Marijn Haverbeke's blog (license)

Extensible Extension Mechanisms

Tuesday, August 27, 2019 architecture

(The details at the bottom of this post no longer reflect the way things work in CodeMirror 6. See this post for an updated description.)

It has become fashionable to structure big systems as a number of separate packages. The driving idea is that it is better to, instead of locking people into your implementation of a feature, provide the feature as a separate package that they can load alongside the core system.

This gets you, roughly...

The cost of this approach is mostly one of complexity. You can provide a batteries-included wrapper package to get users started, but at one point they will probably have to remove the cover and start installing and configuring specific helper packages, which tends to be harder than flipping on a feature in a monolithic library.

This post will try to explore designs for extension mechanisms that support “extension at scale” and unanticipated new extension points.


What do we want from an extensible system? Firstly, of course, it has to allow external code to extend its behavior.

But that is hardly enough. Let me illustrate with an anecdote about a stupid thing I did at some point. I work on editor software. An early version of a code editor allowed client code to set the style of a given line. This was great—now you can selectively style a line.

Except that, as soon as two independent pieces of code try to style a line, they will step on each other's toes. The second extension to touch the line will override the first extension's style. Or, when the first one tries to remove its styling at some later point, it will instead clear the second one's style.

The solution was to make it possible to add (and remove) styling, instead of setting it, so that two extensions can interact with the same line without sabotaging each other.

More generally, you have to make sure that extensions can be combined, even if they are entirely unaware of each other's existence, without causing problematic interactions.

To do this, each extension point must support being acted on by any number of actors. How multiple effects are handled differs by use case. Some strategies that may make sense are:

With many of these, ordering is significant. That means that the precedence with effects are applied should be controllable and predictable.

This is one of the places where imperative, side-effect based extension systems tend to fall short. For example, the browser DOM's addEventListener operation will cause event handlers to be called in the order in which they were registered. This is fine if a single system controls all the calls, or if the ordering happens to be irrelevant, but when you have multiple pieces of software independently adding handlers, it can become very hard to predict which ones will be called first.

A Simple Approach

As a concrete example, I applied the modular strategy for the first time in ProseMirror, a rich text editor system. Its core is pretty much completely useless on its own—it relies on additional packages to describe the structure of documents, to bind keys, to provide the undo history. Though the system is a bit challenging to use, it has seen adoption in systems that need to customize things that classical editors don't allow you to customize.

ProseMirror's extension mechanism is relatively straightforward. When creating the editor, the client code specifies a single array of plugin objects. Each of these plugins can influence various aspects of how the editor works, doing things like adding bits of state data or handling interface events.

All these aspects have been designed to work with an ordered array of configuration values, using one of the strategies outlined in the previous section. For example, when you specify multiple key maps, the ordering in which you specify the instances of the keymap plugin determines their precedence. The first keymap that knows how to handle a given key gets it.

This is usually powerful enough, and people have been making good use of it. But at a certain level of extension complexity it becomes awkward.

CodeMirror version 6 is a rewrite of the code editor by that name, in which I'm trying to take the modular approach further. This requires a more expressive extension system. Let us go over some of the challenges involved in the design of such a system.


It's not hard to design a system that provides full control over the ordering of extensions. It is pretty hard to design such a system which is pleasant to use and allows you to combine independent extension code without a lot of finnicky manual intervention.

One tempting solution, when it comes ordering, it so work with precedence values. An example of this is z-index property in CSS, where you specify a number that determines where an element is placed in the depth stack.

As the comically large z-index values that one often finds in style sheets illustrate, this way of specifying precedence is problematic. A given module, in isolation, doesn't know which precedences other modules are specifying. The options are just points on an undifferentiated numeric range. It can provide a huge (or deeply negative) value in the hope of hitting one of the far ends of the scale, but everything else requires guesswork.

This can be made somewhat better by defining a limited set of well-labeled precedence categories, so that extensions can classify the general “level” of their precedence. You still need some way to break ties within categories.

Grouping and Deduplication

As I mentioned before, once you start heavily relying on extensions you might have some extensions making use of other extensions. Manual dependency management doesn't scale very well, so it would be nice if you could pull in a group of extensions at once.

But, besides making the ordering problem even more pressing, this introduces another issue: Multiple extensions might depend on a given extension, and if extensions are represented as values, you can easily end up loading the same extension multiple times. For some types of extensions, such as keymaps or event handlers, this is okay. For others, like an undo history or a tooltip library, this would be wasteful or even break things.

Thus, it seems that allowing extensions to be composed forces some of the complexity of dependency management onto your extension system. You'll need to be able to recognize extensions that shouldn't be duplicated, and load only one instance of them.

But since, in most cases, extensions can be configured, and thus not all instances of a given extension are the same, we can't just pick one instance and use that—we have to somehow merge those instances in a meaningful way (or report an error when this isn't possible).

A Design

Here I'll outline what we're doing in CodeMirror 6. I'm presenting this as a sketch of a solution, not The Definitive Solution. It is entirely possible that this system will further evolve as the library is stabilized.

The core primitive in this approach is called a behavior. Behaviors are the things that extensions can extend by providing values. An example would be the state field behavior, where extensions can add new fields by providing a field description. Or the browser event handler behavior, where extensions can add their own handlers.

From the point of view of a behavior consumer, behaviors, as configured in a specific instance of the editor, provide an ordered sequence of values, with the higher-precedence ones coming first. Each behavior has a type, and the values provided for it should match that type.

A behavior is represented as a value, which is used both to declare an instance of the behavior and to access the values the behavior has. The library comes with a number of built-in behaviors, but external code can define its own. For example, the extension that defines a line number gutter could define a behavior that allows other code to add additional markers to that gutter.

An extension is a value that can be used to configure the editor. An array of them is passed on initialization. Each extension resolves to zero or more behaviors.

The simplest type of extension is simply an instance of a behavior. When you specify a value for a behavior, it returns an extension value that produces that behavior.

A sequence of extensions can also be grouped into a single extension. For example, an editor configuration for a given programming language might pull in several other extensions, such as a grammar to parse and highlight the language, information about how to indent it, and an autocompletion source that intelligently provides suggestions for that language. So you'd have a language extension that just collects these relevant extensions and groups them together into a single extension value.

A simple version of this system could stop here, and just flatten out the nested extensions into a single array of behavior extensions. These could then be grouped by behavior type, and there you have your ordered sequences of behavior values.

But we still need to address deduplication and provide more control over ordering.

A third type of extension value, unique extensions, are the mechanism for deduplication. Extensions that don't want to be instantiated twice in a single editor provide an extension of this kind. To define one, you must provide a spec type, which is the type of configuration value that the extension constructor expects, and an instantiation function, which takes an array of such spec values, and returns an extension.

Unique extensions somewhat complicate the process of resolving a collection of extensions into a set of behaviors. As long as there are unique extensions in the flattened set of extensions, the resolver must pick one type of unique extension, collect all instances of it, call its instantiation function with their specs, and replace them with (a single instance of) the result.

(There's another catch, in that these must be resolved in the right order—if you first resolve unique extension X, but then later resolving Y yields another X, that would be wrong, since all instances of X should be combined together. Since extension instantiation is pure, the system handles this by trial-and-error, restarting the process—and recording information about what it learned—when it runs into this situation.)

Finally, we must address precedence. The basic approach is still to preserve the ordering in which the extensions were provided. Compound extensions are flattened into that same ordering at the position where they occur. The result of resolving a unique extension is inserted at its first occurrence.

But extensions can assign some of their sub-extensions to a different precedence category. The system defines four such categories: fallback (take effect after the other things), default, extend (higher precedence than the bulk), and override (should probably be on top). Actual ordering happens first by category, then by original position.

So an extension that has a low-priority keymap and a regular-priority event handler could give you a composite extension built out of the result of the keymap extension (without needing to know what behaviors that is made up of) with its priority set to fallback, plus an instance of the event handler behavior.

The way extensions can be combined without worrying about what they are doing internally seems a major win. In the extensions that we've modeled so far, which include a two parsing systems that expose the same syntax behavior, a syntax highlighter, a smart indentation service, the undo history, a line number gutter, bracket matching, keymaps, and multiple selections, it seems to work well. It also anticipates some of the problem that I ran into in ProseMirror, though no one has actually built complex enough setups with this system yet to run into them.

There are a few new concepts that users need to grasp to be able to use this system, and it is decidedly harder to use than the imperative systems that are traditional in the JavaScript community (call method to add/remove effect). Having extensions properly compose seems to provide enough value to offset that cost.