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Fennel + C++ application model over Python ​

Context ​

The project began with a Python scripting layer (assets/python/) driving the C++ engine. Python provided rapid prototyping but introduced performance issues, complex packaging, and a heavy runtime footprint for a 3D application that needed to be responsive and portable.

We needed a scripting language that:

  • Embeds directly into the C++ process with minimal overhead
  • Supports hot-reloading for live development
  • Has good interop with our existing C++ engine
  • Feels natural for a Lisp-heavy codebase (the team had Lisp experience)

Decision ​

Adopt Fennel (a Lua Lisp) as the scripting language, running on LuaJIT via sol2 bindings.

  • All new feature work, tests, and architecture decisions happen in assets/lua/ using Fennel
  • assets/python/ is preserved as historical reference only
  • C++ engine (src/) exposes bindings via sol2 with factory functions instead of constructors
  • Widget system, layout engine, and application model are built in pure Fennel

Consequences ​

Positive:

  • Single-process architecture — no Python subprocess, no IPC overhead
  • Fast startup and hot-reload via LuaJIT
  • Widget/layout system composes cleanly in a functional style
  • Tests run in-process via the same space binary
  • Smaller distribution footprint (no Python dependency)

Negative:

  • Smaller ecosystem than Python — fewer libraries
  • Fennel adds a compile step (though transparent at runtime)
  • New contributors must learn Fennel/Lisp syntax
  • sol2 binding code requires careful lifetime management

Alternatives considered ​

Stick with Python: Rejected — performance and packaging were too painful for a 3D runtime.

Lua without Fennel: Rejected — the team valued Lisp's expressiveness and macros for DSLs (widget builders, layout constraints).

Guile Scheme: Rejected — weaker C++ interop story, larger runtime, less mature embedding.

JavaScript (QuickJS/Duktape): Rejected — didn't match the team's functional/Lisp affinity.