Database.hs

{-# LANGUAGE MultiParamTypeClasses #-}

module DataLib where

class Record a where
  getId :: a -> Int

insert :: Record a => a -> [a] -> [a]
insert r rs = r : rs

update :: Record a => a -> [a] -> [a]
update r rs = map (\r' -> if getId r == getId r' then r else r') rs

delete :: Record a => Int -> [a] -> [a]
delete i rs = filter (\r' -> getId r' /= i) rs


data Person = Person { personId :: Int, name :: String }
instance Record Person where
  getId = personId

data Book = Book { bookId :: Int, title :: String }
instance Record Book where
  getId = bookId

data Database = Database { employees :: [Person], books :: [Book], customers :: [Person] }

class (Record r) => Table r t where
  getRecords :: t -> Database -> [r]
  setRecords :: t -> [r] -> Database -> Database

insertRecord :: (Table r t) => t -> r -> Database -> Database
insertRecord t r db =
  let records = getRecords t db
      newRecords = insert r records
  in setRecords t newRecords db

updateRecord :: (Table r t) => t -> r -> Database -> Database
updateRecord t r db =
  let records = getRecords t db
      newRecords = update r records
  in setRecords t newRecords db

deleteRecord :: (Table r t) => t -> Int -> Database -> Database
deleteRecord t i db =
  let records = getRecords t db
      newRecords = delete i records
  in setRecords t newRecords db

data ET = EmployeeTable
instance (Table Person) ET where
  getRecords t db = employees db
  setRecords t records db = Database records (books db) (customers db)

data BT = BookTable
instance (Table Book) BT where
  getRecords t db = books db
  setRecords t records db = Database (employees db) records (customers db)

data CT = CustomerTable
instance (Table Person) CT where
  getRecords t db = customers db
  setRecords t records db = Database (employees db) (books db) records

db = Database [] [] []
bookTable = BookTable
book = Book { bookId = 0, title = "" }
db1 = insertRecord bookTable book db

Program.cs

using System.Linq;

namespace DataLib {
    interface IRecord { int Id { get; } }
    static class RecordExt {
        static public TRecord[] Insert<TRecord>(this TRecord[] rs, TRecord r) where TRecord : IRecord {
            return rs.Concat(new[] { r }).ToArray();
        }
        static public TRecord[] Update<TRecord>(this TRecord[] rs, TRecord r) where TRecord : IRecord {
            return rs.Select(r1 => r1.Id == r.Id ? r : r1).ToArray();
        }
        static public TRecord[] Delete<TRecord>(this TRecord[] rs, int i) where TRecord : IRecord {
            return rs.Where(r => r.Id != i).ToArray();
        }
    }

    // We make fields public/mutable for brevity, but use is immutably everywhere.
    class Person : IRecord {
        public int Id { get; set; }
        public string Name { get; set; }
    }
    class Book : IRecord {
        public int Id { get; set; }
        public string Title { get; set; }
    }

    class Database {
        public Person[] Employees;
        public Book[] Books;
        public Person[] Customers;
    }

    interface ITable<TRecord> where TRecord : IRecord {
        TRecord[] GetRecords(Database db);
        Database SetRecords(TRecord[] records, Database db);
    }

    static class TableExt {
        static public Database InsertRecord<TRecord>(this ITable<TRecord> t, TRecord r, Database db) where TRecord : IRecord {
            var records = t.GetRecords(db);
            var newRecords = records.Insert(r);
            return t.SetRecords(newRecords, db);
        }
        static public Database UpdateRecord<TRecord>(this ITable<TRecord> t, TRecord r, Database db) where TRecord : IRecord {
            var records = t.GetRecords(db);
            var newRecords = records.Update(r);
            return t.SetRecords(newRecords, db);
        }
        static public Database DeleteRecord<TRecord>(this ITable<TRecord> t, int i, Database db) where TRecord : IRecord {
            var records = t.GetRecords(db);
            var newRecords = records.Delete(i);
            return t.SetRecords(newRecords, db);
        }
    }

    class EmployeeTable : ITable<Person> {
        public Person[] GetRecords(Database db) {
            return db.Employees;
        }
        public Database SetRecords(Person[] records, Database db) {
            return new Database { Employees = records, Books = db.Books, Customers = db.Customers };
        }
    }

    class BookTable : ITable<Book> {
        public Book[] GetRecords(Database db) {
            return db.Books;
        }
        public Database SetRecords(Book[] records, Database db) {
            return new Database { Employees = db.Employees, Books = records, Customers = db.Customers };
        }
    }

    class CustomerTable : ITable<Person> {
        public Person[] GetRecords(Database db) {
            return db.Customers;
        }
        public Database SetRecords(Person[] records, Database db) {
            return new Database { Employees = db.Employees, Books = db.Books, Customers = records };
        }
    }

    static class Program {
        static void Main() {
            var db = new Database { Employees = new Person[0], Books = new Book[0], Customers = new Person[0] };
            var bookTable = new BookTable();
            var book = new Book { Id = 0, Title = "" };
            db = bookTable.InsertRecord(book, db);
        }
    }

}