connection.ts

/* Just a fake NodeJS db api exposed by mysql, mssql, etc...*/
interface FakeTrans {}
interface FakeConnection {
  query(sql: string, trans?: FakeTrans): Promise<ResultSet>;
  beginTransaction(): Promise<FakeTrans>;
  commitTransaction(trans: FakeTrans): Promise<void>;
  rollbackTransaction(trans: FakeTrans): Promise<void>;
}
declare function fakeConnect(): Promise<FakeConnection>;

/* The domain of errors and result of the DB service  */
type QueryError = unknown;
type ConnectionError = unknown;
type ResultSet = any[][];

interface Connection {
  query(query: string): T.Effect<unknown, QueryError, ResultSet>;
  transaction<R, E, A>(
    eff: T.Effect<Has<Connection> & R, E, A>
  ): T.Effect<R, E | QueryError, A>;
}
const Connection = tag<Connection>();

function makeConnection(): T.Effect<unknown, ConnectionError, Connection> {
  return pipe(
    T.promise(fakeConnect), // <== here we create the root connection,
    T.map((connectionHandle) => ({
      query: (sql: string) => // implementation of regular query without trans
        T.promise(() => connectionHandle.query(sql, undefined)), 
      transaction: <R, E, A>(
        eff: T.Effect<Has<Connection> & R, E, A>
      ): T.Effect<R, E | QueryError, A> =>
        T.bracketExit_(
          T.promise(connectionHandle.beginTransaction), // acquire trans
          (trans) => {
            const connectionInTrans: Connection = {
              query: (sql: string) =>
                T.promise(() => connectionHandle.query(sql, trans)),
              transaction: <R, E, A>(
                eff: T.Effect<Has<Connection> & R, E, A>
              ): T.Effect<R, E | QueryError, A> =>
                T.provideService(Connection)(connectionInTrans)(eff), // if already in trans, just use current trans
            };
            return T.provideService(Connection)(connectionInTrans)(eff);
          },
          (trans, exit) => // release trans by rollback if failed or interrupt or commit if done
            T.promise(() =>
              exit._tag === "Failure"
                ? connectionHandle.rollbackTransaction(trans)
                : connectionHandle.commitTransaction(trans)
            )
        ),
    }))
  );
}