Suggestion: if database.c is redone would it be possible to name the functions `database-get' etc rather than `get-database' ? Here is some proof-of-concept code for supporting multiple database-like backends with a generic interface: it supports dbm, pg.el and in-memory/filesystem-persistent backends. The trickiest bit is handling searches uniformly ; billp's syntax could work with SQL and ldap, but for dbm & in-memory would require mapping over the entire database. EIEIO is at <URL:http://www.ultranet.com/~zappo/eieio.shtml> (with stuff like this I occasionally manage to forget I'm programming in a crummy imitation of a Lisp :-).  ;;; db.el -- various types of databases for emacs ;; ;; Copyright: (C) 1999 Eric Marsden ;; Author: Eric Marsden <emarsden(a)mail.dotcom.fr&gt; ;; Time-stamp: <1999-11-16 ecm> ;; ;; ;; quasi-uniform access to different types of databases from emacs: ;; PostgreSQL tables using pg.el, XEmacs dbm/berkeley_db database API, ;; and file+hashtable-based in-memory tables. (require 'cl) (require 'eieio) (defclass db:database () () "The class of objects accessible via the database API") (defclass db:dbm-database (db:database) ((file :initarg :file :documentation "Path to file which contains the data for this database") (access :initarg :access :initform "rw+" :documentation "Access rights to the database (any combination of `r', `w' and `+' for read, write and creation flags)") (mode :initarg :mode :initform 420 :documentation "Permissions on the database") (db :initarg :db :initform nil)) "A dbm database") (defclass db:pg-database (db:database) ((dbname :initform "template1" :documentation "Name of the database") (user :documentation "Username under which to connect to the database") (password :initform "") (port :initform 5432 :documentation "Port where postmaster is listening") (host :initform "localhost") (connection :initform nil)) "A PostgreSQL database, accessed via pg.el") (defclass db:file-database (db:database) ((file :initarg :file :documentation "Path to file which contains the data for this database") (access :initarg :access :initform "rw+" :documentation "Access rights to the database (any combination of `r', `w' and `+' for read, write and creation flags)") (mode :initarg :mode :initform 420 :documentation "Permissions on the database") (db :initarg :db :initform nil)) "A hashtable-based database (persistence via filesystem)") (defgeneric db:open ((db database)) "Initialize a connection to the database") (defgeneric db:close ((db database)) "Close a connection to the database") (defgeneric db:live-p ((db database)) "Is the connection alive?") (defgeneric db:get ((db database) key &optional default) "Extract value corresponding to KEY from the database") (defgeneric db:put ((db database) key value &optional replace) "Add a relation between KEY and VALUE in the database. If REPLACE is non-nil, replace old value if it exists.") (defgeneric db:map ((db database) function) "Apply FUNCTION to each (key, value) pair in the database. Order is unspecified; FUNCTION should take two arguments.") (defgeneric db:remove ((db database) key) "Remove the relation for KEY from the database.")  ;; PostgreSQL methods (using pg.el) (defmethod db:open ((db db:pg-database)) (require 'pg) (setf (slot-value db :connection) (with-slots (db) (pg:connect dbname user password host port)))) (defmethod db:close ((db db:pg-database)) (require 'pg) (pg:disconnect (slot-value db :connection)) (setf (db:pg-database-connection db) nil)) (defmethod db:live-p ((db db:pg-database)) (slot-value db :connection)) (defmethod db:get ((db db:pg-database) key &optional default) (require 'pg) (let* ((con (slot-value db :connection)) (sql (format "SELECT value FROM emacs_db WHERE key = %s" key)) (res (pg:exec con sql))) (pg:result res :tuple 1))) (defmethod db:put ((db db:pg-database) key value &optional replace) ) (defmethod db:map ((db db:pg-database) function) ) (defmethod db:remove ((db db:pg-database) key) )  ;; dbm methods using XEmacs API (when (fboundp 'databasep) (defmethod db:open ((db db:dbm-database)) (setf (slot-value db :db) (with-slots ((f file) (a access) (m mode)) db (open-database f 'dbm nil a m)))) (defmethod db:close ((db db:dbm-database)) (close-database (slot-value db :db))) (defmethod db:live-p ((db db:dbm-database)) (slot-value db :db)) (defmethod db:get ((db db:dbm-database) key &optional default) (get-database key (slot-value db :db) default)) (defmethod db:put ((db db:dbm-database) key val &optional replace) (put-database key val (slot-value db :db) replace)) (defmethod db:map ((db db:dbm-database) function) (map-database function (slot-value db :db))) (defmethod db:remove ((db db:dbm-database) key) (remove-database key (slot-value db :db))) ) ;; (fboundp 'databasep)  ;; hash-table databases (in memory until closed) (defmethod db:open ((db db:file-database)) (let* ((filename (slot-value db :file)) (access (slot-value db :access)) buf data table) (cond ((position ?+ access) ;; start with an empty hash table (setq table (make-hash-table :test #'string=)) (setf (slot-value db :db) table)) (t ;; read data from disk into hash table (unless (file-readable-p filename) (error "database is not readable" filename)) (setq buf (find-file-noselect filename t t)) (setq data (read buf)) (setq table (make-hash-table :test #'string=)) (mapcar #'(lambda (c) (setf (gethash (car c) table) (cdr c))) table) (setf (slot-value db :db) table))))) (defmethod db:close ((db db:file-database)) (let* ((filename (slot-value db :file)) (access (slot-value db :access)) (table (slot-value db :db)) (buf (get-buffer-create " *db-work*")) (alist '())) (when (and (position ?+ access) (file-exists-p filename)) (delete-file filename)) ;; write data to alist (maphash #'(lambda (key val) (push (cons key val) alist)) table) (save-excursion (set-buffer buf) (erase-buffer) (prin1 alist) (write-file filename) (kill-buffer buf)))) (defmethod db:live-p ((db db:file-database)) (slot-value db :db)) (defmethod db:get ((db db:file-database) key &optional default) (gethash key (slot-value db :db) default)) (defmethod db:put ((db db:file-database) key val &optional replace) (setf (gethash key (slot-value db :db)) val)) (defmethod db:map ((db db:file-database) function) (maphash function (slot-value db :db))) (defmethod db:remove ((db db:file-database) key) (remhash key (slot-value db :db)))  ;; testing (defun db:test/dbm () (require 'db) (let ((db (make-instance 'db:dbm-database :file "/tmp/try"))) (db:open db) (loop for i from 1 to 200 for positive = (format "%d" i) for negative = (format "%d" (- i)) do (db:put db positive negative)) (loop for i from 200 downto 1 for positive = (format "%d" i) for negative = (format "%d" (- i)) unless (string= (db:get db positive) negative) do (error "test failed for i = %d" i)) (db:close db))) (defun db:test/file () (require 'db) (let ((db (make-instance 'db:file-database :file "/tmp/zob.db"))) (db:open db) (loop for i from 1 to 200 for positive = (format "%d" i) for negative = (format "%d" (- i)) do (db:put db positive negative)) (loop for i from 200 downto 1 for positive = (format "%d" i) for negative = (format "%d" (- i)) unless (string= (db:get db positive) negative) do (error "test failed for i = %d" i)) (db:close db))) (provide 'db) ;; db.el ends here