where[A1] = Where.And(self, that)

    def || [A1 <: A](that: Where[A1]): Where[A1] = Where.Or(self, that)
  }
  object Where {
    final case class And[A](left: Where[A], right: Where[A]) extends Where[A]
    final case class Or[A](left: Where[A], right: Where[A]) extends Where[A]
    final case class Relation[A, B](expr: Expr[A, B], predicate: Predicate[B]) extends Where[A]
  }

  sealed trait Predicate[-A]
  object Predicate {
    final case class Equals[A](right: A) extends Predicate[A]
    case object IsNull extends Predicate[Any]
    case object IsNotNull extends Predicate[Any]
  }


}