The document tree in a Laika transformation is a generic representation of the document that does not contain any specific semantic or technical coupling to a concrete input or output format. This allows to add custom processing logic only operating on the tree itself, so it can be used with all supported input and output formats unchanged.

Rewriting a tree means traversing it and replacing or removing some of its nodes. A tree model is immutable, so rewriting always creates a new tree, while reusing unmodified branches for efficiency.

The purpose of this facility is to provide a way to support basic tree rewriting needs without introducing additional dependencies. If you have more sophisticated requirements, you can alternatively use Kiama, a language processing library that also supports rewriting. According to their documentation Kiama should be compatible with Laika as they support tree models consisting of Product instances (which all case classes are).

How Rewrite Rules Work

A rewrite rule has the type PartialFunction[Element,Option[Element]]. Laika offers a type alias RewriteRule for convenience.

The partial function expects an Element and returns an Option[Element]. The rules are as follows:

  • If the function is not defined for a particular element the old element is kept in the tree.

  • If the function returns Some(Element) this element is used in place of the old one.

  • If the function returns None the old element is removed from the tree.

  • Processing happens depth-first (bottom-up), so all nodes getting passed to this function already had their children getting processed.

  • The tree is immutable, so new instances are returned when rewriting, but unmodified branches are reused.

  • Therefore, if the rule does not affect any child node, the rule will simply return the old root node.

The following sections show the three ways to apply such a rule.

Applying a Rewrite Rule

The mechanism is slightly different, depending on whether you are using the sbt plugin or Laika embedded in an application. In the latter case you have two choices, one for hooking into a full transformation, the other for operating on nodes obtained by a separate parse operation. All three options are described below.

Using the sbt Plugin

The following example of an sbt build file shows how to turn each Emphasized node into a Strong node while processing everything else with default rules:

import laika.tree.Elements._

laikaRewriteRules += laikaRewriteRule { 
  case Emphasized(content, opts) => Some(Strong(content, opts))

Using the Transform API

When using Laika embedded and all you want to do is to perform a full transformation from some input text to some output format, the Transform API offers a hook to do this in one go, as a step in the transformation process.

Again we replace all Emphasized nodes with Strong nodes:

Transform from ReStructuredText to HTML usingRule {
  case Emphasized(content, opts) => Some(Strong(content, opts))
} fromFile "hello.rst" toFile "hello.html"

Working with the Tree Model

The final option is for splitting the parse and render operations and working with the tree model directly between these operations. For obtaining a document tree see Separate Parsing and Rendering.

Often a rule gets applied to the whole Document instance, but other container types support rewriting, too, so you can also apply it to elements like Paragraph or ListItem.

Once again we are turning all Emphasized nodes in the text to Strong nodes:

val doc: Document = ... // obtained through the Parse API

val newDoc = doc rewrite {
  case Emphasized(content, opts) => Some(Strong(content, opts))

For a slightly more advanced example, let's assume you only want to replace Emphasized nodes inside headers. To accomplish this you need to nest a rewrite operation inside another one:

val newDoc = doc rewrite {
  case h: Header => Some(h rewrite {
    case Emphasized(content, opts) => Some(Strong(content, opts))