4.13.1 Introduction

When there are many solutions to a goal, and a list of all those solutions is desired, one means of collecting them is to write a procedure that repeatedly backtracks into that goal to get another solution. In order to collect all the solutions together, it is necessary to use the database (via assertion) to hold the solutions as they are generated, because backtracking to redo the goal would undo any list construction that had been done after satisfying the goal.

The writing of such a backtracking loop can be avoided by the use of one of the built-in predicates setof/3, bagof/3 and findall/[3,4], which are described below. These provide a nice logical abstraction, whereas with a user-written backtracking loop the need for explicit side effects (assertions) destroys the declarative interpretation of the code. The built-in predicates are also more efficient than those a user could write.

Please note: If the solutions being collected contain attributed variables (see lib-atts) or suspended goals (see ref-sem-sec), then those attributes are not retained in the list of solutions. To retain the attributes, you can use copy_term/3 (see ref-lte-cpt).