When designing Ducktype, I wanted people to be able to extend the syntax, but I wanted extensions to be declared and defined, so we don’t end up with something like the mess of Markdown flavors. So a Ducktype file can start with a @ducktype/ declaration that declares the version of the Ducktype syntax and any extensions in use. For example:

@ducktype/1.0 if/1.0

This declares that we’re using version 1.0 of the Ducktype syntax,that we want an extension called if, and that we want version 1.0 of that extension.

Up until last week, extensions were just theoretical. I’ve now added two extension points to the Ducktype parser, and I plan to add three or four more. Both of these are exercised in the _test extension, which is fairly well commented so you can learn from it.

Let’s look at the extensions we have, plus the ones I plan to add.

Block line parser

This extension is implemented. It allows extensions to handle really any sort of line in block context, adding any sort of new syntax. Extensions only get to access to lines after headings, comments, fences, and a few other things are handled. This is a limitation, but it’s one that makes writing extensions much easier.

Let’s look at an actual example that uses this extension: Mallard Conditionals. You can use Mallard Conditionals in Ducktype just fine without any syntax extension. Just declare the namespace and use the elements like any other block element:

@ducktype/1.0
@namespace if http://projectmallard.org/if/1.0/

= Conditional Example

[if:if test=target:html]
  This is a paragraph only shown in HTML.

But with the if/1.0 Ducktype syntax extension, we can skip the namespace declaration and use a shorthand for tests:

@ducktype/1.0 if/1.0

= Conditional Example

? target:html
  This is a paragraph only shown in HTML.

We even have special syntax for branching with if:choose elements:

@ducktype/1.0 if/1.0

= Conditional Branching Example

??
  ? platform:fedora
    This paragraph is only shown on Fedora.
  ? platform:ubuntu
    This paragraph is only shown on Ubuntu.
  ??
    This paragraph is shown on any other operating system.

(As of right now, you actually have to use if/experimental instead of if/1.0. But that extension is pretty solid, so I’ll change it to if/1.0 along with the 1.0 release of the parser.)

Directive handler

Ducktype files can have parser directives at the top. We’ve just seen the @namespace parser directive to declare a namespace. There is an implemented extension point for extensions to handle parser directives, but not yet a real-world extension that uses it.

Extensions only get to handle directives with a prefix matching the extension name. For example, the _test extension only gets to see directives that look like @_test:foo.

Block element handler

This extension is not yet implemented. I want extensions to be able to handle standard-looking block declarations with a prefix. For example, I want the _test extension to be able to do something with a block declaration that looks like this:

[_test:foo]

In principle, you could handle this with the current block line parser extension point, but you’d have to handle parsing the block declaration by yourself, and it might span multiple lines. That’s not ideal.

Importantly, I want both block line parsers and block element handlers to be able to register themselves to handle future lines, so they can have special syntax in following lines. Here is how an extension for CSV-formatted tables might look:

@ducktype/1.0 csv/1.0

= CSV Table Example

[csv:table frame=all rules=rows]
one, two, three
eins, zwei, drei
uno, dos, tres

Inline element handler

This extension is not yet implemented. Similar to block element handlers, I want extensions to be able to handle standard-looking inline markup. For example, I want the _test extension to be able to do something with inline markup that looks like this:

$_test:foo(here is the content)

For example, a gnome extension could make links to GitLab issue reports easier:

$gnome:issue(yelp#138)

Inline text parser

This extension is not yet implemented. I also want extensions to be able to handle arbitrary inline markup extensions, things that don’t even look like regular Ducktype markup. This is what you would need to create Markdown-like inline markup like *emphasis* and `monospace`.

This extension might have to come in two flavors: before standard parsing and after. And it may be tricky because you want each extension to get a crack at whatever text content was output by other extensions, except you probably also want extensions to be able to block further parsing in some cases.

All in all, I’m really happy with the Ducktype syntax and parser, and how easy it’s been to write extension points so far.

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