Introduction

Attributed variable is a special ECLiPSedata type which represents a variable with some attached attributes. ^16.1

It is a powerful means to implement various extensions of the plain Prolog language. In particular, it allows the system's behaviour on unification to be customised. In most situations, an attributed variable behaves like a normal variable. E.g. it can be unified with other terms and var/1 succeeds on it. The differences compared to a plain variable are:

• an attributed variable has a number of associated attributes
• the attributes are included in the module system
• when an attributed variable occurs in the unification and in some built-in predicates, each attribute is processed by a user-defined handler