5 SWI-Prolog extensions

This chapter describes extensions to the Prolog language introduced with SWI-Prolog version 7 in 2014. The changes bring more modern syntactical conventions to Prolog such as key-value maps, called dicts, as primary citizens and a restricted form of functional notation. They also extend Prolog basic types with strings, providing a natural notation to textual material as opposed to identifiers (atoms) and lists.

These extensions make the syntax more intuitive to new users, simplify the integration of domain specific languages (DSLs) and facilitate a more natural Prolog representation for popular exchange languages such as XML and JSON.

While many programs run unmodified in SWI-Prolog version 7, some require modifications, especially those that pass double quoted strings to general purpose list processing predicates. See section 5.2.4 and section 5.2.5 for information and tools on porting. We provide a tool (list_strings/0) that we used to port a huge code base in half a day.