A place in update, ++ and -- evaluates each of its subexpressions once, and (update place f args ...) stores (f old args ...) back into it

TODO.org

@@ -1959,21 +1959,6 @@ CLOSED: [2026-09-25]
 =put= on an instance still checks a declared slot's type and still inserts an
 undeclared key; only =set= refuses one, since a slot it writes has to exist.
 
-** NEXT update: change a place by applying a function to it
-Decided 2026-09-25: every place evaluates each of its subexpressions once, C's compound-assignment rule, which also fixes =++= and =--=; =update= is built on that. Rules out refusing side effects in a place.
-=(set (.velocity g) (inc (.velocity g)))= names the place twice. Clojure's
-=update= would be a macro over the same two steps, for a struct field and a
-class slot alike.
-
-Blocked on the double-evaluation question, which =++=, =--= and any
-compound assignment share: =(update (at grid (next-index) c) inc)= evaluates
-=(next-index)= twice, and a place with a side effect is then wrong rather than
-slow. Either places get a general single-evaluation rule — bind every
-subexpression of a place to a temp once, which is what C's compound assignment
-does — or the language says a place must be side-effect free and refuses
-otherwise. The first is the real fix and it is a change to how every place
-lowers, not to one macro.
-
 ** TODO A session eval reported (CFn [] ()) does not cross into dyn yet
 At =sand.flan:46:20=, the =:pause= in =(when (get state :pause) (return))=,
 where =state= is a =defclass= instance with a =pause= slot. =(CFn [] ())= is

docs/BUILT.md

@@ -3887,7 +3887,7 @@ before this landed, so `macro-unless.flan` is a test written after the feature.
 ### The line between a special form and a macro
 
 A form the prelude itself relies on is built into the parser: `cond`, `when` and `dotimes`. A form only programs use
-is a prelude macro: `inc`, `++`, `into`, `unless`, `until` and `comment`. The reason is `Macro.reduce`: a prelude
+is a prelude macro: `inc`, `++`, `update`, `into`, `unless`, `until` and `comment`. The reason is `Macro.reduce`: a prelude
 function that calls a macro is left out of the module that runs macros, so a form the prelude's own functions use
 cannot be a macro without taking those functions away from every macro body. `until` peels an optional leading label
 and answers `(while :label (not test) body ...)`.

lib/parse.ml

@@ -449,6 +449,14 @@ and form f mk (head : Form.t) (args : Form.t list) : Ast.expr =
      | [ target; value ] -> mk (Ast.Set (place target, expr value))
      | _ -> fail f "set is (set place value)")
 
+  (* What [update], [++] and [--] expand into: (update~ PLACE g NEW), where
+     NEW is written over the name [g]. The name has a [~] in it so no program
+     can write it; only a prelude macro builds one. See [modify]. *)
+  | Sym "update~" ->
+    (match args with
+     | [ target; { v = Sym g; _ }; value ] -> modify f target g value
+     | _ -> fail f "internal: update~ is (update~ place name value) — a compiler bug")
+
   (* ── (vec-new [u8]) and (map-new string [u8]) ───────────────────────
      The type positions of these two take a type expression. Whether this call
      is the builtin at all is the checker's to know — a program may define its
@@ -1252,6 +1260,65 @@ and place (f : Form.t) : Ast.place =
        (deref p), or a class slot (get inst :slot)"
       (Form.to_string f)
 
+(* A read-modify-write of a place, with every subexpression of the place
+   evaluated once — C's rule for compound assignment. (update (at grid (next)
+   c) inc) calls [next] once, and the read and the write land on the same
+   element.
+
+   Each index, key and pointer is bound to a temp first, outermost and
+   leftmost first. The container a field or an index is taken from is not: it
+   has to stay a path to the storage, since a temp would be a copy of a struct
+   or an array and the write would land in the copy. Only a path made of
+   names, fields, indexes and derefs stays one; anything else in container
+   position — a call answering a Vec, say — is a value, and is bound like an
+   index. Then [g] is bound to the place's current value, and [value], written
+   over [g], is stored back through the same path. *)
+and modify f target g value : Ast.expr =
+  let mk e = { Ast.e; loc = f.loc } in
+  let binds = ref [] in
+  let temp (e : Ast.expr) =
+    match e.Ast.e with
+    | Ast.Int _ | Ast.UInt _ | Ast.Float _ | Ast.Byte _ | Ast.Str _ | Ast.Kw _ -> e
+    | _ ->
+      let t = fresh_temp "place" in
+      binds := { Ast.bname = t; bty = None; bval = e; bloc = e.Ast.loc } :: !binds;
+      { e with Ast.e = Ast.Var t }
+  in
+  let rec path (e : Ast.expr) =
+    match e.Ast.e with
+    | Ast.Var _ -> e
+    | Ast.Field (t, n) -> { e with Ast.e = Ast.Field (path t, n) }
+    | Ast.Call (({ Ast.e = Ast.Var "at"; _ } as h), t :: idx) when idx <> [] ->
+      let t = path t in
+      { e with Ast.e = Ast.Call (h, t :: List.map temp idx) }
+    | Ast.Call (({ Ast.e = Ast.Var "deref"; _ } as h), [ p ]) ->
+      { e with Ast.e = Ast.Call (h, [ temp p ]) }
+    | _ -> temp e
+  in
+  let p =
+    match place target with
+    | Ast.Pvar _ as p -> p
+    | Ast.Pfield (t, n) -> Ast.Pfield (path t, n)
+    | Ast.Pindex (t, idx) ->
+      let t = path t in
+      Ast.Pindex (t, List.map temp idx)
+    | Ast.Pderef p -> Ast.Pderef (temp p)
+    | Ast.Pslot (t, k) ->
+      let t = temp t in
+      Ast.Pslot (t, temp k)
+  in
+  let at e = { Ast.e; loc = target.loc } in
+  let read =
+    match p with
+    | Ast.Pvar n -> at (Ast.Var n)
+    | Ast.Pfield (t, n) -> at (Ast.Field (t, n))
+    | Ast.Pindex (t, idx) -> at (Ast.Call (at (Ast.Var "at"), t :: idx))
+    | Ast.Pderef p -> at (Ast.Call (at (Ast.Var "deref"), [ p ]))
+    | Ast.Pslot (t, k) -> at (Ast.Call (at (Ast.Var "get"), [ t; k ]))
+  in
+  let old = { Ast.bname = g; bty = None; bval = read; bloc = target.loc } in
+  mk (Ast.Let (List.rev (old :: !binds), [ mk (Ast.Set (p, expr value)) ]))
+
 and arms f (items : Form.t list) : Ast.arm list =
   let rec go = function
     | [] -> []

lib/prelude.ml

@@ -2270,16 +2270,12 @@ let source = {flan|
 ;; because a macro does not have a type at all; the expansion is checked at the
 ;; call site as if it had been written there.
 ;;
-;; **++ and -- read the place twice, and that is an accepted cost.** The
-;; expansion is (set PLACE (+ PLACE 1)), so PLACE is evaluated once to read
-;; and once to write. For a variable, a field or a deref that is free and
-;; means nothing. For (at arr (next-index)) — an index with a side effect —
-;; it means next-index runs twice and the read and the write land on different
-;; elements. That is not a bug to be fixed here: macros are non-hygienic by
-;; decision (plan.org, open decision 2), a macro cannot bind a temporary for
-;; the *place* without a reference type it does not have, and
-;; rl/with-drawing and rl/with-mode-2d already take the same trade on their
-;; arguments. Write the index out first if it does anything.
+;; **++ and -- evaluate the place once.** Each index, key and pointer in the
+;; place is bound to a temp before the read, so (++ (at arr (next-index)))
+;; calls next-index once and reads and writes the same element — C's rule for
+;; compound assignment. They are update with + and -, spelled as the form
+;; update~ that update itself expands into (a prelude macro may not call a
+;; macro); lib/parse.ml's [modify] is where the place is taken apart.
 (defmacro inc [& args]
   (if (!= (length args) 1)
     `(inc-takes-one-number)
@@ -2293,12 +2289,31 @@ let source = {flan|
 (defmacro ++ [& args]
   (if (!= (length args) 1)
     `(++-takes-one-place)
-    `(set ~(at args 0) (+ ~(at args 0) 1))))
+    (let [g (gensym)]
+      `(~(Form.Sym {.s "update~"}) ~(at args 0) ~g (+ ~g 1)))))
 
 (defmacro -- [& args]
   (if (!= (length args) 1)
     `(---takes-one-place)
-    `(set ~(at args 0) (- ~(at args 0) 1))))
+    (let [g (gensym)]
+      `(~(Form.Sym {.s "update~"}) ~(at args 0) ~g (- ~g 1)))))
+
+;; ── update: change a place by applying a function to it ────────────────
+;;
+;;     (update (.velocity g) inc)
+;;     (update (at grid r c) + 10)
+;;
+;; (update place f args ...) stores (f old args ...) back into the place, where
+;; old is what the place held. f is written as the head of a call, so it may be
+;; a function, an operator or a macro such as inc. Every place set takes is a
+;; place here too — a name, a field, an element, a deref, a class slot — and
+;; the place is evaluated once, as ++ says above. It answers what set answers.
+(defmacro update [& args]
+  (if (< (length args) 2)
+    `(update-takes-a-place-and-a-function)
+    (let [g (gensym)]
+      `(~(Form.Sym {.s "update~"}) ~(at args 0) ~g
+         (~(at args 1) ~g ~@(form-rest args 2))))))
 
 ;; ── into: a fused transformation, and not a transducer ────────────────
 ;;

spec-syntax.md

@@ -142,7 +142,7 @@ Each item: the proposal, then the reason in one line.
   index, field).
 - **`==` is `=`; `=` is assignment.** `x = v` reads `(set x v)`, `a[i] = v`
   reads `(set (at a i) v)`, `p.x = v` reads `(set (.x p) v)`. `x += v` reads
-  `(set x (+ x v))`; like `++` today, the place is evaluated twice.
+  `(update x + v)`, which evaluates the place once, as `++` does.
 - **A run of the same operator flattens** (variadics, section 3):
   `a + b + c` reads `(+ a b c)`, `a < b < c` reads `(< a b c)` (Flan's chain
   semantics, `test/programs/chain.flan`). This keeps the converter round trip

test/programs/update-place.flan

@@ -0,0 +1,55 @@
+;;;; update, ++ and -- evaluate every subexpression of their place once, as
+;;;; C's compound assignment does. `calls` counts the index function: one call
+;;;; per form, and the read and the write land on the same element.
+
+(defonce calls i32 0)
+
+(defn next-index [] i32
+  (set calls (+ calls 1))
+  (- calls 1))
+
+(defstruct Body [velocity i32 hits [3 i32]])
+
+(defn add [x i32 y i32] i32 (+ x y))
+
+(defclass counter [n i32])
+
+(defn which-slot [] dyn
+  (set calls (+ calls 1))
+  :n)
+
+(defn main [] i32
+  (let [xs [10 20 30]
+        v (vec-new i32)
+        g (Body {.velocity 5})]
+    (push v 1) (push v 2) (push v 3)
+    ;; next-index answers 0, then 1, then 2.
+    (++ (at xs (next-index)))
+    (-- (at v (next-index)))
+    (update (at xs (next-index)) * 3)
+    (println calls)                              ; 3
+    (println (at xs 0) (at xs 1) (at xs 2)) ; 11 20 90
+    (println (at v 0) (at v 1) (at v 2))    ; 1 1 3
+    ;; A field, with a macro as the function, and with arguments after it.
+    (update (.velocity g) inc)
+    (update (.velocity g) add 10)
+    (println (.velocity g))                      ; 16
+    ;; A path through a field into an element: the struct is written in place,
+    ;; not in a copy.
+    (set calls 0)
+    (update (at (.hits g) (next-index)) + 7)
+    (++ (at (.hits g) (next-index)))
+    (println calls)                              ; 2
+    (println (at (.hits g) 0) (at (.hits g) 1)) ; 7 1
+    ;; Through a pointer.
+    (let [p (addr (.velocity g))]
+      (update (deref p) * 2)
+      (println (.velocity g)))                   ; 32
+    (free v))
+  ;; A class slot, with the key computed once.
+  (let [c (counter 4)]
+    (set calls 0)
+    (++ (get c (which-slot)))
+    (update (get c (which-slot)) * 10)
+    (println calls (get c :n)))                  ; 2 50
+  0)

test/test_acceptance.ml

@@ -432,6 +432,13 @@ let () =
       match_enum_out;
     outputs ~dev:true "match over an enum, dev" "programs/match-enum.flan"
       match_enum_out;
+    (* update, ++ and -- evaluate their place's subexpressions once: the
+       counts are the number of calls an index or a key function got. *)
+    let update_out = "3\n11 20 90\n1 1 3\n16\n2\n7 1\n32\n2 50\n" in
+    outputs "update evaluates its place once" "programs/update-place.flan"
+      update_out;
+    outputs ~x86:true "update evaluates its place once, --x86"
+      "programs/update-place.flan" update_out;
     (* The count is [length] so that [len] is left to programs, and this is
        the claim that it really is one: a local holding a count, a
        parameter, and a defn the program calls by its bare name, all of
@@ -4904,6 +4911,17 @@ level "1"
       "(defn main [] i32 (++) 0)" "++-takes-one-place";
     macro_arity "-- with two arguments"
       "(defn main [] i32 (let [a 1 b 2] (-- a b)) 0)" "---takes-one-place";
+    macro_arity "update with no function"
+      "(defn main [] i32 (let [a 1] (update a)) 0)"
+      "update-takes-a-place-and-a-function";
+    (* A place update takes is a place set takes, and is refused the same way. *)
+    macro_arity "update of something that is not a place"
+      "(defn main [] i32 (update 5 inc) 0)" "5 is not assignable";
+    macro_arity "update of a parameter"
+      "(defn f [a i32] () (update a inc))" "a is a parameter";
+    macro_arity "update whose function answers the wrong type"
+      "(defn yes [x i32] bool true)\n(defn f [] () (let [a 1] (update a yes)))"
+      "expected i32";
     (* unless keeps a guard, and it is now the narrower one: a body may be
        missing, a test may not. *)
     macro_arity "unless with no test at all"