Illustration of using Scala macro to statically prevent nested transactions using the scalasql library

preventNestedTransaction.scala

// See https://github.com/com-lihaoyi/scalasql
import scalasql.*

/** Shorthand for delimited transaction:
 *
 * ```
 * db.txn:
 *   val data = runSomeQuery()
 *   doSomeUpdate(...)
 * ```
 */
extension (db: DbClient)
  /** Runs `f` as part of a database transaction */
  inline def txn[T](f: DbApi.Txn ?=> T): T =
    TxnMacros.txnImpl(db, f)

object TxnMacros:
  import scala.quoted.*
  import scala.compiletime.{error, summonFrom}

  def txnImpl[T](db: Expr[DbClient], f: Expr[DbApi.Txn ?=> T])(using _quotes: Quotes, _typeT: Type[T]): Expr[T] =
    '{
      checkNoNestedTransaction()
      $db.transaction(txn => $f(using txn))
    }

  inline def checkNoNestedTransaction(): Unit =
    summonFrom[DbApi.Txn]:
      case _: DbApi.Txn => error("Nested transactions not supported")
      case _            => ()