(** The checker: AST → typed IR.

    Two passes, because top-level names in a package are order-independent
    (plan.org, Modules): the first collects every type, signature and global,
    the second checks bodies against them. Mutually recursive functions need no
    forward declaration, and a struct may be used above where it is declared.

    Checking is *bidirectional*. An expression is checked against an expected
    type when there is one and inferred when there is not, which is what makes
    [None], a bare [0] and a struct literal work without any inference engine:
    the expected type flows in from the function's return type, the parameter
    it is being passed to, or the field it is being stored in.

    The rule from the two misparse bugs applies here too: *anything not yet
    implemented is rejected by name*, never approximated. Milestone 2 is
    calc-me.flan and nothing more (plan.org, Build sequence), so [Vec], [Map],
    [Result]/[try], user unions, closures, [dotimes], [defer], generics and
    cross-package imports are all errors with a message that says which
    milestone they belong to. *)

let fail = Loc.fail

(* [List.map]'s evaluation order is unspecified, and checking allocates frame
   slots as a side effect. Left-to-right is required, not a preference: a later
   let binding sees an earlier one, and slot numbering must be reproducible. *)
let rec map_lr f = function
  | [] -> []
  | x :: rest -> let y = f x in y :: map_lr f rest

let rec map2_lr f xs ys =
  match xs, ys with
  | [], [] -> []
  | x :: xs, y :: ys -> let z = f x y in z :: map2_lr f xs ys
  | _ -> invalid_arg "map2_lr"

(* ── Environments ──────────────────────────────────────────────────── *)

type binding = {
  slot       : int;
  bty        : Types.t;
  assignable : bool;   (* locals are places; parameters are not — spec-memory *)
}

type env = {
  structs : (string, Tast.structure) Hashtbl.t;
  unions  : (string, Tast.union) Hashtbl.t;
  aliases : (string, Ast.texpr) Hashtbl.t;
  consts  : (string, int64) Hashtbl.t;         (* compile-time array lengths *)
  locs    : (string, Loc.t) Hashtbl.t;         (* where each type was declared *)
  (* Enum name -> its members, in declaration order. A keyword at a call site
     resolves against this and nothing else. *)
  enums   : (string, (string * int64) list) Hashtbl.t;
  (* Flan name -> the C symbol it is really called by. A foreign function is an
     ordinary entry in [fns] as well; this only records how to name it. *)
  externs : (string, string) Hashtbl.t;
  fns     : (string, Types.t list * Types.t) Hashtbl.t;
  globals : (string, Types.t * bool) Hashtbl.t;   (* type, is a constant *)
  (* Functions the checker made up: a handler-bind clause is lifted into one,
     because a handler is called from wherever the signal was and cannot be a
     branch in the function that established it. *)
  mutable lifted : Tast.fn list;
}

let new_env () = {
  structs = Hashtbl.create 16;
  unions  = Hashtbl.create 16;
  aliases = Hashtbl.create 16;
  consts  = Hashtbl.create 16;
  locs    = Hashtbl.create 16;
  enums   = Hashtbl.create 8;
  externs = Hashtbl.create 32;
  fns     = Hashtbl.create 32;
  globals = Hashtbl.create 16;
  lifted = [];
}

(* Per-function state. Slots are never reused, so [slots] is also the frame
   size — the interpreter allocates one array of this length per call. *)
type ctx = {
  env           : env;
  ret           : Types.t;
  mutable slots : int;
  (* The type of each slot, newest first. A backend needs it to size the
     frame — nothing else records it, since the IR refers to slots by index. *)
  mutable slot_tys : Types.t list;
  (* The source name of each slot, newest first, parallel to [slot_tys].
     [None] for a slot the checker invented -- see [Tast.fn.snames]. Recorded
     here rather than recovered later because this scope list is the only place
     that ever knows it. *)
  mutable slot_names : string option list;
  mutable scope : (string * binding) list;   (* innermost first *)
  (* Deferred forms, most recently registered first — which is also the order
     they run in. At milestone 4 [defer] is function-scoped (see [check_fn]),
     so this list belongs to the function and not to a block. *)
  mutable defers : Tast.expr list;
  (* Only for the two things a handler clause cannot do. [outer] is the
     establishing function's scope, kept so that a reference to one of its
     locals can be refused for the reason it is really refused for rather than
     as an unknown name. *)
  outer : (string * binding) list;
  mutable in_handler : bool;
  (* True wherever handler or restart frames established by this function are
     on the stack. A [return] from there would leave them pointing into a frame
     that has gone, so it is refused — the same rule as [defer] inside a
     block. *)
  mutable in_frames : string option;
  (* True inside a [defer]'s forms. A defer is the cleanup a transfer runs on
     its way out (§5), so a transfer *starting* there has no answer: this
     function's defers are already half run and the first transfer's target is
     already in hand. Refused where it is written. *)
  mutable in_defer : bool;
  (* The function being checked, so a clause lifted out of it can be named
     after it. The name has to be stable and has to say whose it is: a
     redefinition module emits the clauses belonging to the bodies it is
     replacing, and nothing else in the program can tell it which those are. *)
  owner : string;
}

(* [?name] is the source name, when there is one. It is optional so that the
   several places that allocate a hidden slot say nothing and get [None] --
   a synthesized slot cannot accidentally acquire a name it was never given. *)
let fresh_slot ?name ctx ty =
  let s = ctx.slots in
  ctx.slots <- s + 1;
  ctx.slot_tys <- ty :: ctx.slot_tys;
  ctx.slot_names <- name :: ctx.slot_names;
  s

(* Shadowing is legal -- [(let [v 11] (let [v 22] ...))] is two slots, both
   named [v] -- and the debug info has nowhere to put the distinction. Every
   [!DILocalVariable] is scoped to the subprogram, because the typed IR has no
   block structure for a [!DILexicalBlock] to be built from, so two variables
   called [v] land in one flat scope and lldb answers [p v] with whichever it
   finds first. Measured, not assumed: it answers with the *outer* one, so it
   prints 11 while the body it is stopped in is computing with 22, and the
   inner binding is not listed at all.

   That is the one outcome worse than printing [s3]: a name the debugger is
   confident about and wrong about. So a repeat of a name already bound in this
   function gets a suffix, and both bindings are then visible and unambiguous.
   [~] is the reader's delimiter and cannot occur in a source symbol (the same
   reason [destructure~nth] is spelled that way), so [v~2] is visibly the
   compiler's doing and can never collide with something the programmer wrote.

   This is a way of not lying, not a way of being right: [v] is still the outer
   binding everywhere, including inside the inner one's extent. Scoping the
   variables properly means emitting a [!DILexicalBlock] per [Let] and moving
   the [llvm.dbg.declare]s out of the entry block to the binding sites, which
   needs block structure this IR does not carry. *)
let bind ctx name bty ~assignable =
  let taken n = List.exists (fun s -> s = Some n) ctx.slot_names in
  let name' =
    if not (taken name) then name
    else
      let rec go k =
        let c = Printf.sprintf "%s~%d" name k in
        if taken c then go (k + 1) else c
      in
      go 2
  in
  let slot = fresh_slot ~name:name' ctx bty in
  (* [ctx.scope] keeps the *source* name: the suffix is a debug-info artifact
     and resolving [v] must still find the innermost binding. *)
  ctx.scope <- (name, { slot; bty; assignable }) :: ctx.scope;
  slot

let lookup ctx name = List.assoc_opt name ctx.scope

(* A handler clause is lifted into a function of its own, so the establishing
   function's locals are simply not there. Capturing them is a closure with an
   explicit environment — spec-memory.md's case 2, a non-escaping [fn] capturing
   by value into a stack environment, since a handler frame does not outlive the
   function that pushed it — and until that exists a reference to one is refused
   for the reason it is really refused for, rather than as a name nobody has
   heard of. *)
let captured ctx loc name =
  if ctx.in_handler && List.mem_assoc name ctx.outer then
    raise
      (Loc.Error
         (loc,
          Printf.sprintf
            "a handler cannot see %s: it is a local of the function that \
             established the handler, and a handler runs from wherever the \
             signal was. Use a global, or pass it on the condition." name))

let scoped ctx f =
  let saved = ctx.scope in
  let r = f () in
  ctx.scope <- saved;
  r

(* ── Type resolution ───────────────────────────────────────────────── *)

let unimplemented loc what milestone =
  fail loc "%s is not implemented yet — milestone %d (see plan.org)"
    what milestone

let rec resolve env ?(seen = []) (t : Ast.texpr) : Types.t =
  let loc = t.Ast.tloc in
  match t.Ast.t with
  | Ast.Tname n -> resolve_name env ~seen loc n
  | Ast.Tslice e -> Types.Slice (resolve env ~seen e)
  | Ast.Tarray (l, e) -> Types.Array (array_len env loc l, resolve env ~seen e)
  | Ast.Tmap _ -> unimplemented loc "the Map type" 6
  (* The function *value* is refused where it is written; the annotation was
     not refused anywhere, so [(defn f [g (Fn [] i32)])] type checked and then
     died in emit with "no layout for". Refused here, beside the Map line
     above, which is the same shape of not-yet. *)
  | Ast.Tfn _ -> unimplemented loc "a function type" 5
  | Ast.Tapp (name, args) ->
    (match name, args with
     | "Ptr", [ a ] -> Types.Ptr (resolve env ~seen a)
     | "Option", [ a ] -> Types.Option (resolve env ~seen a)
     | ("Ptr" | "Option"), _ -> fail loc "(%s T) takes exactly one type" name
     | "Vec", _ -> unimplemented loc "(Vec T)" 6
     | "Map", _ -> unimplemented loc "(Map K V)" 6
     | "Result", _ -> unimplemented loc "(Result T E)" 6
     | "Handle", _ -> unimplemented loc "(Handle T)" 6
     | _ ->
       fail loc
         "%s takes no type arguments — generics are milestone 5" name)

(* One edit away from a type that exists — a substitution, an insertion, a
   deletion or a transposition of neighbours. Bounded at one, because two edits
   is no longer a typo, it is a guess. *)
and near_miss env n =
  let one_edit a b =
    let la = String.length a and lb = String.length b in
    if abs (la - lb) > 1 then false
    else begin
      (* Walk both until they diverge, then require the tails to match with the
         single edit applied. *)
      let i = ref 0 in
      while !i < la && !i < lb && a.[!i] = b.[!i] do incr i done;
      let ta s k = String.sub s k (String.length s - k) in
      if la = lb then
        !i < la
        && (ta a (!i + 1) = ta b (!i + 1)
            (* stirng/string: two neighbours swapped. *)
            || (!i + 1 < la && a.[!i] = b.[!i + 1] && a.[!i + 1] = b.[!i]
                && ta a (!i + 2) = ta b (!i + 2)))
      else if la < lb then ta a !i = ta b (!i + 1)
      else ta a (!i + 1) = ta b !i
    end
  in
  let candidates =
    Types.primitive_names
    @ Hashtbl.fold (fun k _ acc -> k :: acc) env.aliases []
    @ Hashtbl.fold (fun k _ acc -> k :: acc) env.structs []
    @ Hashtbl.fold (fun k _ acc -> k :: acc) env.unions []
    @ Hashtbl.fold (fun k _ acc -> k :: acc) env.enums []
  in
  List.find_opt (fun c -> c <> n && one_edit n c) candidates

and resolve_name env ~seen loc n =
  match Types.ikind_of_name n with
  | Some k -> Types.Int k
  | None ->
    match Types.fkind_of_name n with
    | Some k -> Types.Float k
    | None ->
      match n with
      | "bool" -> Types.Bool
      | "string" -> Types.String
      | "Unit" -> Types.Unit
      | "Never" -> Types.Never
      | _ when Hashtbl.mem env.aliases n ->
        if List.mem n seen then
          fail loc "the type alias %s is defined in terms of itself" n
        else resolve env ~seen:(n :: seen) (Hashtbl.find env.aliases n)
      | _ when Hashtbl.mem env.structs n -> Types.Named n
      (* A union has no layout in emit — nothing there mentions unions at all —
         so a union-typed global reached clang as a reference to an undefined
         %"U". Constructing one and reading a field of one are already refused,
         so there is nothing to lower: only a declaration that got through. *)
      | _ when Hashtbl.mem env.unions n ->
        unimplemented loc (Printf.sprintf "the union type %s" n) 6
      | _ when Hashtbl.mem env.enums n -> Types.Enum n
      (* A typo in a primitive is lowercase too, and the type-variable rule
         below would otherwise report [f65] as unimplemented generics and send
         you to plan.org instead of to the character you mistyped. *)
      | _ when near_miss env n <> None ->
        fail loc "unknown type %s — did you mean %s?" n
          (Option.get (near_miss env n))
      (* Lowercase is a type variable, Capitalized is concrete — no sigil
         (plan.org, Types). A variable parses, but nothing at milestone 2 can
         give a value one, so it is rejected here rather than later. *)
      | _ when n <> "" && n.[0] = Char.lowercase_ascii n.[0] ->
        unimplemented loc
          (Printf.sprintf "generic code over the type variable %s" n) 5
      | _ -> fail loc "unknown type %s" n

and array_len env loc = function
  | Ast.Lint n -> n
  | Ast.Lname n ->
    (match Hashtbl.find_opt env.consts n with
     | Some v -> v
     | None ->
       fail loc "%s is not a compile-time integer constant, so it cannot be \
                 an array length" n)

(* ── Small helpers over the AST ────────────────────────────────────── *)

(* Untyped literals: their machine type comes from context, so when one is an
   operand of a binary operator we look at the *other* operand first. *)
let is_literal (e : Ast.expr) =
  match e.Ast.e with Ast.Int _ | Ast.Float _ | Ast.Byte _ -> true | _ -> false

(* [addr] takes the address of a place, but the parser only builds places for
   [set]. Recover one from the expression it parsed instead. *)
let place_of_expr (e : Ast.expr) : Ast.place option =
  match e.Ast.e with
  | Ast.Var s -> Some (Ast.Pvar s)
  | Ast.Field (t, f) -> Some (Ast.Pfield (t, f))
  | Ast.Call ({ Ast.e = Ast.Var "at"; _ }, t :: idx) when idx <> [] ->
    Some (Ast.Pindex (t, idx))
  | Ast.Call ({ Ast.e = Ast.Var "deref"; _ }, [ p ]) -> Some (Ast.Pderef p)
  | _ -> None

let mk loc ty e : Tast.expr = { Tast.e; ty; loc }

let unit_at loc = mk loc Types.Unit Tast.Unit

(* Every integer index into an array or slice is i32 at milestone 2. *)
let index_ty = Types.Int Types.I32

(* A condition's type at run time is a number, and it has to be the *same*
   number in a module compiled later against a program already running. So it
   is a hash of the name and not an index into anything: an index would shift
   the moment a struct were added, and every handler pushed by the old code
   would then match the wrong type. FNV-1a over the name, 32 bits. *)
let type_id name =
  let h = ref 0x811c9dc5 in
  String.iter
    (fun c ->
       h := (!h lxor Char.code c) land 0xffffffff;
       h := (!h * 0x01000193) land 0xffffffff)
    name;
  !h

let expect loc ~want (got : Tast.expr) =
  match want with
  | None -> got
  | Some w ->
    if Types.fits ~expected:w ~actual:got.Tast.ty then got
    else
      fail loc "expected %s, found %s" (Types.to_string w)
        (Types.to_string got.Tast.ty)

(* ── Expressions ───────────────────────────────────────────────────── *)

let rec check ctx ?want (e : Ast.expr) : Tast.expr =
  let loc = e.Ast.loc in
  match e.Ast.e with
  | Ast.Int n -> int_literal loc ~want n
  | Ast.Byte b -> int_literal loc ~want ~default:Types.U8 (Int64.of_int b)
  | Ast.Float x ->
    let k =
      match want with
      | Some (Types.Float k) -> k
      | Some other when other <> Types.Never ->
        fail loc "expected %s, found the float literal %g"
          (Types.to_string other) x
      | _ -> Types.F64
    in
    mk loc (Types.Float k) (Tast.Float (x, k))
  | Ast.Str s -> expect loc ~want (mk loc Types.String (Tast.Str s))
  | Ast.Kw k ->
    (* A keyword resolves at compile time against the enum the site expects,
       and a typo is an error here rather than a wrong number at run time
       (plan.org, settled: keywords at typed call sites). It has no meaning
       without that expectation — there is no keyword type to fall back on. *)
    (match want with
     | Some (Types.Enum name) ->
       let members = Hashtbl.find ctx.env.enums name in
       (match List.assoc_opt k members with
        | Some v -> mk loc (Types.Enum name) (Tast.Int (v, Types.I32))
        | None ->
          fail loc "%s has no member :%s — it has %s" name k
            (String.concat " "
               (List.map (fun (m, _) -> ":" ^ m) members)))
     | Some other ->
       fail loc ":%s is an enum member, but %s is expected here" k
         (Types.to_string other)
     | None ->
       fail loc
         ":%s only means something where an enum type is expected — there is \
          no keyword type" k)
  | Ast.Quote _ ->
    unimplemented loc "a quoted symbol (restart names)" 6
  | Ast.Var name -> var ctx loc ~want name
  | Ast.Do body -> block ctx ?want loc body
  | Ast.Let (bs, body) -> check_let ctx ?want loc bs body
  | Ast.If (c, t, e') -> check_if ctx ?want loc c t e'
  | Ast.While (c, body) ->
    let c = check ctx ~want:Types.Bool c in
    let body = scoped ctx (fun () -> map_lr (fun b -> check ctx b) body) in
    expect loc ~want (mk loc Types.Unit (Tast.While (c, body)))
  | Ast.Return v when ctx.in_frames <> None ->
    ignore v;
    (* The frames are pushed and popped around the body, so an early exit would
       leave them on the handler or restart stack pointing into a frame that
       has gone. Rejected rather than left to corrupt it, the same rule as
       defer inside a block. *)
    fail loc
      "return is not allowed inside %s yet — the frames it established are \
       popped on the way out and an early exit would leave them on the stack"
      (match ctx.in_frames with Some n -> n | None -> assert false)

  | Ast.Return v ->
    let v =
      match v with
      | None ->
        if not (Types.equal ctx.ret Types.Unit) then
          fail loc "this function returns %s, so return needs a value"
            (Types.to_string ctx.ret);
        None
      | Some v -> Some (check ctx ~want:ctx.ret v)
    in
    (* Whatever has been deferred *so far* runs first: a defer written below
       this return has not executed yet and must not fire. *)
    let r = mk loc Types.Never (Tast.Return v) in
    (match ctx.defers with
     | [] -> r
     | ds -> mk loc Types.Never (Tast.Do (ds @ [ r ])))
  | Ast.Set (p, v) ->
    let p, pty = check_place ctx loc p in
    let v = check ctx ~want:pty v in
    expect loc ~want (mk loc Types.Unit (Tast.Set (p, v)))
  | Ast.Field (target, name) ->
    let target, sname = struct_target ctx target in
    let s = Hashtbl.find ctx.env.structs sname in
    (match Tast.field_index s name with
     | None -> fail loc "%s has no field %s" sname name
     | Some i ->
       let fty = (List.nth s.Tast.fields i).Tast.fty in
       expect loc ~want (mk loc fty (Tast.Field (target, i))))
  | Ast.Struct (name, kvs) -> check_struct ctx ~want loc name kvs
  | Ast.Arr items -> check_arr ctx ~want loc items
  | Ast.Match (scrutinee, arms) -> check_match ctx ?want loc scrutinee arms
  | Ast.Call (head, args) -> check_call ctx ~want loc head args
  | Ast.Unwrap (Ast.Usome, v) ->
    (* Unwrap Some, else early-return None from the enclosing function, so the
       enclosing function must itself return an Option (plan.org). *)
    (match ctx.ret with
     | Types.Option _ ->
       let v = check ctx v in
       (match v.Tast.ty with
        | Types.Option t ->
          expect loc ~want (mk loc t (Tast.UnwrapSome v))
        | other ->
          fail loc "some takes an (Option T), found %s" (Types.to_string other))
     | other ->
       fail loc
         "some early-returns None, so the enclosing function must return an \
          Option; this one returns %s" (Types.to_string other))
  | Ast.Unwrap (Ast.Utry, _) -> unimplemented loc "try (Result)" 6
  | Ast.Fn _ -> unimplemented loc "fn values" 5
  | Ast.Dotimes (name, count, body) -> check_dotimes ctx ~want loc name count body
  (* (signal c) : Unit, always — spec-conditions.md §1. A handler that returns
     normally leaves the signalling function to carry on, and with nothing
     matching this is a no-op, so nothing about it alters control flow. That is
     what makes it checkable here rather than needing the transfer machinery
     restart-case will want. *)
  | Ast.Signal (kind, c) ->
    let c = check ctx c in
    let name =
      match c.Tast.ty with
      | Types.Named n -> n
      | t ->
        fail c.Tast.loc
          "a condition is a struct, not %s — matching is by type and there is \
           no condition hierarchy"
          (Types.to_string t)
    in
    (* §1 and §2. [signal] is Unit whatever it finds; [error] is Never,
       because the only way past it is a handler that transfers — one that
       returns normally has not answered it, and the program stops. *)
    let ty, kind =
      match kind with
      | Ast.Ssignal -> (Types.Unit, Tast.Ssignal)
      | Ast.Serror -> (Types.Never, Tast.Serror)
    in
    expect loc ~want (mk loc ty (Tast.Signal (kind, type_id name, c)))

  | Ast.HandlerBind (clauses, body) -> check_handler_bind ctx ?want loc clauses body

  (* spec-conditions.md §3–§6: the transfer. Neither of these is a call — one
     establishes frames around a body, and the other leaves the function it is
     written in — so both are their own nodes all the way down. *)
  | Ast.RestartCase (body, clauses) -> check_restart_case ctx ?want loc body clauses
  | Ast.InvokeRestart name ->
    (* Never: control resumes at the restart-case, which yields the clause's
       value to *its* continuation, so nothing here has a value and nothing
       after it runs. The lookup is at run time because restarts are
       dynamically scoped and named — §4. *)
    if ctx.in_defer then
      fail loc
        "invoke-restart is not allowed inside a defer — a defer is the cleanup \
         a transfer runs on its way out, so starting one there would leave \
         this function's defers half run with two targets and no way to \
         choose";
    expect loc ~want (mk loc Types.Never (Tast.InvokeRestart (type_id name, name, loc)))

  | Ast.Defer _ ->
    (* Registered by [check_fn], which is the only place that sees a form's
       position. A defer anywhere else would run at function exit rather than
       at the exit of the block it is written in — once for a loop body that
       runs a thousand times — so it is rejected instead of quietly differing. *)
    fail loc
      "defer must be a top-level form in a function body — block-scoped defer \
       is not implemented yet (milestone 4)"

and int_literal loc ~want ?(default = Types.I32) n =
  match want with
  | Some (Types.Int k) -> mk loc (Types.Int k) (Tast.Int (in_range loc k n, k))
  (* An untyped integer constant is usable where a float is wanted, as in
     Odin. A float literal is never usable where an integer is wanted. *)
  | Some (Types.Float k) ->
    mk loc (Types.Float k) (Tast.Float (Int64.to_float n, k))
  | Some other when other <> Types.Never ->
    fail loc "expected %s, found the integer literal %Ld"
      (Types.to_string other) n
  | _ -> mk loc (Types.Int default) (Tast.Int (in_range loc default n, default))

(* Arithmetic wraps, but a literal that does not fit its type is a typo, not a
   wrap — 300 is never what someone meant by a u8. *)
and in_range loc k n =
  let bits = Types.bits k in
  let ok =
    if Types.signed k then
      bits = 64
      || (Int64.compare n (Int64.neg (Int64.shift_left 1L (bits - 1))) >= 0
          && Int64.compare n (Int64.shift_left 1L (bits - 1)) < 0)
    else if bits = 64 then
      (* A u64 literal is its 64-bit pattern, so anything at or above 2^63
         arrives here as a negative [int64] and is still in range —
         0xcbf29ce484222325 is a real u64 and not an error. The cost is that a
         negative *decimal* literal is accepted as a u64 too, because the
         reader records only the value and not how it was written. Narrower
         unsigned types keep the strict check, which is where a typo like 300
         for a u8 actually shows up. *)
      true
    else
      Int64.compare n 0L >= 0
      && Int64.compare n (Int64.shift_left 1L bits) < 0
  in
  if ok then n
  else fail loc "%Ld does not fit in %s" n (Types.ikind_name k)

and var ctx loc ~want name =
  match name with
  | "true" | "false" ->
    expect loc ~want (mk loc Types.Bool (Tast.Bool (name = "true")))
  | "None" ->
    (match want with
     | Some (Types.Option t) -> mk loc (Types.Option t) Tast.None_
     | Some other when other <> Types.Never ->
       fail loc "expected %s, found None" (Types.to_string other)
     | _ ->
       fail loc
         "nothing here says what None is an Option of — annotate the \
          function's return type or the binding")
  | _ ->
    match lookup ctx name with
    | Some b -> expect loc ~want (mk loc b.bty (Tast.Local b.slot))
    | None ->
      match Hashtbl.find_opt ctx.env.globals name with
      | Some (ty, _) -> expect loc ~want (mk loc ty (Tast.Global name))
      | None ->
        if Hashtbl.mem ctx.env.fns name then
          unimplemented loc
            (Printf.sprintf "the function value %s (a name used as a value)" name) 5
        else begin captured ctx loc name; fail loc "unknown name %s" name end

and block ctx ?want loc body =
  match body with
  | [] -> expect loc ~want (unit_at loc)
  | _ ->
    let rec go = function
      | [ last ] -> let l = check ctx ?want last in [ l ], l.Tast.ty
      | x :: rest -> let x = check ctx x in
        let rest, ty = go rest in x :: rest, ty
      | [] -> assert false
    in
    let body, ty = go body in
    mk loc ty (Tast.Do body)

(* A handler runs where the *signal* was, not where it was established, so it
   cannot be a branch in the function that wrote it: it is lifted into a
   function of its own and reached through a pointer.

   Which means it cannot see the establishing function's locals. Capturing them
   is a closure with an explicit environment — the non-escaping kind, captured
   by value onto this frame — and until that exists a reference to one is
   rejected by name rather than silently resolving to something else. Globals and the condition itself are
   in scope, which is enough for the accumulation case §1 is about.

   The body may not [return] either. The frames are pushed and popped around
   it, and an early exit would leave them on the stack pointing into a function
   that has gone. *)
and check_handler_bind ctx ?want loc clauses body =
  ignore want;
  let frames =
    List.map
      (fun (c : Ast.hclause) ->
         let ty = resolve ctx.env c.Ast.hty in
         let name =
           match ty with
           | Types.Named n -> n
           | t ->
             fail c.Ast.hloc
               "a handler matches a struct type, not %s" (Types.to_string t)
         in
         (* Its own context: a fresh frame, an empty scope, and no way to reach
            the enclosing one. *)
         let hctx =
           { env = ctx.env; ret = Types.Unit; slots = 0; slot_tys = []; slot_names = [];
             scope = []; defers = []; outer = ctx.scope; in_handler = true; in_frames = None; in_defer = false; owner = "<none>" }
         in
         (* The condition crosses as a pointer, because the handler runs while
            the signalling frame is still alive and there is nothing to copy.
            What the clause binds is the condition itself, though, so the
            pointer is a hidden parameter and the name is a slot loaded from
            it — a handler that passed [c] to something expecting the struct
            would otherwise be handed an address. *)
         let pslot = fresh_slot hctx (Types.Ptr ty) in
         let cslot = bind hctx c.Ast.hname ty ~assignable:false in
         let hbody = map_lr (fun e -> check hctx e) c.Ast.hbody in
         let hbody =
           [ mk c.Ast.hloc Types.Unit
               (Tast.Let
                  ([ (cslot,
                      mk c.Ast.hloc ty
                        (Tast.Deref
                           (mk c.Ast.hloc (Types.Ptr ty) (Tast.Local pslot)))) ],
                   hbody)) ]
         in
         (* Named after the function it came out of, and numbered within it:
            stable against an unrelated handler-bind being added elsewhere,
            which an index into the whole program's lifted list would not be. *)
         let fname =
           Printf.sprintf "handler/%s/%d/%s" ctx.owner
             (List.length
                (List.filter
                   (fun (l : Tast.fn) -> l.Tast.fparent = Some ctx.owner)
                   ctx.env.lifted))
             name
         in
         ctx.env.lifted <-
           { Tast.name = fname; params = [ Types.Ptr ty ];
             slots = Array.of_list (List.rev hctx.slot_tys);
             snames = Array.of_list (List.rev hctx.slot_names);
             ret = Types.Unit; body = hbody; fdefers = [];
             fparent = Some ctx.owner; floc = c.Ast.hloc }
           :: ctx.env.lifted;
         { Tast.htype = type_id name; hfn = fname })
      clauses
  in
  (* The flag is set on [ctx] itself and restored, not on a copy: [ctx.slots]
     and [ctx.slot_tys] are mutable, so a copy would allocate the body's slots
     into a record the function never sees again and the indices would
     collide. *)
  let saved = ctx.in_frames in
  ctx.in_frames <- Some "handler-bind";
  let body = map_lr (fun e -> check ctx e) body in
  ctx.in_frames <- saved;
  mk loc Types.Unit (Tast.Handled (frames, body))

(* (restart-case BODY (name [] BODY-1) ...) — spec-conditions.md §3 and §6.

   Unlike a handler, a clause runs *at* the restart-case, which is where it was
   written, so it is a branch in this function and sees this function's scope.
   What arrives from elsewhere is only the answer to "which clause": a transfer
   names the frame it is aimed at, and this form compares that against the
   frames it itself pushed.

   Every clause body and the body have the same type, and that is the type of
   the whole form — which is what makes the fall-through path visible in the
   source (§1): a restart-case in value position has to produce its type when
   no restart is invoked too. *)
and check_restart_case ctx ?want loc body clauses =
  let saved = ctx.in_frames in
  ctx.in_frames <- Some "restart-case";
  let tbody = check ctx ?want body in
  ctx.in_frames <- saved;
  (* With no expectation from outside, the body's own type is the expectation
     the clauses are checked against — unless it produced no value at all, in
     which case the first clause that does decides. *)
  let want =
    match want with
    | Some _ -> want
    | None -> if tbody.Tast.ty = Types.Never then None else Some tbody.Tast.ty
  in
  let ty = ref (match want with Some t -> Some t | None -> None) in
  let seen = ref [] in
  let clauses =
    map_lr
      (fun (c : Ast.rclause) ->
         (* Two clauses of one name would make §4's "the first frame offering
            the name" pick between them by an order nothing in the source
            shows. *)
         if List.mem c.Ast.rname !seen then
           fail c.Ast.rloc "this restart-case offers %s twice" c.Ast.rname;
         seen := c.Ast.rname :: !seen;
         (* Each is checked against what the form has settled on so far, so a
            clause that disagrees fails where it is written. The first one to
            produce a value is what settles it when nothing outside did. *)
         let b = block ctx ?want:!ty c.Ast.rloc c.Ast.rbody in
         if !ty = None && b.Tast.ty <> Types.Never then ty := Some b.Tast.ty;
         { Tast.rname_id = type_id c.Ast.rname; rname = c.Ast.rname;
           rbody = [ b ] })
      clauses
  in
  let ty = match !ty with Some t -> t | None -> Types.Never in
  mk loc ty (Tast.RestartCase (clauses, tbody))

and check_let ctx ?want loc bs body =
  scoped ctx (fun () ->
    let bs =
      map_lr
        (fun (b : Ast.binding) ->
           let want = Option.map (resolve ctx.env) b.Ast.bty in
           let v = check ctx ?want b.Ast.bval in
           (match v.Tast.ty with
            | Types.Unit | Types.Never ->
              fail b.Ast.bloc "%s would be bound to %s, which is not a value"
                b.Ast.bname (Types.to_string v.Tast.ty)
            | _ -> ());
           (* Locals are assignable places; parameters are not. *)
           let slot = bind ctx b.Ast.bname v.Tast.ty ~assignable:true in
           (slot, v))
        bs
    in
    let body = block ctx ?want loc body in
    mk loc body.Tast.ty (Tast.Let (bs, [ body ])))

(* (dotimes [i n] body...) is a counting loop, not a new IR node: bind [i] to 0
   and the bound to a hidden slot — [n] is evaluated once, before the loop, so
   a body that changes it cannot change the trip count — then step [i] at the
   end of the body. [i] is not assignable, so the step below is the only writer. *)
and check_dotimes ctx ~want loc name count body =
  let count = check ctx ~want:index_ty count in
  scoped ctx (fun () ->
    let i = bind ctx name index_ty ~assignable:false in
    let limit = fresh_slot ctx index_ty in
    let body = map_lr (fun b -> check ctx b) body in
    let iv = mk loc index_ty (Tast.Local i) in
    let one = mk loc index_ty (Tast.Int (1L, Types.I32)) in
    let cond =
      mk loc Types.Bool
        (Tast.Prim (Tast.Lt, [ iv; mk loc index_ty (Tast.Local limit) ]))
    in
    let step =
      mk loc Types.Unit
        (Tast.Set (Tast.Plocal i,
                   mk loc index_ty (Tast.Prim (Tast.Add, [ iv; one ]))))
    in
    let zero = mk loc index_ty (Tast.Int (0L, Types.I32)) in
    let loop = mk loc Types.Unit (Tast.While (cond, body @ [ step ])) in
    expect loc ~want
      (mk loc Types.Unit (Tast.Let ([ (i, zero); (limit, count) ], [ loop ]))))

and check_if ctx ?want loc c t e =
  let c = check ctx ~want:Types.Bool c in
  match e with
  | None ->
    (* A one-armed if produces Unit whatever the branch evaluates to: there is
       no value on the missing side. `when` desugars to this. *)
    let t = scoped ctx (fun () -> check ctx t) in
    expect loc ~want (mk loc Types.Unit (Tast.If (c, t, unit_at loc)))
  | Some e ->
    let t = scoped ctx (fun () -> check ctx ?want t) in
    (* With no expectation the then-branch supplies one for the else-branch,
       unless it diverges, in which case the else-branch decides. *)
    let ewant =
      match want with
      | Some _ -> want
      | None -> if t.Tast.ty = Types.Never then None else Some t.Tast.ty
    in
    let e = scoped ctx (fun () -> check ctx ?want:ewant e) in
    let ty =
      if t.Tast.ty = Types.Never then e.Tast.ty
      else if e.Tast.ty = Types.Never then t.Tast.ty
      else if Types.equal t.Tast.ty e.Tast.ty then t.Tast.ty
      else
        fail loc "the branches of this if have different types: %s and %s"
          (Types.to_string t.Tast.ty) (Types.to_string e.Tast.ty)
    in
    mk loc ty (Tast.If (c, t, e))

and check_struct ctx ~want loc name kvs =
  match Hashtbl.find_opt ctx.env.structs name with
  | None ->
    if Hashtbl.mem ctx.env.unions name then
      unimplemented loc "constructing a union value" 6
    else fail loc "unknown struct %s" name
  | Some s ->
    let seen = Hashtbl.create 8 in
    List.iter
      (fun (k, (v : Ast.expr)) ->
         if Hashtbl.mem seen k then
           fail v.Ast.loc "field %s is given twice" k;
         if Tast.field_index s k = None then
           fail v.Ast.loc "%s has no field %s" name k;
         Hashtbl.add seen k v)
      kvs;
    (* Omitted fields are zeroed — ZII, the same rule as a declaration with no
       initialiser (plan.org, Data model). Every field is present from here on,
       in declaration order, so no backend has to know about omission. *)
    let fields =
      map_lr
        (fun (f : Tast.field) ->
           match Hashtbl.find_opt seen f.Tast.fname with
           | Some v -> check ctx ~want:f.Tast.fty v
           | None -> mk loc f.Tast.fty (Tast.Zero f.Tast.fty))
        s.Tast.fields
    in
    expect loc ~want (mk loc (Types.Named name) (Tast.Make (name, fields)))

and check_arr ctx ~want loc items =
  let elem_want =
    match want with
    | Some (Types.Array (_, t)) -> Some t
    | Some (Types.Slice t) -> Some t
    | _ -> None
  in
  let items = map_lr (fun i -> check ctx ?want:elem_want i) items in
  let n = Int64.of_int (List.length items) in
  let elem =
    match elem_want, items with
    | Some t, _ -> t
    | None, first :: _ -> first.Tast.ty
    | None, [] ->
      fail loc "an empty array literal needs a type — annotate the binding"
  in
  List.iter
    (fun (i : Tast.expr) ->
       if not (Types.fits ~expected:elem ~actual:i.Tast.ty) then
         fail i.Tast.loc "this array's elements are %s, but this one is %s"
           (Types.to_string elem) (Types.to_string i.Tast.ty))
    items;
  (match want with
   | Some (Types.Array (m, _)) when not (Int64.equal m n) ->
     fail loc "expected %Ld elements, found %Ld" m n
   | _ -> ());
  (* [n T] and [T] are distinct in type and in ownership (spec-memory.md), so
     an array literal does not satisfy a slice expectation. *)
  expect loc ~want (mk loc (Types.Array (n, elem)) (Tast.Arr items))

and check_match ctx ?want loc scrutinee arms =
  let s = check ctx scrutinee in
  let elem =
    match s.Tast.ty with
    | Types.Option t -> t
    (* An enum is the one scrutinee that is not a milestone away: it is an i32
       at run time and its members are all known, so the arms would be a chain
       of [=] with an exhaustiveness check over [env.enums] — a desugaring, not
       a new IR node. What blocks it is upstream of here: a keyword has no case
       in [Ast.pattern], and [lib/load.ml] matches that type exhaustively, so
       the variant cannot be added. Said as itself rather than folded into the
       milestone answer below, because the milestone is not the reason. *)
    | Types.Enum n ->
      fail loc
        "match over the enum %s is not implemented — the lowering is a chain \
         of (= k :member), but a keyword has no case in the pattern type yet. \
         Use cond" n
    | other ->
      (* Union matching arrives with unions themselves, at milestone 6. *)
      fail loc "match works on an Option at milestone 2, not on %s"
        (Types.to_string other)
  in
  let want = ref want in
  let saw_some = ref false and saw_none = ref false and saw_wild = ref false in
  let arms =
    map_lr
      (fun (a : Ast.arm) ->
         let ctor, binds =
           match a.Ast.pat with
           | Ast.Pwild -> saw_wild := true; None, []
           | Ast.Pctor ("Some", [ x ]) -> saw_some := true; Some "Some", [ x ]
           | Ast.Pctor ("Some", _) ->
             fail a.Ast.aloc "the Some pattern binds exactly one name"
           | Ast.Pctor ("None", []) -> saw_none := true; Some "None", []
           | Ast.Pctor ("None", _) -> fail a.Ast.aloc "None binds no names"
           | Ast.Pctor (c, _) ->
             fail a.Ast.aloc
               "%s is not a case of Option — the cases are Some and None" c
         in
         scoped ctx (fun () ->
           let binds = List.map (fun n -> bind ctx n elem ~assignable:false) binds in
           let body = block ctx ?want:!want a.Ast.aloc a.Ast.body in
           if !want = None && body.Tast.ty <> Types.Never then
             want := Some body.Tast.ty;
           { Tast.acase = ctor; binds; abody = [ body ] }))
      arms
  in
  if not (!saw_wild || (!saw_some && !saw_none)) then
    fail loc
      "this match is not exhaustive — Option needs both Some and None, or a \
       _ arm";
  let ty = match !want with Some t -> t | None -> Types.Never in
  mk loc ty (Tast.Match (s, arms))

(* ── Places ────────────────────────────────────────────────────────── *)

(* The target of [.field] is a struct, or one level of pointer to one. The
   auto-deref is inserted here as a real node, so no backend re-derives it. *)
and struct_target ctx (target : Ast.expr) : Tast.expr * string =
  let t = check ctx target in
  match t.Tast.ty with
  | Types.Named n when Hashtbl.mem ctx.env.structs n -> t, n
  | Types.Ptr (Types.Named n) when Hashtbl.mem ctx.env.structs n ->
    mk t.Tast.loc (Types.Named n) (Tast.Deref t), n
  | other ->
    fail target.Ast.loc "%s is not a struct, so it has no fields"
      (Types.to_string other)

and check_place ctx loc (p : Ast.place) : Tast.place * Types.t =
  match p with
  | Ast.Pvar name ->
    (match lookup ctx name with
     | Some b ->
       if not b.assignable then
         fail loc
           "%s is a parameter, and parameters are not assignable places \
            (spec-memory.md) — bind a local with let" name;
       Tast.Plocal b.slot, b.bty
     | None ->
       match Hashtbl.find_opt ctx.env.globals name with
       | Some (_, true) -> fail loc "%s is a constant" name
       | Some (ty, false) -> Tast.Pglobal name, ty
       | None -> captured ctx loc name; fail loc "unknown name %s" name)
  | Ast.Pfield (target, name) ->
    let target, sname = struct_target ctx target in
    let s = Hashtbl.find ctx.env.structs sname in
    (match Tast.field_index s name with
     | None -> fail loc "%s has no field %s" sname name
     | Some i -> Tast.Pfield (target, i), (List.nth s.Tast.fields i).Tast.fty)
  | Ast.Pindex (target, idx) ->
    let target = check ctx target in
    let idx, ty = indexed ctx target idx in
    Tast.Pindex (target, idx), ty
  | Ast.Pderef target ->
    let target = check ctx target in
    (match target.Tast.ty with
     | Types.Ptr t -> Tast.Pderef target, t
     | other ->
       fail loc "deref takes a (Ptr T), found %s" (Types.to_string other))

(* An index or a slice bound that is a literal is known now, so it is an error
   now rather than a trap later. Only literals: a [defconst] is a global in the
   typed IR, not a folded constant, so [(at arr size)] still traps at runtime —
   which is what the emitted bounds check is for. A negative literal is wrong
   whatever the target, but a length is static only for [n T]. *)
and static_index loc (ty : Types.t) ~past_end what k =
  if k < 0L then
    fail loc "%s %Ld is negative — indices count from 0" what k;
  match ty with
  (* [past_end] is the difference between an index and a slice bound: the last
     valid index is len - 1, but a slice may end at len. *)
  | Types.Array (n, _) when if past_end then k > n else k >= n ->
    fail loc "%s %Ld is out of bounds for length %Ld" what k n
  | _ -> ()

(* The literal value of a checked expression, if it has one. *)
and literal (e : Tast.expr) =
  match e.Tast.e with Tast.Int (k, _) -> Some k | _ -> None

(* An index is [i32] internally, but a *narrower* integer may be written as
   one: indexing is not arithmetic on the value, so there is nothing for a
   visible cast to warn about, and requiring (i32 c) at every subscript would
   be noise. A u32 is included because it cannot lose a value the bounds check
   would then miss — anything above 2^31 truncates to a negative i32, which
   the unsigned comparison rejects. i64 and u64 are not: 2^32 + 5 truncates to
   5 and would read the wrong element with no trap at all, so those need the
   cast written out. *)
and index_expr ctx (e : Ast.expr) =
  (* No [want]: an expectation of [i32] would reject a [u32] index outright,
     before there is anything here to convert. An untyped literal still
     defaults to [i32] on its own. *)
  let v = check ctx e in
  match v.Tast.ty with
  | Types.Int Types.I32 -> v
  | Types.Int k when Types.bits k <= 32 ->
    { v with Tast.ty = index_ty;
             Tast.e = Tast.Prim (Tast.Cast index_ty, [ v ]) }
  | Types.Int k ->
    fail e.Ast.loc
      "an index is an i32, and %s 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" (Types.ikind_name k)
  | other ->
    fail e.Ast.loc "an index is an integer, found %s" (Types.to_string other)

(* [(at a i)] and [(at grid row col)]: one index per dimension. *)
and indexed ctx (target : Tast.expr) (idx : Ast.expr list) =
  let rec go ty = function
    | [] -> [], ty
    | i :: rest ->
      let elem =
        match ty with
        | Types.Array (_, t) | Types.Slice t -> t
        | other ->
          fail i.Ast.loc "%s cannot be indexed" (Types.to_string other)
      in
      let loc = i.Ast.loc in
      let i = index_expr ctx i in
      (match literal i with
       | Some k -> static_index loc ty ~past_end:false "index" k
       | None -> ());
      let rest, ty = go elem rest in
      i :: rest, ty
  in
  go target.Tast.ty idx

(* ── Calls ─────────────────────────────────────────────────────────── *)

and check_call ctx ~want loc (head : Ast.expr) (args : Ast.expr list) =
  match head.Ast.e with
  | Ast.Var name -> named_call ctx ~want loc name args
  | _ ->
    unimplemented loc "calling something other than a named function" 5

and arity loc name n args =
  if List.length args <> n then
    fail loc "%s takes %d argument%s, given %d" name n
      (if n = 1 then "" else "s") (List.length args)

(* The operators that fold: [+ - * /], [min]/[max] and the three bitwise
   combining operators all take two operands or more, and mean the same thing
   applied left to right. [%] and the shifts are not in that set — a chain of
   remainders or of shifts has no reading a reader would agree on in advance,
   so there the arity error is the useful answer.

   Two is the floor, and the two missing cases are refused rather than
   invented. Zero operands would have to mean an identity element, 0 for + and
   1 for *, and a sum with no terms in it is a typo far more often than it is
   an intent. One operand would have to mean negation for [-] and reciprocal
   for [/], and this language has no unary minus anywhere: the prelude writes
   every negation as [(- 0 n)] or [(- 0.0 x)], and [(- x)] meaning something
   else than the [-] two lines above it is a rule a reader has to carry rather
   than see. *)
and fold_arity loc name args =
  match args with
  | _ :: _ :: _ -> ()
  | [ _ ] when String.equal name "-" ->
    fail loc
      "- takes two arguments or more, given 1 — there is no unary minus; \
       write (- 0 x) to negate, which is what the prelude does"
  | [ _ ] when String.equal name "/" ->
    fail loc
      "/ takes two arguments or more, given 1 — there is no reciprocal; \
       write (/ 1.0 x)"
  | _ ->
    fail loc "%s takes two arguments or more, given %d" name (List.length args)

(* The first two operands decide the type — [binary] picks which of them is
   allowed to, and that decision is not re-made per pair — and every operand
   after them is checked against it. *)
and fold_left_prim ctx ~want loc name p ok what args =
  let x, y, rest =
    match args with x :: y :: rest -> x, y, rest | _ -> assert false
  in
  let a, b = binary ctx name loc ~want:(numeric_want want) [ x; y ] in
  if not (ok a.Tast.ty) then
    fail loc "%s takes %s, found %s" name what (Types.to_string a.Tast.ty);
  let ty = a.Tast.ty in
  let acc =
    List.fold_left
      (fun acc arg ->
         mk loc ty (Tast.Prim (p, [ acc; check ctx ~want:ty arg ])))
      (mk loc ty (Tast.Prim (p, [ a; b ])))
      rest
  in
  expect loc ~want acc

and named_call ctx ~want loc name args =
  let prim p ty args = expect loc ~want (mk loc ty (Tast.Prim (p, args))) in
  match name with
  (* ── arithmetic and comparison ─────────────────────────────────── *)
  | "+" | "-" | "*" | "/" ->
    let p = match name with
      | "+" -> Tast.Add | "-" -> Tast.Sub | "*" -> Tast.Mul
      | _ -> Tast.Div
    in
    fold_arity loc name args;
    fold_left_prim ctx ~want loc name p Types.is_numeric "numbers" args
  (* Remainder stays at two: (% a b c) is (% (% a b) c), which is a thing
     nobody writes on purpose. *)
  | "%" ->
    arity loc name 2 args;
    let a, b = binary ctx name loc ~want:(numeric_want want) args in
    if not (Types.is_numeric a.Tast.ty) then
      fail loc "%s takes numbers, found %s" name (Types.to_string a.Tast.ty);
    prim Tast.Rem a.Tast.ty [ a; b ]
  | "=" | "!=" | "<" | "<=" | ">" | ">=" ->
    let p = match name with
      | "=" -> Tast.Eq | "!=" -> Tast.Ne | "<" -> Tast.Lt
      | "<=" -> Tast.Le | ">" -> Tast.Gt | _ -> Tast.Ge
    in
    arity loc name 2 args;
    let a, b = binary ctx name loc ~want:None args in
    if not (Types.is_comparable a.Tast.ty) then
      fail loc
        "%s compares machine numbers; %s has no built-in comparison \
         (plan.org, Types)" name (Types.to_string a.Tast.ty);
    prim p Types.Bool [ a; b ]
  | "not" ->
    arity loc name 1 args;
    prim Tast.Not Types.Bool [ check ctx ~want:Types.Bool (List.hd args) ]
  (* Bitwise operators are integers-only, and the shift count has the same type
     as the value shifted — there is no implicit widening anywhere else either. *)
  | "bit-and" | "bit-or" | "bit-xor" ->
    let p = match name with
      | "bit-and" -> Tast.BitAnd | "bit-or" -> Tast.BitOr
      | _ -> Tast.BitXor
    in
    fold_arity loc name args;
    fold_left_prim ctx ~want loc name p
      (function Types.Int _ -> true | _ -> false) "integers" args
  (* The shifts stay at two, and not only because a shift chain reads badly:
     each count would be checked against the same width below, so (<< x 30 30)
     would pass two legal shifts and still shift the value away entirely. *)
  | "<<" | ">>" ->
    let p = if String.equal name "<<" then Tast.Shl else Tast.Shr in
    arity loc name 2 args;
    let a, b = binary ctx name loc ~want:(numeric_want want) args in
    (match a.Tast.ty with
     | Types.Int _ -> ()
     | other -> fail loc "%s takes integers, found %s" name
                  (Types.to_string other));
    (* A shift by the operand's own width or more is poison in LLVM, which at
       -O2 turns the whole function into an undefined value rather than into a
       wrong number. A literal count is rejected here — that is the typo — and
       [emit] masks a computed one, so no shift can reach the hardware out of
       range. *)
    (match a.Tast.ty, b.Tast.e with
     | Types.Int k, Tast.Int (n, _) when p = Tast.Shl || p = Tast.Shr ->
       let w = Int64.of_int (Types.bits k) in
       if Int64.unsigned_compare n w >= 0 then
         fail loc
           "%s by %Ld is out of range for %s, which is %d bits wide" name n
           (Types.to_string a.Tast.ty) (Types.bits k)
     | _ -> ());
    prim p a.Tast.ty [ a; b ]
  (* (min a b) and (max a b) evaluate each operand once — hence the slots —
     because a min over two calls must not call either of them twice.

     Which is also why this one does not go through [fold_left_prim]: there is
     no Prim to fold, and the pair it folds is a whole comparison. Each step
     puts *both* of its sides in slots, the accumulated pick included, so the
     three-operand form is two nested lets and still exactly one evaluation of
     each operand — where reusing the previous [If] as an operand of the next
     would have duplicated everything inside it. *)
  | "min" | "max" ->
    fold_arity loc name args;
    let x, y, rest =
      match args with x :: y :: rest -> x, y, rest | _ -> assert false
    in
    let a, b = binary ctx name loc ~want:(numeric_want want) [ x; y ] in
    if not (Types.is_numeric a.Tast.ty) then
      fail loc "%s takes numbers, found %s" name (Types.to_string a.Tast.ty);
    let ty = a.Tast.ty in
    let cmp = if String.equal name "min" then Tast.Lt else Tast.Gt in
    let pick a b =
      let sa = fresh_slot ctx ty and sb = fresh_slot ctx ty in
      let la = mk loc ty (Tast.Local sa) and lb = mk loc ty (Tast.Local sb) in
      let test = mk loc Types.Bool (Tast.Prim (cmp, [ la; lb ])) in
      mk loc ty (Tast.Let ([ (sa, a); (sb, b) ],
                           [ mk loc ty (Tast.If (test, la, lb)) ]))
    in
    expect loc ~want
      (List.fold_left (fun acc arg -> pick acc (check ctx ~want:ty arg))
         (pick a b) rest)
  (* (zeroed) is the all-bytes-zero value of whatever it is being stored into,
     so it only means anything where a type is expected of it. *)
  | "zeroed" ->
    arity loc name 0 args;
    (match want with
     | Some ty when ty <> Types.Never -> mk loc ty (Tast.Zero ty)
     | _ ->
       fail loc
         "zeroed needs to know the type it is zeroing — use it where one is \
          expected, as in (set grid (zeroed))")

  (* The one half of a destructuring [let] that [Parse] cannot do on its own.
     Everything else about a pattern is bindings and field accesses it already
     wrote; the arity is a *type* question — how many elements the value has —
     and there are no types in the parser. So the pattern's shape travels here
     as arguments: which element this binding wants, how many names the pattern
     binds, and whether that count is exact or a minimum (it is a minimum when
     the pattern ends in [& rest]).

     No source symbol can contain a [~] — the reader makes it a delimiter — so
     this name is unspellable and nothing but [Parse] can reach it. *)
  | "destructure~nth" ->
    (match args with
     | [ target;
         { Ast.e = Ast.Int i; _ }; { Ast.e = Ast.Int n; _ };
         { Ast.e = Ast.Int exact; _ } ] ->
       let plural k = if Int64.equal k 1L then "" else "s" in
       let target = check ctx target in
       (match target.Tast.ty with
        | Types.Array (m, elem) ->
          if Int64.equal exact 1L && not (Int64.equal m n) then
            fail loc
              "this pattern binds %Ld name%s, but %s has %Ld element%s — a \
               pattern over a fixed array names every element, or ends in \
               [& rest]"
              n (plural n) (Types.to_string target.Tast.ty) m (plural m);
          if Int64.equal exact 0L && Int64.compare m n < 0 then
            fail loc
              "this pattern binds %Ld name%s before the &, but %s has only %Ld \
               element%s" n (plural n) (Types.to_string target.Tast.ty) m
              (plural m);
          prim Tast.At elem
            [ target; mk loc index_ty (Tast.Int (i, Types.I32)) ]
        (* The asymmetry is real and is the reason this is refused rather than
           lowered to a bounds-checked [at]: a fixed array's length is in its
           type, so [[a b]] over a [[2 f32]] is a claim the checker can settle,
           and over a [[T]] it is a claim about a number that does not exist
           until the program runs. Turning it into a runtime trap would be a
           pattern that type checks and then kills the program, which is the
           trade this language does not make. *)
        | Types.Slice _ ->
          fail loc
            "a pattern cannot destructure %s: a slice's length is a runtime \
             value, so nothing here can check that it has %Ld element%s. Use \
             (at s i) and test (len s) yourself"
            (Types.to_string target.Tast.ty) n (plural n)
        | other ->
          fail loc
            "%s is not a fixed array, so [a b ...] cannot destructure it"
            (Types.to_string other))
     | _ ->
       fail loc
         "destructure~nth is written by the compiler and cannot be called")

  (* ── containers ────────────────────────────────────────────────── *)
  | "len" ->
    arity loc name 1 args;
    let a = check ctx (List.hd args) in
    (match a.Tast.ty with
     | Types.Array _ | Types.Slice _ | Types.String -> ()
     | other -> fail loc "len takes an array, a slice or a string, found %s"
                  (Types.to_string other));
    prim Tast.Len index_ty [ a ]
  | "at" ->
    (match args with
     | target :: idx when idx <> [] ->
       let target = check ctx target in
       let idx, ty = indexed ctx target idx in
       prim Tast.At ty (target :: idx)
     | _ -> fail loc "%s is (%s collection index ...)" name name)
  | "slice" ->
    arity loc name 3 args;
    (match args with
     | [ target; lo; hi ] ->
       let target = check ctx target in
       let elem = match target.Tast.ty with
         | Types.Array (_, t) | Types.Slice t -> t
         | other -> fail loc "slice takes an array or a slice, found %s"
                      (Types.to_string other)
       in
       prim Tast.Slice (Types.Slice elem)
         (let lo_loc = lo.Ast.loc and hi_loc = hi.Ast.loc in
          let lo = check ctx ~want:index_ty lo in
          let hi = check ctx ~want:index_ty hi in
          let ty = target.Tast.ty in
          (* A bound may sit one past the end, so the length is checked against
             lo and hi both, not against the last valid index. *)
          (match literal lo with
           | Some k -> static_index lo_loc ty ~past_end:true "slice bound" k
           | None -> ());
          (match literal hi with
           | Some k -> static_index hi_loc ty ~past_end:true "slice bound" k
           | None -> ());
          (match literal lo, literal hi with
           | Some a, Some b when a > b ->
             fail loc "slice [%Ld %Ld) runs backwards — lo must not exceed hi" a b
           | _ -> ());
          [ target; lo; hi ])
     | _ -> assert false)

  (* ── pointers ──────────────────────────────────────────────────── *)
  | "addr" ->
    arity loc name 1 args;
    let a = List.hd args in
    (match place_of_expr a with
     | None ->
       fail a.Ast.loc
         "addr takes the address of a place — a name, (.field x), (at a i) \
          or (deref p)"
     | Some p ->
       let p, ty = check_place ctx a.Ast.loc p in
       expect loc ~want (mk loc (Types.Ptr ty) (Tast.Addr p)))
  | "deref" ->
    arity loc name 1 args;
    let a = check ctx (List.hd args) in
    (match a.Tast.ty with
     | Types.Ptr t -> expect loc ~want (mk loc t (Tast.Deref a))
     | other -> fail loc "deref takes a (Ptr T), found %s"
                  (Types.to_string other))

  (* ── Option ────────────────────────────────────────────────────── *)
  | "Some" ->
    arity loc name 1 args;
    let inner = match want with Some (Types.Option t) -> Some t | _ -> None in
    let a = check ctx ?want:inner (List.hd args) in
    expect loc ~want (mk loc (Types.Option a.Tast.ty) (Tast.Some_ a))

  (* ── the milestone-2 host primitives (plan.org) ────────────────── *)
  | "bytes" ->
    arity loc name 1 args;
    prim Tast.Bytes (Types.Slice (Types.Int Types.U8))
      [ check ctx ~want:Types.String (List.hd args) ]

  (* (string b): a [u8] seen as a string. The mirror of (bytes s), spelled the
     same way — a type name in head position, like (bytes s) and unlike the
     numeric casts, which go through [is_cast] and really do convert.

     It costs nothing. emit.ml lowers Types.String and Types.Slice _ to the
     same %slice, 16 bytes at align 8, so a string and a [u8] are already the
     identical value at run time; both this and [Bytes] emit as the argument
     itself. What changes is only what the checker will let the value be
     passed to — which is the whole gap: i64->bytes answers a [u8] and every
     declare-c text parameter wants a string, and nothing joined them.

     Two decisions are baked in here.

     1. It does NOT check UTF-8, because `string` does not claim UTF-8. The
        prelude settles this: valid-utf8? is an ordinary function you call when
        you care, decode-rune/rune-at/rune-count all take [u8] rather than
        string, and decode-rune answers {:ok false :width 1} on a malformed
        byte rather than assuming its input is well-formed. The one place the
        runtime treats a string differently from a byte slice is
        flan_escape_bytes, for a string nested in a printed structure, and that
        is a byte-wise escape table with no decoding in it. So there is no code
        that would be wrong about a string of arbitrary bytes, and a check here
        would be the only enforcement point in the language — a claim the rest
        of it does not make.

     2. It does not widen the literal-write hole (NEXT.md, "Writing through a
        string literal"). That hole is the other direction: (bytes "Hi") hands
        you a writable-looking slice over constant data. This direction only
        loses the ability to write — a string is read-only everywhere — so the
        result of (string b) can reach strictly fewer stores than b could.
        Provenance is still what the other direction needs; nothing here
        depends on having it.

     The one sharp edge is not new but is easier to trip over now: the slice
     that i64->bytes / f64->bytes / u64->bytes answer is a view into one shared
     static buffer in the runtime, overwritten by the next such call. Calling
     it a string does not copy it. Use it before formatting the next number;
     you cannot hold two at once. *)
  | "string" ->
    arity loc name 1 args;
    prim Tast.StrOfBytes Types.String [ byte_slice ctx (List.hd args) ]
  | "bytes->f64" ->
    arity loc name 1 args;
    prim Tast.BytesToF64 (Types.Float Types.F64) [ byte_slice ctx (List.hd args) ]
  | "bytes->i64" ->
    arity loc name 1 args;
    prim Tast.BytesToI64 (Types.Int Types.I64) [ byte_slice ctx (List.hd args) ]
  | "f64->bytes" ->
    arity loc name 1 args;
    prim Tast.F64ToBytes (Types.Slice (Types.Int Types.U8))
      [ check ctx ~want:(Types.Float Types.F64) (List.hd args) ]
  | "i64->bytes" ->
    arity loc name 1 args;
    prim Tast.I64ToBytes (Types.Slice (Types.Int Types.U8))
      [ check ctx ~want:(Types.Int Types.I64) (List.hd args) ]
  | "write-stdout" ->
    arity loc name 1 args;
    prim Tast.WriteStdout Types.Unit [ byte_slice ctx (List.hd args) ]

  (* (println x) and (print x): the structural printer, selected on the type
     the argument checked to. plan.org, Milestone 5 — "compiler-provided,
     per concrete type". That is not overloading and needs no type variables:
     there is no dispatch at run time and no user-supplied printer to pick
     between. The walk itself is render.ml, shared with the REPL, which is what
     stops the two from drifting apart.

     [min]/[max]/[zeroed] above dispatch on the resolved argument type the same
     way. The slots the slice arm needs come out of the frame of whatever
     function this call is written in, via [fresh_slot] — allocated once per
     call site, at check time, not once per iteration of a loop around it.

     A string prints raw here and quoted inside a structure. Those are not in
     conflict: (println "hello") has to print hello or it is useless, and
     (println b) where b has a string field has to quote it or the field
     cannot be told from the punctuation. The split is exactly top level vs
     nested, which is why it lives here and not in the walk. *)
  | "print" | "println" ->
    arity loc name 1 args;
    let a = check ctx (List.hd args) in
    let bslice = Types.Slice (Types.Int Types.U8) in
    let write x = mk loc Types.Unit (Tast.Prim (Tast.WriteStdout, [ x ])) in
    let conv pr x = mk loc bslice (Tast.Prim (pr, [ x ])) in
    let emitter : Render.emitter =
      { Render.ebytes = write;
        estr = (fun x -> write (conv Tast.EscapeBytes x));
        ei64 = (fun x -> write (conv Tast.I64ToBytes x));
        eu64 = (fun x -> write (conv Tast.U64ToBytes x));
        ef64 = (fun x -> write (conv Tast.F64ToBytes x)) }
    in
    let rc =
      { Render.structs =
          Hashtbl.fold (fun _ v acc -> v :: acc) ctx.env.structs [];
        enums = Hashtbl.fold (fun k v acc -> (k, v) :: acc) ctx.env.enums [];
        emit = emitter;
        alloc = (fun ty -> fresh_slot ctx ty) }
    in
    let parts =
      match a.Tast.ty with
      | Types.String | Types.Slice (Types.Int Types.U8) ->
        [ write (mk loc bslice (Tast.Prim (Tast.Bytes, [ a ]))) ]
      | _ -> Render.render rc 0 a
    in
    let nl =
      if String.equal name "println" then
        [ write
            (mk loc bslice
               (Tast.Prim (Tast.Bytes, [ mk loc Types.String (Tast.Str "\n") ])))
        ]
      else []
    in
    expect loc ~want (mk loc Types.Unit (Tast.Do (parts @ nl)))
  | "exit" ->
    arity loc name 1 args;
    prim Tast.Exit Types.Never [ check ctx ~want:index_ty (List.hd args) ]
  | "argv" ->
    arity loc name 0 args;
    prim Tast.Argv (Types.Slice Types.String) []

  (* ── casts: (i32 x), (f64 x) ───────────────────────────────────── *)
  | _ when is_cast name && List.length args = 1 ->
    let target = resolve_name ctx.env ~seen:[] loc name in
    let a = check ctx (List.hd args) in
    if not (Types.is_numeric a.Tast.ty) then
      fail loc "%s converts a number, found %s" name
        (Types.to_string a.Tast.ty);
    prim (Tast.Cast target) target [ a ]

  (* ── ordinary calls ────────────────────────────────────────────── *)
  | _ ->
    match Hashtbl.find_opt ctx.env.fns name with
    | Some (params, ret) ->
      if List.length args <> List.length params then
        fail loc "%s takes %d argument%s, given %d" name
          (List.length params)
          (if List.length params = 1 then "" else "s")
          (List.length args);
      let args = map2_lr (fun p a -> check ctx ~want:p a) params args in
      expect loc ~want (mk loc ret (Tast.Call (name, args)))
    | None ->
      if Hashtbl.mem ctx.env.structs name || Hashtbl.mem ctx.env.unions name
      then
        fail loc
          "%s is a type — a struct value is written (%s {:field value ...})"
          name name
      else if String.contains name '/' then
        unimplemented loc
          (Printf.sprintf "the call %s into an imported package" name) 4
      else fail loc "unknown function %s" name

and is_cast name =
  Types.ikind_of_name name <> None || Types.fkind_of_name name <> None

and byte_slice ctx (a : Ast.expr) =
  check ctx ~want:(Types.Slice (Types.Int Types.U8)) a

and numeric_want want =
  match want with Some (Types.Int _ | Types.Float _) -> want | _ -> None

(* Both operands of a binary operator have one type, and there is no implicit
   widening, so one side has to decide it. Check the side that carries the most
   information first: a non-literal over a literal, and a float literal over an
   integer one, since an integer constant converts to a float and not back. *)
and binary ctx name loc ~want args =
  match args with
  | [ x; y ] ->
    let y_decides =
      (is_literal x && not (is_literal y))
      || (match x.Ast.e, y.Ast.e with
          | (Ast.Int _ | Ast.Byte _), Ast.Float _ -> true
          | _ -> false)
    in
    if y_decides then begin
      let b = check ctx ?want y in
      let a = check ctx ~want:b.Tast.ty x in
      a, b
    end else begin
      let a = check ctx ?want x in
      let b = check ctx ~want:a.Tast.ty y in
      a, b
    end
  | _ -> fail loc "%s takes two arguments" name

(* ── Declarations: pass 1, collect ─────────────────────────────────── *)

(* Constant folding, only over integers and only for defconst — enough for an
   array length like (/ screen-height cell-size). *)
let rec const_int env (e : Ast.expr) : int64 option =
  match e.Ast.e with
  | Ast.Int n -> Some n
  | Ast.Byte b -> Some (Int64.of_int b)
  | Ast.Var n -> Hashtbl.find_opt env.consts n
  | Ast.Call ({ Ast.e = Ast.Var op; _ }, [ x; y ]) ->
    (match const_int env x, const_int env y with
     | Some a, Some b ->
       (match op with
        | "+" -> Some (Int64.add a b)
        | "-" -> Some (Int64.sub a b)
        | "*" -> Some (Int64.mul a b)
        | "/" when b <> 0L -> Some (Int64.div a b)
        | "%" when b <> 0L -> Some (Int64.rem a b)
        | _ -> None)
     | _ -> None)
  | _ -> None

let collect env (decls : Ast.decl list) =
  (* One pass over every declaration kind before any of the others, because
     the tables below are per-kind — structs, unions, aliases, enums, functions
     and globals each have their own — and a collision between two of them
     would otherwise be found by LLVM, as [redefinition of function
     '@flan.item'], or not at all. A [defn item] and a [defvar item] are two
     declarations of one name and are rejected here. *)
  let claimed = Hashtbl.create 64 in
  List.iter
    (fun (d : Ast.decl) ->
       match Ast.declared_name d with
       | None -> ()
       | Some n ->
         if Hashtbl.mem claimed n then
           fail d.Ast.dloc "%s is defined twice" n;
         Hashtbl.add claimed n ())
    decls;
  (* Names first, so a struct may mention one declared below it. *)
  List.iter
    (fun (d : Ast.decl) ->
       match d.Ast.d with
       | Ast.Defstruct (n, _) ->
         Hashtbl.replace env.locs n d.Ast.dloc;
         Hashtbl.replace env.structs n { Tast.sname = n; fields = [] }
       | Ast.Defunion (n, _) ->
         Hashtbl.replace env.locs n d.Ast.dloc;
         Hashtbl.replace env.unions n { Tast.uname = n; cases = [] }
       | Ast.Defalias (n, t) -> Hashtbl.replace env.aliases n t
       | _ -> ())
    decls;
  (* Compile-time integer constants next, to a fixpoint, because an array
     length may name a constant declared below it — top-level names in a
     package are order-independent (plan.org, Modules). *)
  let fold_consts () =
    let progress = ref false in
    List.iter
      (fun (d : Ast.decl) ->
         match d.Ast.d with
         | Ast.Defconst (n, _, v) when not (Hashtbl.mem env.consts n) ->
           (match const_int env v with
            | Some i -> Hashtbl.replace env.consts n i; progress := true
            | None -> ())
         | _ -> ())
      decls;
    !progress
  in
  while fold_consts () do () done;
  let field (f : Ast.field) : Tast.field =
    { Tast.fname = f.Ast.fname; fty = resolve env f.Ast.fty }
  in
  (* Constants with no declared type are inferred from their value, which needs
     every other signature in hand — so they are deferred to a pass of their
     own below. *)
  let untyped = ref [] in
  (* Enums come first, in a pass of their own: a signature below may name one,
     and [resolve] has to find it before it resolves that signature. *)
  List.iter
    (fun (d : Ast.decl) ->
       match d.Ast.d with
       | Ast.Defenum (n, members) ->
         let names = List.map fst members in
         if List.length (List.sort_uniq compare names) <> List.length names then
           fail d.Ast.dloc "%s declares the same member twice" n;
         Hashtbl.replace env.enums n members;
         Hashtbl.replace env.locs n d.Ast.dloc
       | _ -> ())
    decls;
  List.iter
    (fun (d : Ast.decl) ->
       let loc = d.Ast.dloc in
       match d.Ast.d with
       | Ast.Package _ -> ()
       | Ast.Defenum _ -> ()
       (* Imports are gone by now: [Load] resolved them into these very decls,
          so one reaching the checker is a driver that skipped that step. *)
       | Ast.Import (alias, _) ->
         fail loc "internal: the import of %s was not resolved before checking"
           alias
       (* [Shim.expand] rewrote every one of these into a [Declare] and a
          [Defn] before [collect] ran, so one arriving here is a driver that
          skipped that step. *)
       | Ast.DeclareC (fn, _) ->
         fail loc "internal: the declare-c of %s was not expanded before checking"
           fn.Ast.name
       | Ast.Declare (fn, csym) ->
         if Hashtbl.mem env.fns fn.Ast.name then
           fail loc "%s is declared twice" fn.Ast.name;
         let params =
           List.map (fun (p : Ast.field) -> resolve env p.Ast.fty) fn.Ast.params
         in
         let ret =
           match fn.Ast.ret with None -> Types.Unit | Some t -> resolve env t
         in
         (* What may cross the boundary. A slice or a string goes as ptr+len,
            a scalar as itself; an aggregate does not go at all, because how
            one is passed differs per target and reproducing that here would
            be three calling conventions to maintain. Pass (Ptr T) instead and
            let the C shim dereference it — that is what the shim is for. *)
         let crossable what (t : Types.t) =
           match t with
           | Types.Int _ | Types.Float _ | Types.Bool | Types.Ptr _
           | Types.Enum _ | Types.Unit -> ()
           | Types.String | Types.Slice _ when what = "a parameter" -> ()
           | _ ->
             fail loc
               "%s of %s is %s, which cannot cross to C directly — pass \
                (Ptr %s) and let the shim read it" what fn.Ast.name
               (Types.to_string t) (Types.to_string t)
         in
         List.iter (crossable "a parameter") params;
         crossable "the return type" ret;
         Hashtbl.replace env.fns fn.Ast.name (params, ret);
         Hashtbl.replace env.externs fn.Ast.name csym
       | Ast.Defalias _ -> ()
       | Ast.Defstruct (n, fs) ->
         let names = List.map (fun (f : Ast.field) -> f.Ast.fname) fs in
         if List.length (List.sort_uniq compare names) <> List.length names then
           fail loc "%s declares the same field twice" n;
         Hashtbl.replace env.structs n
           { Tast.sname = n; fields = List.map field fs }
       | Ast.Defunion (n, vs) ->
         Hashtbl.replace env.unions n
           { Tast.uname = n;
             cases = List.map (fun (v : Ast.variant) ->
               { Tast.vname = v.Ast.vname;
                 vfields = List.map field v.Ast.vfields }) vs }
       | Ast.Defn fn ->
         let params =
           List.map (fun (p : Ast.field) -> resolve env p.Ast.fty) fn.Ast.params
         in
         let ret =
           match fn.Ast.ret with None -> Types.Unit | Some t -> resolve env t
         in
         Hashtbl.replace env.fns fn.Ast.name (params, ret)
       | Ast.Defvar (n, t, _) ->
         let ty = match t with
           | Some t -> resolve env t
           | None -> fail loc "defvar %s needs a type" n
         in
         Hashtbl.replace env.globals n (ty, false)
       | Ast.Defconst (n, Some t, _) ->
         Hashtbl.replace env.globals n (resolve env t, true)
       | Ast.Defconst (n, None, v) -> untyped := (n, v) :: !untyped)
    decls;
  (* Also to a fixpoint, and for the same reason: one untyped constant may be
     defined in terms of another declared after it. A constant that still does
     not check once no progress is left has a real error, so the last round is
     run without swallowing it. *)
  let infer (_, v) =
    (check { env; ret = Types.Unit; slots = 0; slot_tys = []; slot_names = []; scope = []; defers = [];
      outer = []; in_handler = false; in_frames = None; in_defer = false; owner = "<none>" } v).Tast.ty
  in
  let pending = ref (List.rev !untyped) in
  let rec settle () =
    let left =
      List.filter
        (fun ((n, _) as c) ->
           match infer c with
           | ty -> Hashtbl.replace env.globals n (ty, true); false
           | exception Loc.Error _ -> true)
        !pending
    in
    let progressed = List.length left < List.length !pending in
    pending := left;
    if progressed && left <> [] then settle ()
  in
  settle ();
  List.iter (fun c -> ignore (infer c)) !pending

(* A type that contains itself by value has no finite size. [(Ptr T)] and a
   slice are indirections and break the cycle; a fixed array does not, because
   it is inline. Caught here rather than when a backend tries to lay the type
   out or a zero value is built for it — which would not fail, it would hang. *)
let check_finite env =
  let rec walk seen name =
    if List.mem name seen then
      fail (Option.value (Hashtbl.find_opt env.locs name) ~default:Loc.unknown)
        "%s contains itself by value, so it has no size — go through (Ptr %s)"
        name name;
    let seen = name :: seen in
    match Hashtbl.find_opt env.structs name with
    | Some s -> List.iter (fun (f : Tast.field) -> ty seen f.Tast.fty) s.Tast.fields
    | None ->
      match Hashtbl.find_opt env.unions name with
      | None -> ()
      | Some u ->
        List.iter
          (fun (c : Tast.variant) ->
             List.iter (fun (f : Tast.field) -> ty seen f.Tast.fty) c.Tast.vfields)
          u.Tast.cases
  and ty seen = function
    | Types.Named n -> walk seen n
    | Types.Array (_, e) | Types.Option e -> ty seen e
    | _ -> ()
  in
  Hashtbl.iter (fun n _ -> walk [] n) env.structs;
  Hashtbl.iter (fun n _ -> walk [] n) env.unions

(* ── Declarations: pass 2, check bodies ────────────────────────────── *)

let check_fn env (fn : Ast.fn) : Tast.fn =
  let params, ret = Hashtbl.find env.fns fn.Ast.name in
  let ctx = { env; ret; slots = 0; slot_tys = []; slot_names = []; scope = []; defers = [];
                outer = []; in_handler = false; in_frames = None; in_defer = false;
                owner = fn.Ast.name } in
  List.iter2
    (fun (p : Ast.field) ty ->
       if List.mem_assoc p.Ast.fname ctx.scope then
         fail p.Ast.floc "%s has two parameters named %s" fn.Ast.name p.Ast.fname;
       ignore (bind ctx p.Ast.fname ty ~assignable:false))
    fn.Ast.params params;
  let body =
    match fn.Ast.fbody with
    | [] ->
      if Types.equal ret Types.Unit then []
      else fail fn.Ast.nloc "%s returns %s but has no body" fn.Ast.name
             (Types.to_string ret)
    | body ->
      (* The last form is the return value, unless the function returns Unit,
         in which case whatever it evaluates to is discarded. *)
      let want = if Types.equal ret Types.Unit then None else Some ret in
      (* [defer] is recognised here and nowhere else, because this is the only
         place that knows a form is at the top level of the function body. Each
         one is checked in place — so it sees the scope it is written in — and
         then registered on the context; it emits nothing where it stands. *)
      let defer_here (e : Ast.expr) =
        match e.Ast.e with
        | Ast.Defer forms ->
          ctx.in_defer <- true;
          let forms = map_lr (fun d -> check ctx d) forms in
          ctx.in_defer <- false;
          let d = mk e.Ast.loc Types.Unit (Tast.Do forms) in
          ctx.defers <- d :: ctx.defers;
          Some (unit_at e.Ast.loc)
        | _ -> None
      in
      let rec go = function
        | [ last ] ->
          (match defer_here last with
           | Some u -> [ u ]
           | None -> [ check ctx ?want last ])
        | x :: rest ->
          let x = match defer_here x with Some u -> u | None -> check ctx x in
          x :: go rest
        | [] -> assert false
      in
      go body
  in
  (* Function exit runs the defers, innermost first. An explicit [return] ran
     its own (see [check]); this is the fall-off-the-end path. A trap does not
     run them — it is [noreturn] and then [unreachable] — and that is the same
     rule the bounds checks already follow. *)
  let body =
    match ctx.defers with
    | [] -> body
    | ds when Types.equal ret Types.Unit -> body @ ds
    | ds ->
      (* The result is computed before the defers run and returned after, so it
         goes through a slot rather than staying the last form. *)
      let rec split = function
        | [ last ] -> ([], last)
        | x :: rest -> let (init, last) = split rest in (x :: init, last)
        | [] -> assert false
      in
      let init, last = split body in
      let s = fresh_slot ctx ret in
      let loc = last.Tast.loc in
      init @ [ mk loc ret
                 (Tast.Let ([ (s, last) ], ds @ [ mk loc ret (Tast.Local s) ])) ]
  in
  { Tast.name = fn.Ast.name; params;
    slots = Array.of_list (List.rev ctx.slot_tys);
    snames = Array.of_list (List.rev ctx.slot_names);
    (* The same defers again, for the transfer exit path §5 describes. The
       normal path has them spliced into [body] above. *)
    ret; body; fdefers = ctx.defers; fparent = None; floc = fn.Ast.nloc }

let check_global env (d : Ast.decl) : Tast.global option =
  let ctx () = { env; ret = Types.Unit; slots = 0; slot_tys = []; slot_names = []; scope = []; defers = [];
      outer = []; in_handler = false; in_frames = None; in_defer = false; owner = "<none>" } in
  match d.Ast.d with
  | Ast.Defvar (n, _, init) ->
    let ty, _ = Hashtbl.find env.globals n in
    let ginit =
      match init with
      | Ast.Zeroed -> { Tast.e = Tast.Zero ty; ty; loc = d.Ast.dloc }
      | Ast.Uninit -> { Tast.e = Tast.Uninit ty; ty; loc = d.Ast.dloc }
      | Ast.Init v -> check (ctx ()) ~want:ty v
    in
    Some { Tast.gname = n; gty = ty; ginit; gconst = false; gfolded = false }
  | Ast.Defconst (n, _, v) ->
    let ty, _ = Hashtbl.find env.globals n in
    (* [collect] already folded the integer constants, because an array length
       has to be known before any type resolves. Use that value here rather
       than the expression it came from: a global's initialiser has to be a
       compile-time constant, and [(/ screen-height cell-size)] is one — the
       folding pass is the only thing that knows it. *)
    let ginit =
      match Hashtbl.find_opt env.consts n, ty with
      | Some k, Types.Int kind ->
        (* Still range-checked: this path skips [check], and [in_range] is the
           only thing that rejects 300 as a u8. *)
        { Tast.e = Tast.Int (in_range d.Ast.dloc kind k, kind); ty;
          loc = d.Ast.dloc }
      | _ -> check (ctx ()) ~want:ty v
    in
    (* [env.consts] holds exactly the constants the folding pass consumed, so
       membership is the question "is this value in the program's shape?" *)
    Some { Tast.gname = n; gty = ty; ginit; gconst = true;
           gfolded = Hashtbl.mem env.consts n }
  | _ -> None

(* The entry point, plan.org: (defn main [args [string]] i32), with both the
   parameter and the return type optional. *)
let check_main env =
  match Hashtbl.find_opt env.fns "main" with
  | None -> ()   (* a library, or a file being checked on its own *)
  | Some (params, ret) ->
    let ok_params =
      match params with
      | [] -> true
      | [ Types.Slice Types.String ] -> true
      | _ -> false
    in
    if not ok_params then
      fail Loc.unknown
        "main takes no parameters or one [string], not (%s)"
        (String.concat " " (List.map Types.to_string params));
    if not (Types.equal ret Types.Unit || Types.equal ret (Types.Int Types.I32))
    then
      fail Loc.unknown "main returns i32 or nothing, not %s"
        (Types.to_string ret)

(* The environment as well as the program. A session needs it to check an
   expression typed at a REPL against the program the process is running — and
   it has to be this one rather than anything rebuilt from declarations,
   because [program] prepends the prelude and no accumulated AST contains it. *)
let program_with_env (decls : Ast.decl list) : Tast.program * env =
  let env = new_env () in
  let decls = Parse.program (Prelude.forms ()) @ decls in
  (* Before anything is collected: every (declare-c ...) becomes an ordinary
     flattened [declare] with a Flan [defn] over it, and the C that does the
     flattening comes back to be compiled into the build. Nothing below this
     line knows the form exists. *)
  let decls, cshim = Shim.expand decls in
  collect env decls;
  check_finite env;
  check_main env;
  let globals = List.filter_map (check_global env) decls in
  let fns =
    List.filter_map
      (fun (d : Ast.decl) ->
         match d.Ast.d with
         | Ast.Defn fn -> Some (check_fn env fn)
         | _ -> None)
      decls
  in
  (* The handler clauses lifted out along the way. They are ordinary functions
     from here down; nothing in the backend knows they were written inside
     something else. *)
  let fns = fns @ List.rev env.lifted in
  (* Sorted, so the emitted IR is reproducible build to build: a Hashtbl's
     fold order is not. *)
  let values name tbl =
    Hashtbl.fold (fun _ v acc -> v :: acc) tbl []
    |> List.sort (fun a b -> String.compare (name a) (name b))
  in
  let externs =
    Hashtbl.fold
      (fun name esym acc ->
         let eparams, eret = Hashtbl.find env.fns name in
         { Tast.ename = name; esym; eparams; eret } :: acc)
      env.externs []
    |> List.sort (fun (a : Tast.extern) b -> String.compare a.Tast.esym b.Tast.esym)
  in
  ({ Tast.structs = values (fun (s : Tast.structure) -> s.Tast.sname) env.structs;
     unions = values (fun (u : Tast.union) -> u.Tast.uname) env.unions;
     globals; externs; fns; cshim },
   env)

let program (decls : Ast.decl list) : Tast.program = fst (program_with_env decls)

(* One expression, checked against a program that is already running. The
   frame is empty — a REPL expression has no parameters and no enclosing
   function — so the slots it needs are whatever its own [let]s allocate. *)
let expression env (e : Ast.expr) :
  Tast.expr * Types.t array * string option array =
  let ctx =
    { env; ret = Types.Unit; slots = 0; slot_tys = []; slot_names = []; scope = []; defers = [];
      outer = []; in_handler = false; in_frames = None; in_defer = false; owner = "<none>" }
  in
  let t = check ctx e in
  (t, Array.of_list (List.rev ctx.slot_tys),
   Array.of_list (List.rev ctx.slot_names))