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package fj.control.db;

import fj.F;
import fj.Function;

import java.sql.Connection;
import java.sql.SQLException;
import java.util.concurrent.Callable;

/**
 * The DB monad represents a database action, or a value within the context of a database connection.
 */
public abstract class DB {

  /**
   * Executes the database action, given a database connection.
   *
   * @param c The connection against which to execute the action.
   * @return The result of the action.
   * @throws SQLException if a database error occurred.
   */
  public abstract A run(final Connection c) throws SQLException;

  /**
   * Constructs a database action as a function from a database connection to a value.
   *
   * @param f A function from a database connection to a value.
   * @return A database action representing the given function.
   */
  public static  DB db(final F f) {
    return new DB() {
      public A run(final Connection c) {
        return f.f(c);
      }
    };
  }

  /**
   * Returns the callable-valued function projection of this database action.
   *
   * @return The callable-valued function which is isomorphic to this database action.
   */
  public final F> asFunction() {
    return c -> () -> run(c);
  }

  /**
   * Map a function over the result of this action.
   *
   * @param f The function to map over the result.
   * @return A new database action that applies the given function to the result of this action.
   */
  public final  DB map(final F f) {
    return new DB() {
      public B run(final Connection c) throws SQLException {
        return f.f(DB.this.run(c));
      }
    };
  }

  /**
   * Promotes any given function so that it transforms between values in the database.
   *
   * @param f The function to promote.
   * @return A function equivalent to the given one, which operates on values in the database.
   */
  public static  F, DB> liftM(final F f) {
    return a -> a.map(f);
  }

  /**
   * Constructs a database action that returns the given value completely intact.
   *
   * @param a A value to be wrapped in a database action.
   * @return A new database action that returns the given value.
   */
  public static  DB unit(final A a) {
    return new DB() {
      public A run(final Connection c) {
        return a;
      }
    };
  }

  /**
   * Binds the given action across the result of this database action.
   *
   * @param f The function to bind across the result of this database action.
   * @return A new database action equivalent to applying the given function to the result of this action.
   */
  public final  DB bind(final F> f) {
    return new DB() {
      public B run(final Connection c) throws SQLException {
        return f.f(DB.this.run(c)).run(c);
      }
    };
  }

  /**
   * Removes one layer of monadic structure.
   *
   * @param a A database action that results in another.
   * @return A new database action equivalent to the result of the given action.
   */
  public static  DB join(final DB> a) {
    return a.bind(Function.>identity());
  }
}