Introduction

## 9.1 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, 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
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 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
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.