Introduction

10.1 Introduction

This chapter describes primitives that allow one to break the normal logic programming rules in two ways:

information can be saved across logical failures and backtracking;
information can be accessed by naming rather than by argument passing.

Obviously, these facilities must be used with care and should always be encapsulated in an interface that provides logical semantics.

ECLiPSe provides several facilities to store information across backtracking. The following table gives an overview. If at all possible, the handle-based facilities (bags, record, shelves and stores) should be preferred because they lead to cleaner, reentrant code (without global state) and reduce the risk of memory leaks.

Facility	Type	Access	See
bags	unordered bag	by handle	bag_create/1
records	ordered list	by handle	record_create/1
shelves	array	by handle	shelf_create/2,3
stores	hash table	by handle	store_create/1
named records	ordered list	by name	record/1,2
named shelves	array	by name	shelf/2
named stores	hash table	by name	store/1
non-logical variables	single cell	by name	variable/1
non-logical arrays	array	by name	array/1,2
dynamic predicates	ordered list	by name	dynamic/1, assert/1

The other facility described here, Global References, does not store information across failure, but provides a means to give a name to an otherwise logical data structure. See section 10.8.