11.2 Collecting solutions

There may be many solutions to a query. For example, suppose we are working with the database

child(martha,charlotte).
child(charlotte,caroline).
child(caroline,laura).
child(laura,rose).
 
descend(X,Y) :- child(X,Y).          
 
descend(X,Y) :- child(X,Z),          
                descend(Z,Y).

Then if we pose the query

descend(martha,X).

there are four solutions (namely X=charlotte, X=caroline, X=laura, and X=rose).

However Prolog generates these solutions one by one. Sometimes we would like to have all the solutions to a query, and we would like them handed to us in a neat, usable, form. Prolog has three built-in predicates that do this: findall, bagof, and setof. Basically these predicates collect all the solutions to a query and put them in a list, but there are important differences between them, as we shall see.

• 11.2.1 findall/3

• 11.2.2 bagof/3

• 11.2.3 setof/3