Joseph Ferano
ce3b62340b
A defvar typed at the top of a file could not be evaluated with C-x C-e. The key was wired straight to eval-expr, so the parser met a declaration where an expression was required and said so: "defvar is a top-level declaration, not an expression". The only way to get the var installed was C-c C-c. That split was never a property of the compiler, which has had both evaluators side by side in lib/dev.ml all along — one installs a body under a name, the other wraps an expression in a thunk because there is no name to install into. It was a property of the keymap. So the key now dispatches on the form it would actually send. Two questions, both of which have to answer yes before the declaration path is taken: the depth at the form's open delimiter says whether anything encloses it, and the head says whether it is a declaration. Requiring the first is what keeps an inner expression inside a defn body evaluating as an expression — the enclosing form being a declaration is not a reason to reinstall it, and the cursor's position in the file was never the question. Requiring it also leaves a defn written inside a let to the parser, which refuses it correctly, rather than quietly installing it as a top-level definition. The head list is spelled out in flan-dev.el and points at the arm of Parse.expr that is its authority; package is deliberately left out of it, because this is the set of heads that fail when sent to the expression evaluator and not the set that introduces a name. The echo area is now the only thing that says which evaluator ran, so it has to say it. An expression still reports "=> 42". A declaration reports what landed, and reports it by name: a defvar installs no bodies, so leading the sentence with the generic label left it reading "form installed in 4 ms (also ticks)" — the one name that changed, parenthesised as an afterthought. Where there are no functions the names are what changed, so they are what the sentence is about, and the aside is then empty rather than a repeat of it. C-c C-c is untouched and stays the explicit "reload the definition I am standing in", which is still the command to use from inside a body.
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
VecandMapcontainers, 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.
Checking it
dune test the suite. Seconds. Run it constantly.
dune build @checks everything else that can fail. A couple of minutes.
dune build @sanitize the corpus under ASan and UBSan.
dune build @valgrind the corpus under memcheck. Tens of minutes.
dune test means "the language still works". @checks — which is @page,
@x86 and @cells — means "and everything written down about it is still
true": the reference page's examples still print what the page says, the
hand-written x86 backend still agrees with LLVM, and a --dev build still calls
through its indirection cells.
The two are separate on purpose. A suite that goes red because prose drifted
teaches you to skim past red. But @checks only helps if it is run, and nothing
runs it for you — there is no CI here. The convention that has to carry it is
that a lane's handoff quotes @checks, the way the x86 handoffs already quote
the survey's counts. Both of this repository's silent failures — two backend
refusals that sat for a month, a page of examples that stopped compiling for two
days — were found by accident, and neither would have survived one person
typing one command.
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.