Writing Parser Extensions

Implementing a custom parser is one of two possible ways to extend the syntax of a text markup language.

The second option is Implementing Directives and in many cases that approach offers more convenience. It is based on a common syntax for declaring a directive and its attributes and body elements. Therefore directives can be implemented without writing a custom parser and without getting familiar with Laika's parser combinators.

There are usually only two scenarios where you may prefer to write a parser extension instead:

Multi-Pass Markup Parsing

Before delving deeper into the practicalities of writing a parser extension, it is helpful to understand how markup parsing works under the hood in Laika.

In contrast to Laika's HOCON and CSS parsers which are single-pass, markup parsing is always a multi-pass operation.

At the minimum it is parsing in two passes: the first looks for markup which is significant for demarcating a block type (like a blockquote, an ordered list, a code block or a regular paragraph for example). It then further parses the text of each block to look for inline markup.

But it may require any number of additional passes, as blocks or spans themselves can be nested inside each other. An example for a recursive block structure is a list, where each list item may contain nested lists. A span may be recursive where an element like a link allows further inline markup for the link text.

The distinction between block and span parsers is also reflected in the APIs. The document AST comes with base traits Block and Span (both extending the root Element trait) and registration of span and block parsers (which produce Block or Span AST nodes respectively) is separate and comes with slightly different configuration options as shown in the following sections.


The content of this chapter builds on top of concepts introduced in other chapters, therefore reading those first might help with following through the examples.

First, all parsers build on top of Laika's Parser Combinators. Having its own implementation helps with keeping all functionality tightly integrated and adding some optimizations useful for the specific use case of markup parsing right into the base parsers.

Second, since the parsers we discuss in this chapter all produce AST nodes, it might help to get familiar with The Document AST first.

Span Parsers

Span parsers participate in inline parsing. Examples for built-in span parsers are those for emphasized text, literal spans or links.

Adding a span parser to an existing text markup language requires two steps:

Let's go through this step by step with an example. We are going to build a simple span parser for a ticket reference in the form of #123.

Span Parser Implementation

We require a mandatory # symbol followed by one or more digits.

import laika.ast._
import laika.parse.implicits._

val ticketParser: PrefixedParser[Span] = 
  ("#" ~> someOf(CharGroup.digits)).map { num =>
    val url = s"http://our-tracker.com/$num"
    SpanLink(Seq(Text("#" + num)), ExternalTarget(url))

This first example hard-codes the base URL. If your parser extension is for internal use, this may not be a problem. But if you require a configurable base URL, we later show an enhanced example that has access to the configuration.

Registering a Span Parser

For bundling all your Laika extensions, you need to extend ExtensionBundle. In our case we only need to override the parsers property and leave everything else at the empty default implementations.

object TicketSyntax extends ExtensionBundle {

  override val parsers: ParserBundle = ParserBundle(
    spanParsers = Seq(SpanParser.standalone(tickerParser))


The SpanParser.standalone method can be used in cases where your parser does not need access to the parser of the host language for recursive parsing.

Finally you can register your extension together with any built-in extensions you may use:

laikaExtensions := Seq(
val transformer = Transformer

Access to Configuration

This will enhance the previous example by making the base URL configurable.

Access to configuration or other AST nodes is not possible in the parsing phase itself, as each parser executes in isolation and multiple input documents are processed in parallel.

But your parser can return an instance that implements SpanResolver instead of directly producing a link. Such a resolver will then participate in the AST transformation phase where its resolve method will be invoked:

case class TicketResolver (num: String, options: Options = NoOpt) extends SpanResolver {

  def resolve (cursor: DocumentCursor): Span = {
      error => InvalidElement(s"Invalid base URL: $error", "#" + num).asSpan,
      baseURL => SpanLink(Seq(Text("#"+num)), ExternalTarget(s"$baseURL$num"))

The DocumentCursor passed to the resolve method provides access to the project configuration which we use in this example, but also to all ASTs of the input tree, including other documents. It can therefore be used for advanced functionality like producing a table of contents.

The API of the cursor.config property is documented in Config.

In our case we expect a string, but we also need to handle errors now, as the access might fail when the value is missing or it's not a string. We return an InvalidElement for errors, which is a useful kind of AST node as it allows the user to control the error handling. The presence of such an element will by default cause the transformation to fail with the provided error message shown alongside any other errors encountered. But users can also switch to a "visual debugging" mode by rendering all errors in place. See Error Handling for details.

The options property of our implementation is a mandatory property of each Span or Block element. Options are a generic hook that allows users to add an id or multiple styles to a node. It's always best to have an empty default argument like in our example.

With this change in place, the user can now provide the base URL in the builder of the Transformer:

laikaConfig := LaikaConfig.defaults
  .withConfigValue("ticket.baseURL", "https://example.com/issues")
val transformer = Transformer
  .withConfigValue("ticket.baseURL", "https://example.com/issues")

The original ticket parser then only needs to be adjusted to return our resolver instead:

val ticketParser: PrefixedParser[Span] = 
  ("#" ~> someOf(CharGroup.digits)).map(TicketResolver(_))

The registration steps are identical to the previous example.

Detecting Markup Boundaries

The examples so far came with a major simplification to keep the code concise. The logic we implemented would also match on a link embedded in markup for example: http://somwhere.com/page#123. In such a case we would not want the #123 substring to be recognized as a ticket link.

For this reason text markup languages usually define a set of rules which define how to detect markup boundaries. In reStructuredText these are defined in a very clear and strict way (rst-markup-recognition-rules), while in Markdown the definition is somewhat more fuzzy.

Laika's parser combinators come with convenient helpers to check conditions on preceding and following characters without consuming them. Let's fix our parser implementation:

val ticketParser: PrefixedParser[Span] = {
  val letterOrDigit: Char => Boolean = { c => 
    Character.isDigit(c) || Character.isLetter(c)
  val delim = delimiter("#").prevNot(letterOrDigit)
  val ticketNum = someOf(CharGroup.digits)
  val postCond = nextNot(letterOrDigit)
  (delim ~> ticketNum <~ postCond).map(TicketResolver(_))

Instead of just using a literal parser for the "#" we use the delimiter parser which comes with convenient shortcuts to check conditions before and after the delimiter.

The end of our span has no explicit delimiter, so we use a standalone nextNot condition.

Checking that neither the preceding nor the following character is a letter or digit increases the likeliness we only match on constructs the user actually meant to be ticket references.

Recursive Parsing

In our example we used the SpanParser.standalone method for registration. In cases where your parser needs access to the parser of the host language for recursive parsing we need to use the SpanParser.recursive entry point instead:

SpanParser.recursive { recParsers =>
  ('*' ~> recParsers.recursiveSpans(delimitedBy("*"))).map(Emphasized(_))

This parser parses markup between two asterisk. It also detects and parses any span kinds provided by either the host markup language or other installed extensions between these two delimiters. The result from the parser obtained with the call to recursiveSpans is always of type Seq[Span].

Only this entry point gives you access to a parser that is fully configured with all extensions the user has specified. This parser needs to be injected as you would otherwise hard-code a concrete, fixed set of inline parsers.

Block Parsers

Block parsers participate in parsing block level elements. Examples for built-in block parsers are those for lists, headers, tables or code blocks.

Adding a block parser to an existing text markup language requires two steps:

Let's again go through this step by step with an example. We are going to build a block parser for a quoted block. In practice you would not need such an extension, as both supported markup languages in Laika already contain a quoted block element. But it's a good example as the syntax is so simple. In our case we require that each line of such a block starts with the > character:

> This line is part of the quotation
> This line, too.
But this line isn't

Block Parser Implementation

Let's look at the implementation and examine it line by line:

import laika.ast._
import laika.parse.implicits._

val quotedBlockParser = BlockParser.recursive { recParsers =>

  val decoratedLine = ">" ~ ws  // '>' followed by whitespace
  val textBlock = BlockParsers.block(decoratedLine, decoratedLine)

Like with span parsers, for blocks which are not recursive you can use the BlockParser.standalone entry point.

Registering a Block Parser

For bundling all your Laika extensions, you need to extend ExtensionBundle. In our case we only need to override the parsers property and leave everything else at the empty default implementations.

object QuotedBlocks extends ExtensionBundle {

  override val parsers: ParserBundle = 
    ParserBundle(blockParsers = Seq(quotedBlockParser))


Finally you can register your extension together with any built-in extensions you may use:

laikaExtensions := Seq(
val transformer = Transformer

Base Parsers for Block Elements

Laika offers a BlockParsers object with convenience methods for creating a typical block parser.

One of the most common patterns is parsing a range of lines while removing any decoration that just serves as markup identifying the block type, e.g. the * starting a Markdown list or the > starting a quotation. And then using the parser result to continue parsing recursively, either nested blocks or the inline elements of the block.

For these kind of block elements Laika offers the following method for convenience, which is the utility we used in our example for parsing a quoted block:

def block (firstLinePrefix: Parser[Any], 
           linePrefix: Parser[Any]): Parser[List[String]]

It expects two parsers, one for parsing the prefix of the first line, one for parsing it for all subsequent lines. These parsers may be identical, like in our example for the quoted block. They are both of type Parser[Any] as the result will be discarded anyway.

The result of this parser is of type List[String] and contains all lines where the specified conditions were met, minus the input consumed by the prefix parser. The prefix parsers are not required to consume any input though, if the logic for a particular block does not require stripping off decoration, you can alternatively pass parsers that only check some pre-conditions, but leave all input for the result.

The method above always stops parsing when encountering a blank line on the input, which is common for many types of block elements. For cases where parsing may need to continue beyond blank lines, there is a second overload of this method that allows this:

def block (firstLinePrefix: Parser[Any], 
           linePrefix: Parser[Any], 
           nextBlockPrefix: Parser[Any]): Parser[List[String]]

It simply adds a third prefix parser that gets invoked on the beginning of a line following a blank line and is expected to succeed when the line should be treated as a continuation of the block element.

Finally there is a second utility that can be used for indented blocks:

def indentedBlock (minIndent: Int = 1,
    linePredicate: => Parser[Any] = success(()),
    endsOnBlankLine: Boolean = false,
    firstLineIndented: Boolean = false,
    maxIndent: Int = Int.MaxValue): Parser[String]

Like the other utility it allows to specify a few predicates. This method is not used for parsing Markdown's indented blocks, though, as Markdown has a special way of treating whitespace.


Both, block and span parsers can specify a precedence:

BlockParser.recursive { implicit recParsers =>
  ??? // parser impl here

The default is that extension parsers have higher precedence than host language parsers, which means that they will be tried first and only when they fail the host language parsers will be invoked.

When setting low precedence like in the example above, this is reversed.

Note that precedence only refers to the ordering between the parsers of your extension and those of the host language. The precedence between your extensions, in case you install multiple, is determined by the order you specify them in.

override val parsers: ParserBundle = ParserBundle(
  spanParsers = Seq(parserA, parserB)

In this case parserB will only be invoked when parserA fails. Normally the syntax between inline constructs is different enough that the precedence does not matter. But in some cases extra care is needed. If, for example, you provide parsers for spans between double asterisk ** and between single asterisk *, the former must be specified first, as otherwise the single asterisk parser would 100% shadow the double one and consume all matching input itself.

Internal Design & Performance

For span parsers the parser you are passing to the standalone or recursive methods must be of type PrefixedParser[Span], which is a subtype of the base trait Parser[T].

This is a trait implemented by all parsers with a stable prefix that can be optimized, like literal string parsers or many other base parsers like the someOf or delimiter parsers. In most cases this is nothing you need to worry about, as most of the building blocks you use already implement this trait. In the rare occasions where this is not the case, you might need to wrap your inline parser in a custom implementation of the PrefixedParser trait.

This restriction is necessary as inline parsers cause checks to be performed on each character. This is therefore the key hot spot for performance optimizations. For details see Performance Optimizations in the chapter about Laika's parser combinators.

For block parsers this restriction is lifted, you can pass a regular Parser[Block] to the registration entry points. One reason is that optimizing block parsers is much less critical, as they are only invoked (more or less) after a blank line has been read, not on each input character. Another reason is that the nature of some block elements makes this type of optimization impractical. An underlines header for example does not have any concrete set of start characters.

But even though it is not required, if you do actually pass PrefixedParser[Block] it will nevertheless get optimized under the hood, just that the impact of this optimization will be smaller than for span parsers.