Getting started with Jetbrains MPS: how to define a simple language (screencast)

I am blessed (or cursed) with having many different interests in software development. However my deepest love is with language engineering. There are different tools and techniques but Jetbrains MPS is particularly fascinating.

To help people getting started or just get the feeling of how it is to develop with Jetbrains MPS I recorded this short screencast.



If you are not familiar with this tool here there are the basic ideas.

Jetbrains MPS is a projectional editor: that means that it internally stores the information contained in the different nodes.

If you create a model, then change how certain nodes should look, and you reopen the model it will be automatically using the new “look & feel” that you just. This is simply because what is stored is the pure information, while the appearance is recreated each time you open a model. In other words when you open a model it is projected in a form that you defined and that should be easy to understand for a human being.

This is very different from what happens with text language: suppose you create your language and then decide to replace braces with brackets. You have now to manually go over all your existing source files and modify them, because in that case what is saved is not the pure information, the abstract syntax tree, but its representation: the textual form.

In Jetbrains MPS you work on the different concerns of your language separately: you first define the single concept and the information they contain, then you work on the other aspects. There are several like typesystem, generation, dataflow, etc. In this short screencast we just look at the concept definition and the editor aspect.

One of the advantages of a projectional editor is that you can change the appearance of your nodes without invalidating your existing models. That is great to incrementally improve the appearance of the single nodes and the way they can be edited. In the screencast we start with the default representation of the concept we defined, as provided by default by Jetbrains MPS. Then we refine the editors in a couple of steps until we have a representation which is good enough.

At the end of the screencast we have a language we can use to define very simple todo lists. The possibility to improve it are endless:

  • we could add more concepts: like priority, deadlines and tags for each single task
  • we could add the possibility to mark a task as done and add a nice shortcut to do so
  • we could integrate the language with some other tool: what if we could synchronize our models with Trello or with workflowy?
  • we could support subtasks

Question: have you ever thought of creating your own language, either a general purpose language or a domain specific language? If so did you actually get started?


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