Introduction

## 18.1 Introduction

This chapter introduces the control facilities that distinguish the ECLiPSe language from Prolog by providing a computation rule that is more flexible than simple left-to-right goal selection. The core feature is the ability to suspend the execution of a goal at some point during execution, and resume it under certain conditions at a later stage. Together with attributed variables, these facilities are the prerequisites for the implementation of constraint propagation and similar data-driven algorithms.