Doobie Raw sql interpolation

RawSqlInterpolator.scala

import doobie.util.param.Param
import doobie.util.pos.Pos

/**
 * Custom Interpolator with support for raw sql.
 * `Composite.toList` is used retrieve and insert raw sql.
 */
final class RawSqlInterpolator(private val sc: StringContext)(implicit pos: Pos) {
  private def mkFragment[A](a: A)(implicit ev: Param[A]): Fragment = {
    val interleave = ev.composite.toList(a).map {
      case SiteListTable.RawSql(sql) => sql
      case _ => "?"
    }

    val sql = sc.parts
      .zipAll(interleave, "", "")
      .map { case (a, b) => a + b }
      .mkString("")

    Fragment(sql, a, Some(pos))(ev.composite)
  }

  object frRaw extends ProductArgs {
    def applyProduct[A: Param](a: A): Fragment = mkFragment(a)
  }
}

object RawSqlInterpolator {
  import doobie.imports._

  case class RawSql(value: String)

  /**
   * This is an empty composite (length 0, empty Meta).
   * The RawSql is exposed using `toList` which in the latest version is only used for logging.
   * (Cf: https://github.com/tpolecat/doobie/blob/series/0.5.x/modules/core/src/main/scala/doobie/util/composite.scala#L43)
   */
  trait RawSqlComposite[T] extends doobie.util.composite.Composite[T]{
    val length: Int = 0
    val meta: List[(doobie.util.meta.Meta[_], doobie.enum.nullability.NullabilityKnown)] = Nil
    val set: (Int, T) => doobie.free.preparedstatement.PreparedStatementIO[Unit] ={
      (n, a) => FPS.raw( ps => () )
    }
    val update: (Int, T) => doobie.free.resultset.ResultSetIO[Unit] = {
      (n, a) => FRS.raw( rs => () )
    }
  }

  val rawSqlComposite = new RawSqlComposite[RawSql] {
    def toList(sql: RawSql): List[Any] = List(sql)
    val unsafeGet: (java.sql.ResultSet, Int) => RawSql = { (_, _) => RawSql("NO GET ON RawSql") }
  }

  def rawSqlComposite[T](t: T, sql: String) = new RawSqlComposite[T] {
    def toList(a: T): List[Any] = List(RawSql(sql))
    val unsafeGet: (java.sql.ResultSet, Int) => T = { (_, _) => t }
  }
  
  implicit val rawSqlParam = new doobie.util.param.Param(rawSqlComposite)
  implicit def rawSqlInterpolator(sc: StringContext)(implicit pos: Pos): RawSqlInterpolator = new RawSqlInterpolator(sc)(pos)
}

SiteListTable.scala

object SiteListTable {
  import doobie.imports._
  import RawSqlInterpolator._
   
  val tableName = RawSql("site_list")
  
  def selectQuery(ioId: IOId): Update0 = frRaw"""
    SELECT * FROM $tableName WHERE io_id = $ioId
  """.query[SiteList]
}