Flan

A statically typed Lisp for native games and interactive development.

Flan is an experimental, ahead-of-time compiled Lisp for programs that need predictable memory use and a fast edit–run loop. It combines S-expressions, static types, explicit ownership, and a development session that can replace a function in a running program without resetting its state.

It is being built around games, but the interesting part is broader: a compiled language where the running program remains available for inspection, experimentation, and small changes.

In practical terms: you get parentheses, a debugger that would like to have a conversation, and no garbage collector quietly choosing the dramatic moment to join your frame loop.

What it has

• Native compilation through LLVM, plus an in-progress direct x86-64 backend.
• C-like data layout: structs, fixed arrays, pointers, slices, and explicit allocation. There is no garbage collector.
• Owned Vec and Map containers, plus checked moves and borrowing-oriented slice operations.
• Generics, algebraic unions, enums, macros, packages, defer, and a C FFI.
• Conditions and restarts for recoverable failures and interactive debugging.
• A raylib package and a collection of ported raylib examples.
• Native, WASI, and web build targets. The cross targets are useful but less complete than the native development workflow.

The project is exploratory software, not a stable language release. Some features are deliberately refused while their semantics are still undecided; the compiler aims to say why rather than quietly accepting a partial version. It has opinions, but at least they arrive as error messages.

Quick start

Building requires a current OCaml/Dune toolchain, LLVM/Clang, and the native C toolchain. Raylib is only needed for programs that use the bundled graphics package.

dune build
dune exec ./bin/main.exe -- run web/examples/hello.flan

To build a standalone native executable:

dune exec ./bin/main.exe -- build web/examples/hello.flan -o hello
./hello

The falling-sand demo uses raylib:

dune exec ./bin/main.exe -- run sand.flan

Once you are iterating regularly, put the built executable on your PATH if you want to use the shorter flan commands shown below.

The live development loop

Start a long-lived development session:

flan dev sand.flan

The program runs normally and publishes a local socket beside the source file. The bundled Emacs mode can attach to it, evaluate expressions in the live process, inspect a stopped program, and recompile a top-level function from the buffer. A body change takes effect on the next call; changing a function's signature is intentionally rejected. The program keeps its state, which is especially nice when you have finally arranged the sand into something almost worth saving.

To set up the mode:

(add-to-list 'load-path "~/path/to/flan/emacs")
(require 'flan-mode)

Then use M-x flan-dev to start and attach, or C-c C-z to attach to a session started in a terminal. The editor workflow is documented in emacs/MANUAL.md.

A small example

(defstruct AssetMissing [id i32])

(defn load-asset [id i32] i32
  (signal (AssetMissing {.id id}))
  100)

(defn asset-or-placeholder [id i32] i32
  (restart-case (load-asset id)
    (use-placeholder [] -1)))

(defn main [] ()
  (handler-bind [(AssetMissing [_] (invoke-restart 'use-placeholder))]
    (println (asset-or-placeholder 7))))

Here a missing asset signals a typed condition. The handler chooses a restart, so execution continues with a placeholder instead of requiring error values to be threaded through every caller. See web/examples/restart.flan for a runnable version.

Commands

flan check <file.flan>                         type-check a program
flan run <file.flan> [args...]                 build and run it
flan build <file.flan> [-o out] [options]      build a native executable
flan dev <file.flan> [-s socket]               start a live development session

Useful build options include --debug, --sanitize, --no-bounds-checks, --x86, and --target=wasm32-wasi|web. run is native-only; cross-built output should be run with an appropriate WASI runtime or browser. A .wasm file is not a tiny native executable in a trench coat.

Project map

• web/index.html — language reference and fuller examples.
• spec-memory.md — ownership, containers, and generics.
• spec-conditions.md — conditions, handlers, and restarts.
• emacs/MANUAL.md — the interactive editor workflow.
• docs/BUILT.md — implementation rationale.
• NEXT.md — current work and known limits.

License

Flan is released under the MIT License. Third-party material under vendor/ is distributed under its own licenses.