% File:         ECMI01.MINIDB
% Updated:	15 October 84 11:10
% Author:	Peter Ross
% Purpose:	a mini database system, needing improvement in every way
% Level:	introductory.

% This is extremely feeble. Learning exercise: improve it.
%
% It uses the following built-in predicates:
%	assert(Term)		- to add to the database
%	retract(Term)		- to remove from the database
%	read(Term)		- to read data (in the form of a term) or
%				  replies (also in the form of a term) from
%				  the user.
%	abolish(Term,Arity)	- to flush out the database
%	bagof(X, Term, List)    - to collect up all the Xs fitting the Term
%
% NOTES: (1) all database items are held as terms of the form  dbitem(Item).
%	 (2) all your input must be valid Prolog terms.
%	 (3) the program is atypical, because backtracking NEVER happens in it.
%	     It can bomb out, if you attempt to query an empty database.

go :-
	abolish(dbitem,1),
	db.

db :-
	write('Type    add.    to add to the database,'), nl,
	write('        delete. to delete items,'), nl,
	write('        query.  to ask a question.'), nl,
	read(Reply),
	process(Reply),
	db.

process(add) :-
	write('Enter what you want to add.'), nl,
	read(Term),
	assert(dbitem(Term)),
	recheck(add).
process(delete) :-
	write('Enter what you want to delete.'), nl,
	read(Term),
	retract(dbitem(Term)),
	recheck(delete).
process(query) :-
	write('What do you want to look up?'), nl,
	read(Term),
	bagof(Term, dbitem(Term), List),
	write(List), nl,
	recheck(query).
process(SomethingElse) :-
	write('I didn''t understand that.'), nl.

recheck(Operation) :-
	write('Another (yes/no)?'), nl,
	read(Reply),
	reaction(Reply, Operation).

reaction(yes, Operation) :-
	process(Operation).
reaction(no, _).
reaction(_, Operation) :-
	write('Answer yes or no, clot! '),
	recheck(Operation).