API for creating directives, the extension mechanism of reStructuredText.

The API did not aim to mimic the API of the original Python reference implementation. Instead the goal was to create an API that is idiomatic Scala, fully typesafe and as concise as possible. Yet it should be flexible enough to semantically support the options of the Python directives, so that ideally most existing Python directives could theoretically get ported to Laika.

Comparison with Laika Directives

Extensions defined in the way described in this chapter could still be used when parsing the markup documents with a different reStructuredText implementation, as they are fully compatible with the original specification.

If this is not a requirement you may alternatively use the Laika variant of directives. This would give you the following advantages:

- The syntax definition is simpler, while offering the same flexibility. - The directive may be used in other parsers, too, like in the Markdown parser. - The directive may also be used in templates. For details on these alternative directive types see http://typelevel.org/Laika/latest/05-extending-laika/03-implementing-directives.html.

Implementing a Directive

Entry points are the BlockDirective and SpanDirective objects. The Python reference parser does not make this distinction on the API level, but does this internally based on the context a directive is parsed in. Since Laika APIs are typesafe, the distinction is necessary since block level and span level directives create different types of document tree nodes. A SpanDirective can only be used in a substitution definition which can then be used within flow elements. A BlockDirective can be used directly in any location other block level content like paragraphs or lists can be used.

A directive may consist of any combination of arguments, fields and body elements:

.. myDirective:: arg1 arg2
 :field1: value1
 :field2: value2

 This is the body of the directive. It may consist of any standard or custom
 block-level and inline markup.

In the example above arg1 and arg2 are arguments, field1 and field2 are fields, and followed by body elements after a blank line. If there are no arguments or fields the blank line may be omitted. For the full specification, see http://docutils.sourceforge.net/docs/ref/rst/restructuredtext.html#directives.

For each of these directive elements, the API offers a method to specify whether the element is required or optional, and an optional function to convert or validate the parsed value.

Basic Example

Consider the following simple example of a directive with just one argument and a body:

.. note:: This is the title

 This is the body of the note.

The implementation of this directive could look like this:

case class Note (title: String,
                 content: Seq[Block],
                 options: Options = NoOpt) extends Block
                                           with BlockContainer[Note]

object MyDirectives extends RstExtensionRegistry {
  val blockDirectives = Seq(
    BlockDirective("note") {
      (argument(withWS = true) ~ blockContent).map { case title ~ content => Note(title, content) }
    }
  )
  val spanDirectives = Nil
  val textRoles = Nil
)

val transformer = Transformer
  .from(ReStructuredText)
  .to(HTML)
  .using(MyDirectives)
  .build

The argument() method specifies a required argument of type String (since no conversion function was supplied). We need to set the withWS flag to true as an argument cannot have whitespace per default. The blockContent method specifies standard block content (any block-level elements that are supported in normal blocks, too) which results in a parsed value of type Seq[Block]. Finally you need to provide a function that accepts the results of the specified directive elements as parameters (of the corresponding type). Here we created a case class with a matching signature so can pass it directly as the target function. For a block directive the final result has to be of type Block which the Note class satisfies. Finally the directive gets registered with the ReStructuredText parser.

Adding Converters and Validators

If any conversion or validation is required on the individual parts of the directive they can be passed to the corresponding function:

def nonNegativeInt (value: String) =
  try {
    val num = value.toInt
    Either.cond(num >= 0, num, s"not a positive int: $num")
  }
  catch {
    case e: NumberFormatException => Left(s"not a number: $value")
  }

case class Message (severity: Int,
                    content: Seq[Block],
                    options: Options = NoOpt) extends Block
                                              with BlockContainer[Message]

object MyDirectives extends RstExtensionRegistry {
  val blockDirectives = Seq(
    BlockDirective("message") {
      (argument(nonNegativeInt) ~ blockContent).map {
        case severity ~ content => Message(severity, content)
      }
    }
  )
  val spanDirectives = Nil
  val textRoles = Nil
)

The function has to provide an Either[String, T] as a result. A Left result will be interpreted as an error by the parser with the string being used as the message and an instance of InvalidBlock containing the validator message and the raw source of the directive will be inserted into the document tree. In this case the final function (Message) will never be invoked. A Right result will be used as an argument to the final function. Note how the case class now expects an Int as the first parameter.

Optional Elements

Finally arguments and fields can also be optional. In case they are missing, the directive is still considered valid and None will be passed to your function:

case class Message (severity: Option[Int],
                    content: Seq[Block],
                    options: Options = NoOpt) extends Block
                                              with BlockContainer[Message]

object MyDirectives extends RstExtensionRegistry {
  val blockDirectives = Seq(
    BlockDirective("message") {
      (optArgument(nonNegativeInt) ~ blockContent).map {
        case severity ~ content => Message(severity.getOrElse(0), content)
      }
    }
  )
  val spanDirectives = Nil
  val textRoles = Nil
}

The argument may be missing, but if it is present it has to pass the specified validator.

In case of multiple arguments, the order you specify them is also the order in which they are parsed from the directive markup, with the only exception being that required arguments will always be parsed before optional ones, and arguments with whitespace need to come last.