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A defstruct whose fields introduce $t or an array length $n is a generic struct, each application of it an ordinary struct copy that generic functions bind against, and a printed call is evaluated once

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This commit is contained in:
Joseph Ferano 2026-09-25 16:08:14 +07:00
parent 680b8e7e59
commit 4054c4921b
17 changed files with 1111 additions and 191 deletions
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13
TODO.org
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@ -750,15 +750,10 @@ depth it gave up at. The bare depth number is a backstop that also prints the
chain. Before any of it, the compiler hung rather than failed, which wedges =C-c
C-c= with nothing to show.
** NEXT Generic types
Decided 2026-09-25: the freeze is lifted for this; build both type and length parameters.
=(defstruct Pair [a $t b $t])= cannot be spelled, and neither can a length
parameter. =Types.Named= is a bare string with no room for parameters; giving it
some changes the type, the layout calculator, both backends, the renderer and the
DWARF path. Same price for one as for both. Decided and unblocked, deliberately
not started — it is a language feature under a freeze, and it was stopped once
already for that reason. The motivating case is Odin's =Small_Array=: a
fixed-capacity array with a count and no allocation.
** DONE Generic types
CLOSED: [2026-09-25]
A struct's parameters are its fields' $-names in first-written order, a length by position; there is no
explicit parameter vector. Each application is an ordinary struct under a key, so no backend sees a parameter.
** WAIT A value predicate over a length parameter
Decided 2026-09-25: waits until a program wants one.

3
lib/ast.ml
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@ -24,6 +24,9 @@ and texpr_kind =
them identically — the difference is a fact about the value, and it is
[Check.resolve] that turns it into one. *)
| Tfn of bool * texpr list * texpr
(* An integer written as a generic struct's argument, the 8 in
(Small 8 i32). Parsed only there; it is not a type anywhere else. *)
| Tlen of int64
(* An array length is an integer or a compile-time constant's name. *)
and len =

984
lib/check.ml
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File diff suppressed because it is too large Load Diff

1
lib/cimport.ml
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@ -407,6 +407,7 @@ let rec ty_source (t : Ast.texpr) =
| Ast.Tname n -> n
| Ast.Tapp (n, args) ->
Printf.sprintf "(%s %s)" n (String.concat " " (List.map ty_source args))
| Ast.Tlen n -> Int64.to_string n
| Ast.Tslice (c, e) ->
Printf.sprintf "[%s%s]" (if c then "const " else "") (ty_source e)
| Ast.Tarray (Ast.Lint n, e) -> Printf.sprintf "[%Ld %s]" n (ty_source e)

21
lib/dev.ml
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@ -1819,8 +1819,27 @@ let defs t =
~loc:(Loc.to_string loc) ())
classes
in
(* A generic struct is listed by its template, as [(Pair $t)]; its copies
are struct names only the compiler wrote. *)
let structs =
Hashtbl.fold
(fun name _ acc ->
if Hashtbl.mem env.Check.copies name then acc
else entry ~name ~kind:"struct" ~sign:name ~loc:"" () :: acc)
env.Check.structs []
@ Hashtbl.fold
(fun name (g : Check.gstruct) acc ->
entry ~name ~kind:"struct"
~sign:
(Printf.sprintf "(%s %s)" name
(String.concat " "
(List.map (fun (p, _) -> "$" ^ p) g.Check.gparams)))
~loc:"" ()
:: acc)
env.Check.gstructs []
in
List.sort compare
(of_table "struct" env.Check.structs
(structs
@ datas @ classes
@ of_table "union" env.Check.unions
@ of_table "enum" env.Check.enums

7
lib/emit.ml
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@ -370,7 +370,7 @@ let rec ll (t : Types.t) =
integer spelling costs no casts and keeps the emitter honest about not
knowing whether the bits are a pointer. *)
| Types.Dyn -> "i64"
| Types.Var _ ->
| Types.Var _ | Types.Len _ | Types.LArray _ ->
(* The checker rejects it by name — nothing reaches here. *)
internal "no layout for %s" (Types.to_string t)
@ -638,7 +638,8 @@ let rec lay m (t : Types.t) : int * int =
| Some u -> union_lay m u
| None -> internal "no layout for struct %s" n)
| Types.Dyn -> 8, 8
| Types.Var _ -> internal "no layout for %s" (Types.to_string t)
| Types.Var _ | Types.Len _ | Types.LArray _ ->
internal "no layout for %s" (Types.to_string t)
(* Size, alignment, and the offset of every member. *)
and lay_fields m tys =
@ -1175,7 +1176,7 @@ let rec dty m d (t : Types.t) : int =
reading: it prints, and the person reading it can hand it to the
runtime's own printer. *)
| Types.Dyn -> basic "dyn" 64 "DW_ATE_unsigned"
| Types.Var _ ->
| Types.Var _ | Types.Len _ | Types.LArray _ ->
internal "no debug type for %s" (Types.to_string t)
in
Hashtbl.replace d.dtys key n;

2
lib/js.ml
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@ -249,6 +249,8 @@ let rec refuse_ty loc (t : Types.t) =
host's own, and that work has not been done"
| Types.Var n ->
at loc "a type variable (%s) reached the backend, which cannot happen" n
| Types.Len _ | Types.LArray _ ->
at loc "a length variable reached the backend, which cannot happen"
(* Aggregates in the sense that matters here: the types whose assignment
copies in Flan and would alias in JS. A slice is deliberately not one —

8
lib/load.ml
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@ -205,8 +205,11 @@ let rec rename_texpr owned alias (t : Ast.texpr) : Ast.texpr =
Ast.Tarray (rename_len owned alias l, rename_texpr owned alias e)
| Ast.Tmap (k, v) ->
Ast.Tmap (rename_texpr owned alias k, rename_texpr owned alias v)
(* The head too, when it is a generic struct the package declares. *)
| Ast.Tapp (n, args) ->
let n = if List.mem n owned then qualify alias n else n in
Ast.Tapp (n, List.map (rename_texpr owned alias) args)
| Ast.Tlen _ as k -> k
| Ast.Tfn (env, ps, r) ->
Ast.Tfn (env, List.map (rename_texpr owned alias) ps,
rename_texpr owned alias r)
@ -792,8 +795,11 @@ let rec texpr_uses acc (t : Ast.texpr) =
(match l with Ast.Lname n -> acc := (n, t.Ast.tloc) :: !acc | Ast.Lint _ -> ());
texpr_uses acc e
| Ast.Tmap (k, v) -> texpr_uses acc k; texpr_uses acc v
| Ast.Tapp (_, args) -> List.iter (texpr_uses acc) args
| Ast.Tapp (n, args) ->
acc := (n, t.Ast.tloc) :: !acc;
List.iter (texpr_uses acc) args
| Ast.Tfn (_, ps, r) -> List.iter (texpr_uses acc) ps; texpr_uses acc r
| Ast.Tlen _ -> ()
let rec expr_uses acc (e : Ast.expr) =
let go = expr_uses acc in

20
lib/parse.ml
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@ -136,7 +136,25 @@ let rec texpr (f : Form.t) : Ast.texpr =
mk (Ast.Tfn (env, List.map texpr params, texpr ret))
| _ -> fail f "a function type is (%s [T ...] R)" which)
| List ({ v = Sym name; _ } :: args) when args <> [] ->
mk (Ast.Tapp (name, List.map texpr args))
(* An integer argument is a generic struct's length, and a type
constructor is capitalised. A lowercase head is a body form in the
return slot — (+ x 1) — and its integer is the type parser's reason to
give up, which is the refusal that slot is built on. *)
let capitalised =
let base =
match String.rindex_opt name '/' with
| Some i -> String.sub name (i + 1) (String.length name - i - 1)
| None -> name
in
base <> "" && Char.uppercase_ascii base.[0] = base.[0]
&& Char.lowercase_ascii base.[0] <> base.[0]
in
let arg (a : Form.t) =
match a.v with
| Int n when capitalised -> { Ast.t = Ast.Tlen n; tloc = a.loc }
| _ -> texpr a
in
mk (Ast.Tapp (name, List.map arg args))
| _ -> fail f "expected a type, found %s" (Form.to_string f)
and len (f : Form.t) : Ast.len =

11
lib/session.ml
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@ -597,7 +597,7 @@ let compatible ~loc (old_ : Tast.program) (new_ : Tast.program) =
if not same then
fail loc
"%s changes layout. Restart to change it."
s.Tast.sname
(Types.to_string (Types.Named s.Tast.sname))
| None -> ())
new_.Tast.structs
@ -2610,10 +2610,12 @@ let eval_expr ?(origin = "<eval>") ?(pause = false) t src : change =
let placed =
List.filter (fun (f : Tast.fn) -> List.mem f.Tast.name own) placed
in
let copies = Check.fresh_copies t.env t.program.Tast.structs in
let program =
{ t.program with
Tast.fns = t.program.Tast.fns @ fresh @ placed;
structs = t.program.Tast.structs @ Check.env_structs t.env lifted;
structs =
t.program.Tast.structs @ copies @ Check.env_structs t.env lifted;
externs = t.program.Tast.externs @ externs }
in
let ir =
@ -2639,7 +2641,10 @@ let eval_expr ?(origin = "<eval>") ?(pause = false) t src : change =
caller closes that half by taking a [held] before this and restoring it
when either fails — a copy the session holds and no module defines is a
null cell exactly as a stranded declaration is. *)
t.program <- { t.program with Tast.fns = t.program.Tast.fns @ fresh };
t.program <-
{ t.program with
Tast.fns = t.program.Tast.fns @ fresh;
structs = t.program.Tast.structs @ copies };
{ ir; x86 = t.x86; names = []; fns = []; installs = true; stale = [] }
(* ── What a macro call expands to ──────────────────────────────────── *)

1
lib/shim.ml
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@ -256,6 +256,7 @@ let rec cty env ~needed ~loc ~what (t : Ast.texpr) : string =
fail loc "%s is a function type, and a C callback is not implemented" what
| Ast.Tapp (n, _) ->
fail loc "%s is %s, which is not a type this shim generator knows" what n
| Ast.Tlen n -> fail loc "%s is %Ld, which is not a type" what n
(* ── What one parameter does at the boundary ────────────────────────── *)

26
lib/types.ml
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@ -106,6 +106,15 @@ type t =
| Fn of t list * t (* (Fn [T ...] R) *)
| CFn of t list * t (* (CFn [T ...] R) *)
| Var of string (* a type variable — milestone 5 *)
(* The two halves of a length parameter, and neither is the type of a value.
[Len] is a length standing where a generic struct's argument goes — the 8
in (Small 8 i32) — and what a length variable is bound to. [LArray] is a
fixed array whose length is a variable, [[$n $t]], and exists only in a
generic signature, as the pattern a call site binds [n] from. A generic
body is checked with its lengths at [Check.abstract_len], so neither ever
reaches a backend. *)
| Len of int64
| LArray of string * t
(* [dyn]: one machine word whose contents the runtime knows and this module
does not. It is a written type — [(defonce x dyn 5)] boxes the 5 — and it
is also what an unannotated [defn] parameter means, which is why it is a
@ -206,8 +215,20 @@ let rec equal a b =
&& List.for_all2 equal ps ps'
&& equal r r'
| Var x, Var y -> String.equal x y
| Len x, Len y -> Int64.equal x y
| LArray (n, x), LArray (m, y) -> String.equal n m && equal x y
| _ -> false
(* How a generic struct's copy is spelled to a reader. The copy is an
ordinary struct under a symbol-safe key — [Small-8-i32] — and this is the
key's written form, [(Small 8 i32)], filled in as each copy is made. Global
rather than on a checker's env because every message that prints a type
comes through here with no env in hand. The key determines the spelling,
so an entry left from an earlier program in the same process is wrong only
for a struct that program's successor declares under a copy's key by hand,
and then only in how a message spells it. *)
let display : (string, string) Hashtbl.t = Hashtbl.create 16
let rec to_string = function
| Int k -> ikind_name k
| Float k -> fkind_name k
@ -215,7 +236,8 @@ let rec to_string = function
| String -> "string"
| Unit -> "()"
| Never -> "Never"
| Named n | Enum n -> n
| Named n -> (match Hashtbl.find_opt display n with Some d -> d | None -> n)
| Enum n -> n
| Slice (Mut, t) -> "[" ^ to_string t ^ "]"
| Slice (Const, t) -> "[const " ^ to_string t ^ "]"
| Array (n, t) -> Printf.sprintf "[%Ld %s]" n (to_string t)
@ -232,6 +254,8 @@ let rec to_string = function
Printf.sprintf "(CFn [%s] %s)"
(String.concat " " (List.map to_string ps)) (to_string r)
| Var n -> "$" ^ n
| Len n -> Int64.to_string n
| LArray (n, t) -> Printf.sprintf "[$%s %s]" n (to_string t)
| Dyn -> "dyn"
let is_numeric = function Int _ | Float _ -> true | _ -> false

1
lib/x86.ml
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@ -532,6 +532,7 @@ let is_agg (t : Types.t) =
the arithmetic. *)
| Types.Dyn -> false
| Types.Var v -> unsupported "type variable %s" v
| Types.Len _ | Types.LArray _ -> unsupported "length variable"
let is_void (t : Types.t) = match t with Types.Unit | Types.Never -> true | _ -> false
let is_float (t : Types.t) = match t with Types.Float _ -> true | _ -> false

89
test/programs/generic-struct.flan Normal file
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@ -0,0 +1,89 @@
;;;; Generic structs, end to end: type parameters and length parameters.
;;;;
;;;; A defstruct whose fields introduce $t is a template, and each set of
;;;; arguments it is given is a copy — an ordinary struct. A parameter is a
;;;; length when it stands in an array's length slot, and a type anywhere else;
;;;; the arguments are written in the order the fields first introduce them.
;;;;
;;;; Small is Odin's Small_Array: a fixed-capacity array with a count, and no
;;;; allocation anywhere.
(defstruct Small [items [$n $t] count i32])
;; A generic function over a generic struct binds both of its parameters from
;; the argument, and reads the length back as a value.
(defn append! [s (Ptr (Small $n $t)) x $t] bool
(if (< (.count s) n)
(do (set (at (.items s) (.count s)) x)
(set (.count s) (+ (.count s) 1))
true)
false))
(defn pop! [s (Ptr (Small $n $t))] (Option $t)
(if (= (.count s) 0)
None
(do (set (.count s) (- (.count s) 1))
(Some (at (.items s) (.count s))))))
(defn capacity [s (Ptr (Small $n $t))] i32 n)
(defn total [s (Ptr (Small $n $t))] $t {:where (numeric? $t)}
(let [acc (the $t 0)]
(dotimes [i (.count s)]
(set acc (+ acc (at (.items s) i))))
acc))
;; A type parameter alone, built positionally with the type read off the
;; fields, and returned under a variable.
(defstruct Pair [a $t b $t])
(defn swapped [p (Pair $t)] (Pair $t) (Pair (.b p) (.a p)))
;; A copy that names itself through a pointer, and a literal field that
;; takes its width from the one beside it.
(defstruct Node [v $t next (Option (Ptr (Node $t)))])
(defn sum-list [n (Ptr (Node i64))] i64
(loop [at n acc (the i64 0)]
(let [acc (+ acc (.v at))]
(match (.next at)
(Some p) (recur p acc)
None acc))))
;; A template naming another at its own parameters.
(defstruct Twice [x (Small $m $u) y (Small $m $u)])
;; A length variable straight on an array parameter.
(defn len-of [a [$k $e]] i32 k)
(defconst cap 3)
(defn main [] i32
(let [s (the (Small 4 i32) (zeroed))
f (the (Small cap f64) (zeroed))]
(append! (addr s) 10)
(append! (addr s) 20)
(append! (addr s) 30)
(println (total (addr s)) (.count s) (capacity (addr s)))
(append! (addr f) 1.5)
(append! (addr f) 2.5)
(append! (addr f) 3.5)
(println (append! (addr f) 4.5) (total (addr f)) (capacity (addr f)))
(println (pop! (addr f)) (pop! (addr f)) (.count f))
(let [p (Pair 1 2)
q (swapped p)
r (swapped (Pair {.a 1.5 .b 2.5}))]
(println (.a q) (.b q) (.a r) (.b r)))
(let [c (the (Node i64) {.v 3})
b (Node 2 (Some (addr c)))
a (Node 1 (Some (addr b)))]
(println (sum-list (addr a))))
(let [w (the (Twice 2 u8) (zeroed))]
(append! (addr (.y w)) 7)
(println (.count (.x w)) (.count (.y w)) (capacity (addr (.x w)))))
(println (len-of [1 2 3]) (len-of [1.5 2.5]))
(let [v (vec-new (Pair i32))]
(push v (Pair 5 6))
(println (.b (at v 0)))
(free v))
0))

12
test/test_acceptance.ml
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@ -3466,6 +3466,18 @@ let () =
outputs "generics" "programs/generics.flan" generics_out;
outputs ~opt:"-O0" "generics, -O0" "programs/generics.flan" generics_out;
(* Generic structs — see the program's header. The third line is two pops
printed in one call, which is also the pin for a printed call being
evaluated once: the walk reads an option's tag and then its payload,
and each read used to make the call again. *)
let generic_struct_out =
"60 3 4\nfalse 7.5 3\n(some 3.5) (some 2.5) 1\n2 1 2.5 1.5\n6\n\
0 1 2\n3 2\n6\n"
in
outputs "generic structs" "programs/generic-struct.flan" generic_struct_out;
outputs ~x86:true "generic structs, --x86" "programs/generic-struct.flan"
generic_struct_out;
(* integer?, end to end — see the program's own header. The first eight
lines are the collapsed abs at six widths and both signed minimums
(which answer themselves; the negation wraps). The [0 0] after them is

83
test/test_flan.ml
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@ -1470,10 +1470,10 @@ let () =
can actually be written there; the parameter-vector suggestion survives
where it works, which the return-type pin further down exercises. *)
rejects_check "a real type variable at a field" "(defstruct Holder [x elem])"
~needle:"a field is built at one type for every value";
~needle:"in a defstruct's fields that makes the struct generic over it";
rejects_check "and the field message offers what a field can hold"
"(defstruct Holder [x elem])"
~needle:"Write a concrete type here, or dyn to hold any value";
~needle:"Write $elem, a concrete type, or dyn to hold any value";
rejects_check "an unknown concrete type" "(defn f [x Widget] ())"
~needle:"unknown type Widget";
@ -2871,13 +2871,11 @@ let () =
(* [(Pair i32)] in a defonce falls down the value fork now that the third
element takes either reading, and the generics answer the type fork gave
it has to be reachable from here too. *)
(* A capitalised head with arguments is a *type* given type arguments, and
that is the half of generics that is not built — Types.Named is a bare
string with no room for parameters. The sentence says which half, since
generic functions are here and pointing at them is the useful part. *)
(* A capitalised head with arguments is a *type* given type arguments; with
no such struct declared, the sentence says how one is. *)
rejects_check "a capitalised call with arguments is a generic type"
"(defonce x (Pair i32)) (defn f [] i32 0)"
~needle:"is a generic type, which is not there yet";
~needle:"no struct or generic struct Pair is declared";
accepts "and the generic function it points at is"
"(defn pair-fst [a $t b $u] $t (do b a))\n\
(defn main [] () (println (pair-fst 1 true)))";
@ -6701,9 +6699,74 @@ let () =
"(defn f [a $t b $u] i32 (do a b (let [v (vec-new $w)] (free v) 0)))";
(* Where no variable is in scope there is none to name, and the answer is
the rule: a sigil binds, and only a defn signature is a binding site. *)
rejects_check "a sigil in a struct field, where nothing can bind one"
~needle:"only a defn signature can"
"(defstruct S [v $t])";
rejects_check "a sigil in a data case's field, where nothing can bind one"
~needle:"only a defn signature or a defstruct's fields can"
"(defdata D [(C [v $t])])";
(* ── Generic structs: what is refused, and where ─────────────────── *)
rejects_check "a generic struct given the wrong number of arguments"
~needle:"Pair takes 1 argument, (Pair $t), and this gives 2"
"(defstruct Pair [a $t b $t]) (defn f [p (Pair i32 i64)] i32 0)";
rejects_check "a generic struct named with no arguments"
~needle:"Pair is generic, and a type only once it is given its arguments"
"(defstruct Pair [a $t b $t]) (defn f [p Pair] i32 0)";
rejects_check "a type where a length argument goes"
~needle:"Small's $n is a length"
"(defstruct Small [items [$n $t] count i32]) \
(defn f [p (Small i32 4)] i32 0)";
rejects_check "a length where a type argument goes"
~needle:"Small's $t is a type, and 4 is a length"
"(defstruct Small [items [$n $t] count i32]) \
(defn f [p (Small 4 4)] i32 0)";
rejects_check "a negative length argument"
~needle:"-1 is negative"
"(defstruct Small [items [$n $t] count i32]) \
(defn f [p (Small -1 i32)] i32 0)";
rejects_check "one variable as both a length and a type"
~needle:"$t stands for a length in one place here and a type in another"
"(defstruct Bad [x $t y [$t i32]])";
rejects_check "a length variable where a type goes"
~needle:"n is a length, not a type"
"(defn f [a [$n i32]] i32 (let [x (the n 0)] 0))";
rejects_check "a where clause over a length variable"
~needle:"$n is a length, and a where clause takes type predicates only"
"(defn f [a [$n i32]] i32 {:where (numeric? $n)} 0)";
rejects_check "a generic struct that contains itself by value"
~needle:"(Loop $t) contains itself by value"
"(defstruct Loop [next (Loop $t)])";
rejects_check "a generic struct that asks for bigger copies of itself"
~needle:"Grow names a copy of itself at a type built around its own"
"(defstruct Grow [next (Ptr (Grow [$t]))]) (defn f [p (Grow i32)] i32 0)";
rejects_check "a copy whose key is already a struct's name"
~needle:"Pair at these arguments is called Pair-i32, and Pair-i32 is \
already defined"
"(defstruct Pair [a $t b $t]) (defstruct Pair-i32 [x i32]) \
(defn f [p (Pair i32)] i32 0)";
rejects_check "a generic struct literal whose fields decide nothing"
~needle:"Pair's $t is not decided by the fields given here"
"(defstruct Pair [a $t b $t]) (defn f [] i32 (let [p (Pair {})] 0))";
rejects_check "two fields that disagree about the variable"
~needle:"(Pair $t)'s .b is i32 here, and this is f64"
"(defstruct Pair [a $t b $t]) \
(defn f [] i32 (let [p (Pair (the i32 1) (the f64 2.5))] 0))";
accepts "a literal field takes its width from a typed one beside it"
"(defstruct Pair [a $t b $t]) \
(defn f [] f64 (let [p (Pair 1 (the f64 2.5))] (.a p)))";
rejects_check "a generic struct as a condition"
~needle:"Pair is generic, and a condition struct is not"
"(defstruct Pair :parent Error [a $t])";
rejects_check "an operator a generic body's struct field does not support"
~needle:"+ over the type variable $t"
"(defstruct Pair [a $t b $t]) (defn f [p (Pair $t)] $t (+ (.a p) (.b p)))";
accepts "the same body with the predicate declared"
"(defstruct Pair [a $t b $t]) \
(defn f [p (Pair $t)] $t {:where (numeric? $t)} (+ (.a p) (.b p))) \
(defn main [] i32 (f (Pair 1 2)))";
accepts "a copy wanted where it is built takes its type from there"
"(defstruct Pair [a $t b $t]) (defn f [] (Pair i64) (Pair 1 2))";
accepts "a defonce of a generic struct's copy"
"(defstruct Pair [a $t b $t]) (defonce g (Pair i32)) \
(defn main [] i32 (.a g))";
(* ── The builtin table against the arms it describes ──────────────
[Check.builtins] is what the editor's C-c C-v and M-. read for a name no

20
test/test_session.ml
View File

@ -354,6 +354,26 @@ let () =
| exception Loc.Error { Loc.dmsg = m; _ } ->
fail "the session was poisoned by a bad expression: %s" m);
(* A generic struct's copy first named by an expression typed at the
session: the module built for it has to lay the copy out, and the
session keeps it, as it keeps a generic function's copy. *)
(let gt, _ = Session.create ~file:"programs/reload.flan" () in
(match Session.eval gt "(defstruct Pair [a $t b $t])" with
| _ -> ()
| exception Loc.Error { Loc.dmsg = m; _ } ->
fail "a generic struct was refused at the session: %s" m);
match Session.eval_expr gt "(println (.b (Pair 7 8)))" with
| e ->
if not (has e.Session.ir "%\"Pair-i32\" = type") then
fail "the expression's module did not carry the struct copy";
if not
(List.exists
(fun (s : Tast.structure) -> String.equal s.Tast.sname "Pair-i32")
gt.Session.program.Tast.structs)
then fail "the session did not keep the struct copy an expression made"
| exception Loc.Error { Loc.dmsg = m; _ } ->
fail "an expression building a generic struct was refused: %s" m);
(* The other half of "a refusal costs nothing", and the half that used to be
missing: a form can check and *then* fail, in the build or at the agent,
and the session that already accepted it has no way to hear about it