Introduction

This chapter describes primitives that allow 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 and shelves) should be preferred because they lead to cleaner, reentrant code (without global state) and reduce the risk of memory leaks.

Facility	Type	Access	See
shelves	array	by handle	shelf_create/2,3
bags	unordered bag	by handle	bag_create/1
stores	hash table	by handle	store_create/1
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
records	ordered list	by name	record/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 9.7.