A statically typed Lisp for game development. Clojure's brackets, C's memory and value model, no garbage collector.

Flan compiles s-expressions to LLVM IR and then to a native binary. There are no object headers, so a Flan struct is exactly its C struct. There is no collector, so nothing runs between frames that you did not write. And a running program can be edited: a function recompiled in Emacs is installed into the live process at its next frame boundary, in about twenty milliseconds.

This page describes the compiler as it is, not as it is planned. Where something is designed but not built, it says so and gives the message the compiler prints for it. Every Flan program on this page is a file in web/examples/ with its output recorded beside it; sh web/examples/check.sh runs them all and compares, and quotes.sh re-derives the blocks that are transcripts rather than programs.

What Flan is

A minimal Lisp for games. In one line: Odin with a Lisp frontend and a live REPL. Types are mandatory and inference makes them feel optional; memory is manual; the frontend is OCaml, the backend writes LLVM IR as text and hands it to clang.

What it is not:

• Not a Common Lisp and not a Clojure. No numeric tower, no CLOS, no format, no lazy seqs, no persistent collections, no JVM.
• No immutable collection types. Structure sharing destroys clear ownership, and clear ownership is the only thing that removes the need for a collector. Value structs that copy on assignment replace them.
• No dynamic typing. A tag word on every value is exactly the header cost that dropping the GC was meant to avoid.
• No consoles, and no live-image development at SBCL's level.

Getting started

You need OCaml with dune, and a clang on PATH. Build the compiler, then run something:

$ dune build
$ ./_build/default/bin/main.exe run calc-me.flan "1 + 2 * (3 - 0.5) / 2"
3.5

Call that binary flan. Its subcommands:

$ flan
usage: flan (read|parse|check|emit|shim) <file.flan>...
       flan build <file.flan> [-o out] [--no-bounds-checks] [--dev] [--debug] [--target=wasm32-wasi]
       flan run <file.flan> [args...]
       flan reload <program.flan> <forms.flan> [-o out.so]
       flan dev <program.flan> [-s socket]

read, parse, check, emit and shim each stop the pipeline one stage further along and print what it produced. run builds to a temporary file and execs it.

The smallest program:

(defn main []
  (print-line "hello from flan"))

The entry point is (defn main [args [string]] i32). Both the parameter and the return type are optional: omitting args means the program ignores argv, and omitting the return type means Unit and an exit status of 0.

Values and memory

There is no garbage collector and no hidden allocation. Every local is a stack slot; reading one is a load, assigning to one is a store, and a store of an aggregate is the copy. A struct is its C layout with nothing added.

Four rules carry most of the model:

• Zero is initialisation. A declaration with no initialiser is all-bytes-zero, so a global array is BSS and costs nothing to start. A struct literal that omits a field zeroes it. (zeroed) re-zeroes something later — a memset, not an allocation.
• Fixed arrays are values. [n T] is inline storage and copies on assignment and on pass-by-value.
• Slices are views. [T] is ptr+len and owns nothing. Copying a slice copies the view, never the elements.
• Pointers are visible. (addr x) takes the address of any assignable place and gives (Ptr T). It does not extend anything's lifetime, and keeping one past its frame is your contract to honour — there is no borrow checker.

A value struct is shared mutably by passing its address down the call chain. No heap is involved.

Places — the forms set accepts — are a fixed list, not an extensible setf:

(defstruct Enemy [hp i32  name string])

(defvar spawned i32)
(defconst room-size 4)
(defvar room [room-size i32])

;; `set` takes a fixed list of forms, not an extensible setf.
(defn main []
  (let [e (Enemy {:hp 10 :name "slime"})
        p (addr e)]
    (set spawned (+ spawned 1))    ; a local or a defvar
    (set (.hp e) 7)                ; a struct field
    (set (.hp p) 8)                ; through a (Ptr Enemy) — derefs one level
    (set (at room 2) 5)            ; a fixed array or slice element
    (set (deref p) (Enemy {:hp 3 :name "wisp"}))   ; a whole-object store

    (print-i64 (i64 (.hp e))) (newline)
    (print-line (.name e))
    (print-i64 (i64 (at room 2))) (newline)
    (print-i64 (i64 spawned)) (newline)))

3
wisp
5
1

.field and at dereference exactly one pointer level, so (set (.hp p) 8) above is legal when p is a (Ptr Enemy). The whole-object store through p overwrote e itself, so hp reads 3 and not 8.

Bounds are checked

(defconst xs [3 i32] [1 2 3])

;; (at xs 7) with a literal index does not reach the backend at all: check.ml
;; rejects it. This one goes through a local, so it is the runtime check that
;; catches it — the same message, and the program stops where it happened.
(defn main []
  (let [i 7]
    (print-line "before")
    (print-i64 (i64 (at xs i)))
    (print-line "unreachable")))

$ flan run bounds.flan
before
bounds.flan:9:28: index 7 is out of bounds for length 3
$ echo $?
134

at and slice emit a comparison and a branch to a cold block that names the source location and stops. Checks are on by default and are not tied to the optimisation level. --no-bounds-checks turns them off. Measured cost on a 50-million-iteration dependency chain over a 1024-element array: 0.11–0.12s checked against 0.12–0.13s unchecked.

Types

Types are annotated at function boundaries and inferred everywhere else. Every type notation reads as exactly one data item.

There is no implicit widening. Both operands of a binary operator have one type, and every conversion is written as a cast:

(defn main [] i32
  (let [n 40                       ; i32, inferred
        big (i64 n)                ; every widening is written
        x 1.5]                     ; f64
    (print-i64 (+ big 2)) (newline)
    (print-f64 (* x 2.5)) (newline)
    (print-i64 (i64 (bit-xor (<< 1 8) 255))) (newline)
    0))

42
3.75
511

An untyped integer literal is i32 and an untyped float literal is f64, so (defconst gravity f32 0.05) names the type when something narrower is wanted. One caveat: a whole-numbered float prints without its fraction, so 3.0 comes out as 3.

Arithmetic wraps. Shifts are bounded two ways: a literal count at or past the operand's width is a compile error, and a computed one is masked to the width. >> is arithmetic on a signed type and logical on an unsigned one.

An index converts from a narrower integer and never from a wider one. A u32 index is fine — anything above 2³¹ truncates to a negative i32 and the unsigned bounds check rejects it. An i64 index is refused:

an index is an i32, and i64 is wider — write (i32 …), because a value that does
not fit truncates to one that does and would read the wrong element without
tripping the bounds check

Structs and enums

A defstruct is a list of inline name/type pairs. A struct literal names its fields, and omitted fields are zeroed.

(defstruct Cursor
  [src [u8]      ; a non-owning slice
   pos i32])     ; no initialiser means zeroed

(defn peek [c (Ptr Cursor)] u8
  (if (< (.pos c) (len (.src c)))
    (at (.src c) (.pos c))
    0))

(defn advance [c (Ptr Cursor)]
  (set (.pos c) (+ (.pos c) 1)))   ; field access derefs one level

(defn main []
  (let [c (Cursor {:src (bytes "hi")})]   ; pos omitted, so pos is 0
    (print-i64 (i64 (peek (addr c)))) (newline)
    (advance (addr c))
    (print-i64 (i64 (peek (addr c)))) (newline)))

104
105

Those are the bytes h and i. There is no character type — a byte is a u8 — but there is a byte literal, so \h is 104 and \space is 32, and the prelude's digit? reads as (and (>= b \0) (<= b \9)). To see a byte as a letter rather than as a number, print a slice of them with print-bytes.

An enum is an i32 at run time and its own type in the checker. A keyword at a call site resolves against the parameter's enum type at compile time, so a typo is an error there rather than a wrong number later.

(defenum Key
  [space 32  escape 256  left 263  right 262])

(defn key-name [k Key] string
  (cond
    (= k :space)  "space"
    (= k :escape) "escape"
    :else         "an arrow"))

(defn main []
  ;; :space resolves against the parameter's enum at compile time.
  ;; A typo is an error here, not a wrong number later.
  (print-line (key-name :space))
  (print-line (key-name :left)))

space
an arrow

A keyword means nothing where no enum is expected. There is no keyword type to fall back on, and there is no way to name a member other than as a keyword in a position that expects that enum.

Struct and enum names share one top-level namespace with functions, globals and type aliases. A second declaration of a name is rejected whatever kind either one is.

Functions

(defn name [param Type ...] ReturnType? body ...). The parameters are inline name/type pairs, as in let and defstruct. An omitted return type means Unit. There is no separate declare form for a function with a body — declare is kept only where there is none.

Top-level names are order-independent within a package, so mutually recursive functions need no forward declaration. Globals come in two kinds:

(defconst cell-size 5)                 ; a compile-time constant
(defconst gravity f32 0.05)            ; with its type named
(defvar current-color i32)             ; zeroed storage
(defconst rows 3)
(defconst cols 4)
(defvar grid [rows [cols u32]])        ; BSS, rows*cols*4 bytes

(defn main []
  (print-i64 (i64 cell-size)) (newline)
  (print-f64 (f64 gravity)) (newline)
  (print-i64 (i64 current-color)) (newline)
  (print-i64 (i64 (at grid 2 3))) (newline))

5
0.05
0
0

A defconst the checker consumed — an array length, for instance — is part of the shape of the program. One it did not is only ever bytes in memory. The two reload differently; see the dev loop.

let binds name/value pairs and takes no type annotation, so a constant whose type matters is named at the top level rather than written inline.

Control flow

if, when, unless, cond, do, and, or, not, while, until, dotimes, return, match. and and or short-circuit. :else is cond's catch-all.

(defconst nums [5 i32] [1 3 8 9 10])

(defn classify [n i32] string
  (cond
    (< n 0)  "negative"
    (= n 0)  "zero"
    :else    "positive"))

(defn countdown [n i32]
  (let [i n]
    (while (> i 0)
      (print-i64 (i64 i))
      (print-str " ")
      (set i (- i 1)))
    (newline)))

(defn first-even [s [i32]] (Option i32)
  (dotimes [i (len s)]
    (when (= 0 (% (at s i) 2))
      (return (Some (at s i)))))
  None)

(defn main []
  (print-line (classify -3))
  (countdown 4)
  (unless false
    (print-line "unless runs when the test is false"))
  (match (first-even (slice nums 0 (len nums)))
    (Some n) (do (print-i64 (i64 n)) (newline))
    None     (print-line "none")))

negative
4 3 2 1 
unless runs when the test is false
8

dotimes evaluates its bound once into a hidden slot before the loop, so a body that changes it cannot change the trip count, and the loop variable is not assignable.

Loops are imperative, with while, until and return. There is no loop/recur. There is no break or continue yet either; both refuse by name:

break is not implemented yet (see the build sequence in plan.org)

An early exit out of a loop is return, as first-even does above.

Option, match and some

(Option T) is how absence is spelled: a lookup miss, an empty collection, the end of a stream. match works on an Option and on nothing else today. some unwraps Some and early-returns None from the enclosing function.

(defconst nums [4 i32] [4 8 15 16])

;; `some` unwraps Some and early-returns None from *this* function.
(defn doubled-first [s [i32]] (Option i32)
  (Some (* 2 (some (index-of-i32 s 15)))))

(defn main []
  (match (doubled-first (slice nums 0 4))
    (Some i) (do (print-i64 (i64 i)) (newline))   ; 4
    None     (print-line "not found"))
  (match (index-of-i32 (slice nums 0 4) 99)
    (Some i) (do (print-i64 (i64 i)) (newline))
    None     (print-line "not found")))

4
not found

defer

A defer runs at function exit, innermost first. An explicit return runs the ones registered above it — a defer written below a return has not executed yet and must not fire.

(defn work [n i32] i32
  (defer (print-line "second"))
  (defer (print-line "first"))     ; innermost-first at exit
  (when (< n 0)
    (return 0))                    ; runs both defers above it
  (print-line "body")
  n)

(defn main []
  (print-i64 (i64 (work 3)))
  (newline))

body
first
second
3

defer is function-scoped and is rejected inside a let, a loop or a branch. Block scoping it is not done:

defer must be a top-level form in a function body — block-scoped defer is not implemented yet (milestone 4)

Arrays and slices

at and nth are the same operation and take any number of indices, so (at grid r c) indexes a two-dimensional fixed array directly. len works on a fixed array, a slice or a string. (slice s lo hi) takes a half-open range and never copies.

(defconst rows 3)
(defconst cols 4)

;; A fixed array is a value: inline storage, copies on assignment.
(defconst palette [4 u32] [0xE6B800FF 0x3B6E8CFF 0xA83232FF 0xCC6B1FFF])

;; No initialiser means all-bytes-zero, so this is BSS and costs nothing.
(defvar grid [rows [cols i32]])

(defn main []
  (set (at grid 1 2) 7)
  (print-i64 (i64 (at grid 1 2))) (newline)    ; 7
  (print-i64 (i64 (len palette))) (newline)    ; 4

  ;; A slice is ptr+len and non-owning: it views the array, it does not copy it.
  (let [row (slice (at grid 1) 0 cols)]
    (set (at row 0) 5)
    (print-i64 (i64 (at grid 1 0))) (newline)  ; 5 — the same storage
    (print-i64 (sum-i32 row)) (newline))       ; 12

  ;; (zeroed) is a memset, not an allocation.
  (set grid (zeroed))
  (print-i64 (i64 (at grid 1 2))) (newline))   ; 0

7
4
5
12
0

A reversed range — lo greater than hi — traps, rather than yielding a huge unsigned length.

The prelude

The prelude is written in Flan, all but one line of it, and prepended to every program, so nothing in it needs importing. Printing is deliberately not a primitive: write-stdout is the one output primitive and everything above it is ordinary Flan.

The RNG is ours, not libc's — PCG-XSH-RR 32, written in Flan — because a grid hash is only a regression test if the sequence is byte-identical on native and on wasm32. The parsers are ours too: strtoll answers 0 for "", 0 for "abc" and 12 for "12x", which are three wrong answers a caller cannot tell from a real 12.

sqrt-f32 is the one function in the file that is not Flan: (declare sqrt-f32 [x f32] f32 "sqrtf"). Every other number here is reachable from the four operations and a cast; a square root is not, and the usual trick of seeding Newton's method from the exponent bits needs a bit-cast between f32 and u32 that the language does not have. IEEE-754 makes sqrt correctly rounded, so libm gives the same bit pattern on both targets anyway. Every link carries -lm.

There is no println. There is no overloading yet, so each printer names its type. (println 1) is unknown function println.

The primitives underneath are few — a primitive is the only thing implemented twice per backend: argv, write-stdout, exit, len, at, slice, bytes, bytes->f64, bytes->i64, f64->bytes, i64->bytes, addr, and arithmetic, comparison and casts.

Packages

The directory is the package. Every file in a directory shares one top-level scope; files within a package do not import each other, and their order does not matter. The package declaration is optional and the name is inferred from the directory, so a loose file in a scratch directory is a package of one with no manifest and no ceremony.

;; geom/vec.flan — no package declaration: the name comes from the directory.
(defstruct V2 [x f32  y f32])

(defn add [a V2  b V2] V2
  (V2 {:x (+ (.x a) (.x b))  :y (+ (.y a) (.y b))}))

;; geom/len.flan — a second file in the same directory shares one top-level
;; scope: it does not import vec.flan, and the order of the two does not matter.
(defn length [v V2] f32
  (sqrt-f32 (+ (* (.x v) (.x v)) (* (.y v) (.y v)))))

;; pkg.flan — the directory is the package, and everything it declares
;; arrives qualified by the alias this import chose.
(import g "geom")

(defn main []
  (let [v (g/add (g/V2 {:x 3.0 :y 0.0})
                 (g/V2 {:x 0.0 :y 4.0}))]
    (print-f64 (f64 (g/length v)))
    (newline)))

5

There is one form and one meaning: (import alias "path"), and everything from the package is qualified alias/name. There is no unqualified-import mode, no :refer, no ns form and no per-file namespace object. A path with no collection prefix is relative to the importing file; vendor: and core: are collections, resolved by walking up from the importing file until a directory of that name is found.

Importing is a rename. Every top-level name the package declares becomes alias/name, and every use of one — in a type, in a body, in a struct literal, in an array length — is rewritten to match. Nothing downstream knows a package existed.

Three more rules:

• A package may be a single .flan file named outright, for the program that is also a library. sand.flan shares a directory with three other loose programs, so naming its directory would import all four.
• A package may import a package, and the qualification flattens to the inner alias: raylib imported by a package that is itself imported is still rl/…. A directory is keyed by its real path and read once, so a cycle ends there. The same directory under two different aliases is refused.
• main is not exported. A package carrying one would collide with the importer's, and main is a reachability root, so an imported one would keep everything it calls alive. Writing sand/main is refused at the line that wrote it.

Visibility is that one rule and no more: there is no package-private marker for anything other than main yet.

A package may carry the C it binds to. Every .c file in the directory is compiled into the build, and a file named link lists extra linker arguments. Whether those reach the build is decided after checking, from the program rather than from the import list: the compiler starts at main, follows every call, and a package none of whose externs survive contributes no C and no linker argument, so a file that imports raylib still builds for wasm32.

Conditions and restarts

A condition is a struct. There is no class hierarchy; matching is by type. The signalling end says here is something notable, here is the data, and an outer caller decides what to do about it — or decides nothing, in which case the signaller carries on.

(signal c)                  ; Unit. Handler returns -> carry on. No handler -> no-op.
(error  c)                  ; Never. Only a transfer gets past; else the program stops.

(handler-bind [(Type [c] body ...) ...] body ...)     ; match by type, no hierarchy

(restart-case BODY          ; BODY and every clause have the same type = the form's
  (name [] CLAUSE) ...)

(invoke-restart 'name)      ; Never. Innermost frame offering the name wins.

signal has type Unit, always. A handler that returns normally leaves the signaller to carry on — the accumulation case:

(defstruct AssetMissing [id i32])

(defvar seen i64)

(defn load-all []
  (signal (AssetMissing {:id 1}))     ; Unit — the caller carries on
  (signal (AssetMissing {:id 2})))

(defn main []
  (load-all)                          ; no handler: a no-op
  (print-i64 seen) (newline)          ; 0

  ;; A handler that returns normally accumulates and lets the signaller run on.
  (handler-bind [(AssetMissing [c] (set seen (+ seen (i64 (.id c)))))]
    (load-all))
  (print-i64 seen) (newline))         ; 3

0
3

A handler that invokes a restart instead transfers control outward to the restart-case that offers the name, and the clause's value becomes that form's value. Every defer between the invoke and the target runs, innermost first, before the clause body starts.

(defstruct AssetMissing [id i32])

(defvar cleanups i64)

(defn load [n i32] i32
  (signal (AssetMissing {:id n}))
  100)

(defn middle [n i32] i32
  (defer (set cleanups (+ cleanups 1)))   ; runs on the transfer too
  (+ (load n) 1))

(defn fetch [n i32] i32
  (restart-case (middle n)                ; its value if nothing transfers
    (use-placeholder [] -1)
    (retry           [] 7)))

(defn main []
  (print-i64 (i64 (fetch 1))) (newline)   ; 101 — nothing handled it

  (handler-bind [(AssetMissing [c] (invoke-restart 'use-placeholder))]
    (print-i64 (i64 (fetch 2))) (newline)) ; -1

  (print-i64 cleanups) (newline))         ; 2 — the defer ran both times

101
-1
2

Restart lookup walks the dynamic restart stack from innermost outward and takes the first frame offering the name, so an inner restart-case shadows an outer one for the duration of its body. An inner parser's skip-form is found before an outer one's.

How a transfer is lowered

A transfer is not platform unwinding. Every Flan signature carries a transfer channel — one pointer appended as an out-parameter — which invoke-restart writes and every call site checks. A callee writes the target into its caller's slot; each frame checks, runs its defers and returns early. The disassembly is the release one plus a guard after each call.

Three consequences:

• wasm32 works with no exception proposal, and native and wasm builds of the same program agree.
• Every function carries the channel, release builds included. A hot-reload cell holds a bare pointer, so the honest answer to "what can this call?" is "anything". A later optimisation may stop a function checking the channel; it may not drop the parameter.
• A transfer cannot cross a foreign frame. A handler installed across an FFI boundary must return normally.

Gotchas

• A handler closes over nothing. A clause is lifted into a function of its own, because it runs from wherever the signal was. Accumulate into a global, or put the value on the condition. A reference to an enclosing local is refused for that reason rather than reported as an unknown name.
• A restart re-runs whatever sits between it and the target. Control resumes at the restart-case, so a retry repeats the side effects after it. Put the restart-case where re-entry is safe.
• An unknown restart name is a hard stop — a located runtime error. There is no find-restart to test with yet.
• No supertype, so nothing can say "any condition".
• signal cannot hand a value back. Deliberate: the alternative forces every signal site to declare a default value and a result type.
• return is refused inside a handler-bind or restart-case body, and so is invoke-restart inside a defer. In each case a bare exit would leave frames on the stack pointing into a function that has gone.
• Restarts take no parameters in this version.

The break loop

An error nothing handles does not kill the program. It stops on the frame that erred, with nothing unwound, so the condition and every restart between there and the top are still live:

flan: unhandled Missing — stopped, not dead.
  restart: retry
  restart: use-placeholder

From there you fix the function, install it, and take a restart. Control never left the erring frame, so retry calls through the indirection cell and reaches the new body. Installing while stopped is allowed; there is no frame in progress.

The break loop lives in vendor/agent, which is an optional package. A program that does not import it leaves the hook null and stops the old way — the message and an exit status of 134:

(defstruct Missing [id i32])

(defn load [n i32] i32
  (restart-case
    (do (error (Missing {:id n}))   ; Never — only a transfer gets past
        0)
    (use-placeholder [] -1)
    (retry           [] 7)))

(defn main []
  (print-i64 (i64 (load 1)))
  (newline))

$ flan run boom.flan
unhandled Missing
$ echo $?
134

The FFI

There are two declaration forms, and they are two forms rather than one because no structural rule could tell them apart.

declare names a C symbol in a signature Flan can already spell. Nothing is generated; a Flan string crosses as ptr+len, exactly as it is stored.

(declare cos-f64 [x f64] f64 "cos")

(defn main []
  (print-f64 (cos-f64 0.0)) (newline))

1

That is 1.0, printed by the same rule as before.

declare-c names the C library's own function in the C library's own signature, and the compiler writes the wrapper. This is what raylib's package is made of — one line per binding:

(declare-c unload-texture [texture Texture2D] "UnloadTexture")

(declare-c draw-texture
  [texture Texture2D  x i32  y i32  tint Color]
  "DrawTexture")

An aggregate's calling convention is not part of its layout. On x86-64, clang gives raylib's own prototypes <2 x float> for a returned Vector2, i32 for a Color argument, and { i64, i64 } for a returned Rectangle — none of which is the struct's own LLVM type, and arm64 and wasm32 classify differently again. Reproducing that in the backend would be three classifiers to write and keep correct forever, and a mistake would show up as a field full of garbage rather than as a link error.

So the boundary has one wrapper per binding, each flattening the aggregates: a struct returns through an out-pointer, a struct argument is passed by pointer, and clang classifies all of it, per target, for free. flan shim <file> prints the whole generated file, of which this is the end — the rest is the typedefs and a comment saying not to edit it:

(defstruct Vector2 [x f32  y f32])

(declare-c get-mouse-position [] Vector2 "GetMousePosition")

typedef struct flan_ty_Vector2_1bebc5ae_s flan_ty_Vector2_1bebc5ae;

struct flan_ty_Vector2_1bebc5ae_s {  /* Vector2 */
  float x;
  float y;
};

/* get-mouse-position */
extern flan_ty_Vector2_1bebc5ae GetMousePosition(void);
void flan_shim_get_mouse_position_5ad0e205(flan_ty_Vector2_1bebc5ae *out) {
  *out = GetMousePosition();
}

No library header is read, deliberately, so a build needs the shared library to be linkable and not the -devel package to be installed. Guaranteed: the C typedef and the Flan struct come from the same defstruct, so they cannot disagree, and clang type-checks the wrapper against the generated prototype. Trusted: that the defstruct matches the library's real struct, and that the declare-c signature is the function's real signature. A scalar's width now carries ABI weight — f64 where the library says float emits double, and the library reads garbage.

Everything the boundary cannot represent is refused by name with the reason, rather than half-supported: an Option, a union, a fixed array, a map, a returned string, a callback, a slice parameter, an unknown type, an unrepresentable struct field, and two Flan names for one C symbol. An aggregate in a plain declare is refused too, so the narrow boundary cannot quietly acquire one.

One known edge: a REPL redefinition that introduces a new declare-c cannot work, because the reload path compiles no C and the wrapper would not exist in the running process. Editing the body of a function that calls an existing binding is unaffected.

The dev loop

Edit the code, keep the sand.

$ flan dev sand.flan

That builds the program, launches it, holds a session beside it, and listens on .flan-dev.sock next to the source. From Emacs, C-c C-c on a function recompiles it and installs it into the running process at that process's next frame boundary. The window does not blink and the grid does not reset.

How it works

Four pieces, each runnable on its own.

The reload primitive. llc → ld -shared → dlopen → call. Measured in this codebase:

About 19ms end to end. The clang driver on the same IR is 50ms, so the dev path never invokes it. The driver forks a second process and re-does argument and target resolution; codegen is not the cost.

Indirection cells. Loading a new body is not installing it. A call bound at link time cannot be made to notice one, so a --dev build routes every Flan-to-Flan call through a cell — a mutable global holding the address of the function that is current.

Here is the whole of hello.flan through flan emit --dev:

@"flan.cell.print-line" = global ptr @"flan.print-line"

define {} @"flan.main"(ptr %xfer) {
entry:
  %t1 = load ptr, ptr @"flan.cell.print-line"
  %t2 = call {} %t1(%slice { ptr @".str.36", i64 15 }, ptr %xfer)

The call site loads the cell rather than naming @"flan.print-line" directly. The signature carries ptr %xfer — the transfer channel from conditions, on every Flan function, release builds included.

Redefinition is then one store, below a microsecond. Three rules follow. A redefinition module declares every global external, so globals live in the host and survive a reload. Every other function is a declare, so a redefined settle calls the host's move-grain rather than freezing a private copy of it. And nothing is ever dlclosed: a cell holds an address inside a module's text, so unloading it would leave call sites pointing at unmapped memory. Old code is never unloaded, so a thread mid-execution finishes safely in the old version.

The agent. vendor/agent is a package like any other: a listener thread, a single-producer ring, and three calls. This is the whole of agent.flan — no aggregate crosses the boundary, so a plain declare does it and there is no shim.

(declare start-raw [path string] i32 "flan_agent_start")
(declare poll-raw [] i32 "flan_agent_poll")
(declare wait-raw [ms i32] i32 "flan_agent_wait")

(defn start [path string] i32 (start-raw path))
(defn poll [] i32 (poll-raw))
(defn wait [ms i32] i32 (wait-raw ms))

(agent/start path) listens on a unix socket, once, at startup. (agent/poll) installs whatever has arrived and returns how many. (agent/wait ms) is the same but waits for something first. A headless test uses it so that a reload is deterministic rather than a race against the frame rate.

Loading and installing are separate. dlopen relocates a module and takes the loader lock — milliseconds, unbounded — so it happens on the listener thread. Installing is one store per function and must not land while a redefined function is on the stack, so it happens on the game thread, at the top of the frame, when the program asks. A game loop calls agent/poll at the top of its frame and ignores the result.

The session and the daemon. flan dev holds the declarations the running process was built from plus every change accepted since, and it owns the build, so its rules describe the process that is actually running. Re-checking the whole program on every evaluation costs under 10ms, less than the llc that follows. An evaluation is transactional: a form that fails to check mutates nothing.

The protocol is one s-expression per message, length-framed by a decimal byte count. It is not nREPL: eval there is string-in/string-out with no slot for which form, from which file, and once the editor client is ours too there is no CIDER to be compatible with.

What a running process cannot be told

Some changes are refused with a reason rather than loaded, because the alternative is a silent mismatch against memory the process has already laid out:

A defvar's initial value is deliberately not on that list: its storage holds state the program moved past long ago. And a defconst the checker never consumed can be changed, so a colour table can be tuned live while an array length stays refused. A dev build emits those as mutable globals, so LLVM cannot fold a read of one.

The signature row is a stopgap. The design is versioned functions with their own trampolines, so that new callers resolve the new version while existing ones keep the old. None of the three parts exists yet, and the alternative to refusing is a silent argument mismatch.

Evaluating an expression

C-x C-e is a different primitive from redefining a name. There is no name to install a body into, so the expression is wrapped in a thunk with nowhere to be called from. The module says run this once, and the agent calls it after the install, on the game thread, at a frame boundary. An expression reading the program's state therefore sees a consistent one.

Nothing is marshalled back, because nothing could be: a Flan value carries no header, so no code at run time can say what it is. The compiler knows the type and renders it there, in the thunk. What comes back looks like this:

big                       18446744073709551615
col                       :blue
(.pos b)                  (V {:x 1.5 :y 0})
b                         (Blob {:id 7 :name "sandy \"quoted\"" :pos (V {:x 1.5 :y 0}) :tags [ 0 42 0]})
(slice (.tags b) 0 3)     [ 0 42 0]
(rl/get-color 0x11223344) (rl/Color {:r 17 :g 34 :b 51 :a 68})
sim/grid                  [ [ 0 0 0 0 0 0 0 0 ...] [ 0 ... ] ...]

A pointer is never followed; it renders as <ptr>. Following one would make the walk cycle, and dereferencing a pointer a REPL was handed is not safe. The walk is bounded at depth 4 and 8 elements, and the output truncates at 4K. Map, function values and type variables refuse by name.

The thunk's module is unloaded afterwards. Nothing points into its text once it has returned. Sixteen expression evaluations retain zero mappings, where each redefinition retains three, permanently and correctly.

Emacs

emacs/flan-mode.el derives from prog-mode with lisp-mode's syntax table, so sexp motion, paren matching and indentation are already right. It adds Flan's brackets — [ and { are brackets, not symbol characters, since every binding list and every type is written with them — and the characters a Flan name may contain. emacs/flan-dev.el is the client; there is no parser in it.

C-c C-k sends one module rather than a form at a time on purpose: a defvar and the function that uses it have to arrive in the same load, or the first refers to storage that does not exist yet.

eldoc, completion and M-. all read one cached reply rather than asking per keystroke, refreshed at the two moments the answer can have changed: on connect, and after an evaluation the daemon accepted. The modeline says whether there is a program on the other end, and says stopped when there is one sitting in the break loop — a stopped program looks exactly like a running one from anywhere else in Emacs.

An error comes back with a location and the client draws an overlay there, cleared the next time that buffer's evaluation is accepted. The repl buffer is comint-derived and every line goes through the same request C-x C-e uses; it is program-scoped, so in sand you write sim/settle and not settle.

Targets and builds

Native x86-64 is the development target. --target=wasm32-wasi produces a module, and the headless sand acceptance program prints the same 64-bit hash under it as it does natively:

$ flan run test/programs/sand-headless.flan
-2851001042534928384
$ flan build test/programs/sand-headless.flan --target=wasm32-wasi -o sand.wasm
$ node --no-warnings test/wasm-run.mjs sand.wasm
-2851001042534928384

The RNG is written in Flan rather than called from libc for that number: a grid hash is only a regression test if the sequence is byte-identical on both targets. It holds at -O2 and at -O0. The wasm side needs a wasm32 builtins archive — from wasi-sdk, or emscripten's substituting for it — and the compiler names every path it looked in when it cannot find one.

Dev and release builds are deliberately different. --dev means indirection cells and -rdynamic, which exports those cells for a loaded module to bind to. Release builds call directly, emit constants as constants, and get all the folding back. Dev builds are not pruned by reachability: what a REPL may redefine next is not a function of what has been called so far.

--debug is a third flag beside --dev and the optimisation level. --dev asks whether you can redefine the program while it runs; --debug asks whether you can stop it and read it. It emits DWARF, sets -O0, and is refused by name for wasm32. lldb needs no plugin to read a Flan struct: the struct is its C struct.

Some things are refused by name rather than half-supported, and both cross-target refusals say why:

$ flan run hello.flan --target=wasm32-wasi
flan run: --target is refused — a cross-built module is not something this host
can exec. Use flan build --target=... and a wasm runtime.

$ flan build hello.flan --dev --target=wasm32-wasi
wasm32: --dev is native only — the reload path is dlopen, which wasm32 has no
equivalent of

Build time for calc-me.flan is about 110ms, of which the frontend — read, parse, load, check, emit — is under 10ms. Every C translation unit goes through an object cache keyed by a digest of the source text, the compiler, the optimisation level and the target flags, so it never needs invalidating by hand.

Not implemented yet

The house rule is that anything which binds a name, alters control flow, or is not yet implemented must be recognised explicitly and rejected. Each of these refuses by name, with the milestone it belongs to, and the tests assert on the reason.

Beyond that list, and just as true: there is no allocator and no context; there is no println and no overloading; restarts take no parameters; match works on an Option and nothing else; defer is function-scoped; there is no package-private marker other than main not being exported; there are no threads in the language; and the managed class facility that plan.org describes is a plan and not a feature.

The vocabulary in spec-memory.md is normative but largely unbuilt: clone, as-slice, push, get, put, resolve and allocator-aware operations belong to Vec and Map, and arrive with them.

Two of these are settled rather than pending. There is no interpreter and there is not going to be one: the compiled path is the only backend. The instrumentation-based step debugger that wanted one is cut, and compiled redefinition at ~19ms is perceptually instant for expression evaluation too. And the macro expander is blocked on unions rather than on itself — a macro is a function from Form to Form, which needs Form to exist as a Flan union value first.

Further reading

The repository's own documents, in the order they are worth reading:

• plan.org — the design, the build sequence, and the open decisions.
• NEXT.md — the project's memory, and the authority on what is actually built.
• spec-memory.md — ownership, containers, places, generics, function values. Frozen.
• spec-conditions.md — conditions and restarts, operational semantics. Frozen.
• conditions.org — a cheatsheet for driving conditions.
• calc-me.flan, sand.flan, test/programs/ — real programs that run.