Creates a new directive with the specified name and part specification.

When the result of the directive is a Left, the directive will produce an invalid AST element with the string as the error message.

Provides the basic building blocks for defining directives, Laika's extension mechanism for creating custom tags for both, templates or text markup.

This object is used as part of the concrete objects Blocks.dsl, Spans.dsl and Templates.dsl respectively.

It contains several simple combinators that allow to specify the expected attributes and body elements of the directive, optional converters for these elements and the function responsible for producing the final node element.

In contrast to custom tag hooks in other template engines the result of a directive is not a string. In the same way as markup documents get transformed into a tree of elements before rendering, a directive produces a node of the tree to render. As a result, the directive can be used independent from the output format.

Entry points of the API are the Templates, Blocks and Spans objects for the three different directive types.

A directive may consist of any combination of attributes and body elements:

@:myDirective { arg1 = value1, arg2 = 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 attributes, followed by a body element enclosed in curly braces.

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

Consider the following simple example of a directive with just one argument and a body, for specifying a specially formatted inline note:

@: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 DirectiveRegistry {
  val blockDirectives = Seq(
    Blocks.create("note") {
      (defaultAttribute.as[String], parsedBody).mapN(Note(_,_))
    }
  )
  val spanDirectives = Seq()
}

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

The defaultAttribute combinator specifies a required attribute of type String and without a name. The parsedBody combinator specifies standard block content (any block elements that are supported in normal markup, 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 Markdown parser. It can be registered for a reStructuredText parser, too, without any changes.

If any conversion of attributes is required it can be performed with the as[T] method:

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

val blockDirectives = Seq(
  Blocks.create("message") {
    (defaultAttribute.as[Int], blockContent).mapN(Message(_,_))
  }
)

In the example above the built-in Int decoder gets passed to the defaultAttribute combinator, but you can easily create and use your own instances of ConfigDecoder[T].

If required attributes or bodies are missing or any type conversion fails, an instance of InvalidBlock containing the error 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.

Finally attributes 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: Int,
                    content: Seq[Block],
                    options: Options = NoOpt) extends Block
                                              with BlockContainer[Message]

val blockDirectives = Seq(
  Blocks.create("message") {
    (defaultAttribute.as[Int].optional, blockContent).mapN {
      (severity, content) => Message(severity.getOrElse(0), content)
    }
  }
)

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