I did check on FMJ many years ago when researching about Visual Programming Languages. I recall the very inspiring notes on the FMJ website (http://www.fmjlang.co.uk/fmj/FMJ.html).
(1) The type system is inspired by TypeScript. It has generics that are resolved when unit pins are connected to concrete types. Right now there’s no way to define new types. Hopes are that type inference alone would be good enough for a start. Likewise, new interfaces cannot be created. Only the system defined interfaces can be exposed, such as channel `CH` and media stream `ST`. That is interesting because it influences the design of machines in terms of common interfaces and protocols for inputs and outputs.
(2) The visual representation is a graph specification stored in JSON. I think JSON works well with current version control systems in terms of diff readability, but I don’t see why custom unit components couldn’t be created to show the diff in graph, visually.
(3) The unit will stop the data flow and emit an “error” event. To catch this event particularly, there’s a `catch unit.
(4) Yes, there’s a few examples of recursive units, such as a `merge sort` in the open source system. The `editor` unit can’t create them from scratch right now though… but it can be done with a meta trick…
(5) The graph is loaded and saved as a JSON. The JSON has to be read by an interpreter that will instantiate all the primitives and make all the connections. Unit specification is language agnostic, but currently the project is all written in JavaScript (works with Web and NodeJS).