Programming

XTS is a programming language for typesetting. A layout file is not a static template that gets filled in: it is a program that runs, command by command, as the PDF is built. This section explains that execution model and the language features that go with it – the data/action distinction, typed values, variables, XPath, functions, templates, and control flow.

If you are looking for how to make a page look a certain way (fonts, tables, images, the grid), see the Layout Guide. If you want the terse, per-command facts, see the Reference.

The mental model first

Two chapters set up the way of thinking that the rest build on. Read them first.

• Execution model – why XTS runs commands in order, keeps live layout state, and is therefore imperative (not a one-shot tree-to-PDF renderer).
• Data vs. action – every command either builds a value or performs an action. Knowing which is which explains almost everything else.

The language

• Values and types – as= sequence types, the queryable data band, and when things are evaluated.
• Variables – setting, reading, scope, evaluation time, building up content, dynamic names.
• XPath basics – a practical primer on the XPath expression language: paths, predicates, operators, functions.
• XPath in XTS – where XPath is used and the sd: layout functions.
• Maps and arrays – XPath 3.1 structured data: maps, arrays, and the ? lookup operator.
• Functions – reusable, side-effect-free <Function> definitions that return data.
• Templates – reusable behaviour with <Template> / <CallTemplate>, including layout actions.
• Control flow – ForAll, Loop, While/Until, Switch/Case.
• Records and dispatch – matching data elements to records, modes, and priority.
• Data files – structuring the XML you feed to XTS.