Merge master into the arity lane.

TODO.org

@@ -45,6 +45,21 @@ Decided (133): =x?= is a bool; =if x?=, =elif x?=, =while x?= and the rest of an
 make a local Option its payload in the block, in place (not a copy). Assigning an Option
 to it there is refused rather than ending the narrowing; =e? as g= names what a test
 found. Rules out =if let g = x= over a plain name, which is refused toward these.
+** DONE A T is wrapped where a T? is wanted
+CLOSED: [2026-09-26]
+Decided (138), like Swift: one level per boundary, the literal built at T first; not
+inside a container. Rules out implicit unwrapping: a T? where a T is wanted stays refused.
+** DONE A kept when over an Option body flattens one level
+CLOSED: [2026-09-26]
+Decided (140), reversing 125a: a kept =when=, else-less =if=/=elif= or =if let= chain whose
+arm is already a =T?= is a =T?=, and =T= beside =T?= arms is =T?=; a =T??= arm stays =T??=.
+Where =T??= is wanted the arm is Some of it. Rules out telling "no branch matched" apart
+from "a branch gave None" without asking for =T??=.
+** DONE e as g inside an and chain
+CLOSED: [2026-09-26]
+Decided (136): =e as g= (or =e? as g=) is any test of a condition's =and= chain, binding =g=
+for the rest of the chain and the block, and a kept =when= with one is one flat Option. Rules
+out =as= as a cast (only an Option or a dyn), and a binding under =or=, =not= or =until=.
 ** TODO The stepper does not step inside an optional chain
 =Ast.step_expr= treats a =Chain= as a leaf (its catch-all), so nothing in a chain's
 body gets a step point of its own.
@@ -70,8 +85,8 @@ Option or a dyn holds (130); over any other type, and =_=, it is refused toward
 ** DONE when as a value, and get as a checked lookup
 CLOSED: [2026-09-26]
 Every one-armed =if= (and a =cond= with no =:else=) is a =when=; kept — a =let= value, a
-call's argument, a lambda's return — it is =Option(T)=, nested over an Option body
+call's argument, a lambda's return — it is =Option(T)=, and body-or-nil where a dyn is
-(Rust's =bool::then=), and body-or-nil where a dyn is wanted. A =_=-inferred return's
+wanted. Over an Option body it was nested (Rust's =bool::then=, 125a); 140 reversed that. A =_=-inferred return's
 last form is not kept. =get= over dyn text or vec is nil when out of range; =.field= still traps.
 
 ** NEXT str and String
@@ -820,6 +835,10 @@ One spelling for one operation; != stays, and not= is refused with a suggestion
 of !=.
 
 * Checker
+** TODO An error in a callee's condition adds a bogus one at main
+=fn f(a)= with a refused condition (=if n + 1= over an i32), and =fn main()= calling =f(3)=
+last, also reports "main returns i32 or nothing, not Never": the recovered body reads as
+Never and main's last form inherits it.
 ** TODO A u64 above the i64 maximum becomes -1 when it crosses into dyn
 =(+ z u)= and =(max u 0 z)= with u = u64 max read u as -1, silently. It should trap at
 the crossing, as a u64 field read through a view already does.
@@ -1633,6 +1652,9 @@ are a dyn vector, except numbers with no common type, which are refused. Rules
 out the first element typing the rest.
 
 * Dev loop
+** TODO eval-in-frame can assign names the program cannot
+A loop index or a pattern binding in a stopped frame takes =x = …= through eval-expr, where
+the program refuses it; only =as= names are marked read-only (=Tast.fn.as_slots=).
 ** WAIT Three dev-session tests fail intermittently under load
 Parked 2026-09-26 until the features land: "a restart accepted with a read queued behind
 it" (test_agent), "a parked session with no main" (test-flan.el), and a connect ENOENT in

emacs/flan-fln-mode.el

@@ -1854,6 +1854,19 @@ it, so a block pasted at another depth stays one block."
 (defun flan-fln--fallback-re (heads)
   (concat "^" (regexp-opt heads t) "(" flan-fln--name-re))
 
+(defun flan-fln--as-matcher (limit)
+  "Find the next `as' of a condition up to LIMIT, `if e as g and ...': an
+`as' before a name, on a line an `if', `elif', `while' or `when' comes first on."
+  (let (found)
+    (while (and (not found)
+                (re-search-forward "[ \t]\\(as\\)[ \t]+[^][ \t\n(){},;\":]" limit t))
+      (setq found (save-excursion
+                    (save-match-data
+                      (goto-char (match-beginning 1))
+                      (re-search-backward "\\_<\\(?:if\\|elif\\|while\\|when\\)\\_>"
+                                          (line-beginning-position) t)))))
+    found))
+
 (defun flan-fln--return-type-matcher (limit)
   "Find the next return type up to LIMIT: after the `->' of a fn header, a
 lambda or a `Fn(...)' type, and not after a match arm's."
@@ -1931,8 +1944,9 @@ lambda or a `Fn(...)' type, and not after a match arm's."
     ;; The words inside a line: `for i in range(n)', `if c then a else b', a
     ;; `where' constraint.
     ("[ \t]\\(then\\|else\\|in\\|where\\)[ \t]" 1 font-lock-keyword-face)
-    ;; A test's `as', `if e? as g'.
+    ;; A test's `as', `if e? as g', and a condition's, `if e as g and ...'.
     ("?[ \t]+\\(as\\)[ \t]" 1 font-lock-keyword-face)
+    (flan-fln--as-matcher 1 font-lock-keyword-face)
     ;; `if let Some(g) = x', and a value's `if' or `when', `x = when c then a'.
     ("\\_<\\(?:el\\)?if[ \t]+\\(let\\)[ \t]" 1 font-lock-keyword-face)
     ("[ \t=(,]\\(if\\|when\\)[ \t]" 1 font-lock-keyword-face)

emacs/test-flan-fln.el

@@ -967,6 +967,16 @@ defconst(k, 3)
   (search-forward "g!")
   (backward-char 1)
   (test-flan-fln--is "the name at x! is x" (thing-at-point 'symbol t) "g"))
+;; A condition's `as' with no `?' before it, twice on a line; an `as' outside
+;; a condition is left alone.
+(test-flan-fln--in "fn f()\n  left = when get(grid, r) as g and b(g) as h then g\n  x = as y\n"
+  (font-lock-ensure)
+  (let ((face (lambda (needle)
+                (save-excursion (goto-char (point-min)) (search-forward needle)
+                                (get-text-property (match-beginning 0) 'face)))))
+    (test-flan-fln--is "a condition's as is a keyword" (funcall face "as g") 'font-lock-keyword-face)
+    (test-flan-fln--is "and a second one" (funcall face "as h") 'font-lock-keyword-face)
+    (test-flan-fln--is "an as outside a condition is not" (funcall face "as y") nil)))
 (test-flan-fln--is "after if x? as g, one level deeper"
                    (test-flan-fln--tabs "fn f() -> ()\n  if o? as g\n|" 1) 4)
 (with-temp-buffer

lib/ast.ml

@@ -91,6 +91,10 @@ and expr_kind =
      (Option T) tested by [x?] in the condition above it, read as its
      payload ([if x?] narrowing, decision 133). *)
   | Narrow of string list * expr
+  (* Made by the checker, never read: [body] with each [(g, h)] reading [g]
+     as the binding of the hidden name [h], what an [as] in the condition
+     above found (decision 136). *)
+  | Alias of (string * string) list * expr
   | Struct of string * (string * expr) list   (* (Cursor {.src s}) *)
   (* {.src s .pos 0} with no type written in front of it. The fields alone do
      not name a type, so this node carries no name and is only checkable where
@@ -485,6 +489,7 @@ let map_children f (e : expr) : expr =
     | IfLet (s, a, e) -> IfLet (ex s, arm a, Option.map ex e)
     | Chain (n, v, b) -> Chain (n, ex v, ex b)
     | Narrow (ns, b) -> Narrow (ns, ex b)
+    | Alias (ps, b) -> Alias (ps, ex b)
     | Struct (n, fs) -> Struct (n, List.map (fun (n, v) -> (n, ex v)) fs)
     | Bare fs -> Bare (List.map (fun (n, v) -> (n, ex v)) fs)
     | MapLit (tag, kvs) -> MapLit (tag, List.map (fun (k, v) -> (ex k, ex v)) kvs)
@@ -609,7 +614,14 @@ let mark_pause ?fn ~line ~col (ds : decl list) : decl list option =
          reports, and the line DWARF names, nowhere. *)
       { e with e = Do [ pause_call ?fn e.loc; e ] }
     end
-    else map_children walk e
+    else
+      match e.e with
+      (* A call's name is not a form of its own: a mark on it, the [?] of
+         [x?] or the [>] of [(> a b)], stops before the call. *)
+      | Call ({ e = Var _; loc = hl }, _) when at hl ->
+        hit := true;
+        { e with e = Do [ pause_call ?fn e.loc; e ] }
+      | _ -> map_children walk e
   in
   let body es = List.map walk es in
   let decl (d : decl) =
@@ -637,3 +649,29 @@ let mark_pause ?fn ~line ~col (ds : decl list) : decl list option =
   in
   let ds = List.map decl ds in
   if !hit then Some ds else None
+
+(* The names the [as] tests of condition [c] bind (decision 136), for the
+   block [c] guards. [Parse.as_chain] reads the chain to this shape: each
+   test an [If] with [false] for its else, each [as] an [IfLet] over a plain
+   name with the rest of the chain as its body and [false] for its else. *)
+let as_name g =
+  g <> "" && (match g.[0] with 'A' .. 'Z' -> false | _ -> true)
+  && g <> "true" && g <> "false"
+
+(* [x] when [c] is [x] with a pause mark in front of it, [Do [pause; x]]:
+   [mark_pause] wraps whatever starts at the column marked, a test of a
+   condition's chain included, and the chain still means what it did. *)
+let unpause (c : expr) =
+  match c.e with
+  | Do [ { e = Call ({ e = Var _; _ }, []); loc }; x ] when loc = x.loc -> Some x
+  | _ -> None
+
+let rec as_binds (c : expr) =
+  match unpause c with
+  | Some x -> as_binds x
+  | None ->
+  match c.e with
+  | If (_, q, Some { e = Var "false"; _ }) -> as_binds q
+  | IfLet (_, { pat = Pctor (g, []); body = [ q ]; _ }, Some { e = Var "false"; _ })
+    when as_name g -> g :: as_binds q
+  | _ -> []

lib/emit.ml

@@ -4881,7 +4881,7 @@ let emit_startup m ?(hidden = false) (globals : Tast.global list) =
        own to live in. *)
     List.iter (emit_global m ~hidden) flags;
     emit_fn m ~hidden
-      { Tast.name = ".init-globals"; params = []; slots = [||]; snames = [||];
+      { Tast.name = ".init-globals"; params = []; slots = [||]; snames = [||]; as_slots = [];
         ret = Types.Unit; body; fdefers = []; fenv = None; fparent = None;
         floc = (List.hd computed).Tast.ginit.Tast.loc };
     true
@@ -5072,6 +5072,7 @@ declare void @flan_alloc_set_budget(ptr, i64)
 ; ever looks inside one, so every operation on a dyn value is one of these.
 declare i64 @flan_dyn_nil()
 declare i64 @flan_dyn_from_i64(i64)
+declare i64 @flan_dyn_from_u64(i64, ptr, i64)
 declare i64 @flan_dyn_from_f64(double)
 declare i64 @flan_dyn_from_bool(i32)
 declare i64 @flan_dyn_from_bytes(ptr, i64)
@@ -5153,6 +5154,8 @@ declare ptr @flan_dev_literal(ptr, i64)
 declare i64 @flan_dyn_need_i64(i64)
 declare double @flan_dyn_need_f64(i64)
 declare i32 @flan_dyn_need_bool(i64)
+declare double @flan_dyn_need_f64_at(i64, ptr, i64)
+declare i32 @flan_dyn_need_bool_at(i64, ptr, i64)
 declare i32 @flan_dyn_need_i32(i64, ptr, i64)
 declare i64 @flan_dyn_need_int(i64, i32, ptr, i64)
 declare i64 @flan_dyn_int_of(i64)

lib/indent_reader.ml

@@ -1005,7 +1005,7 @@ let no_place (e : Form.t) =
     failk "chain-assign" e.loc
       "%s is an optional chain, and a chain cannot be assigned to: when it \
        holds nothing there is no place to write. Test it first: if %s?, and \
-       in the block %s is what it holds, or if %s? as g, then assign through g"
+       in the block %s is what it holds, or if %s as g, then assign through g"
       (text_of e) r r r
   | Form.List [ { v = Form.Sym "!!"; _ }; x ] ->
     let r = text_of x in
@@ -1456,6 +1456,13 @@ and primary p : Form.t * int =
       (match (peek p).tok with
        | RP -> ignore (advance p)
        | EOF -> unclosed p '(' l0
+       | NAME "as" ->
+         failk "as-paren" (peek p).loc
+           "as names what a test found for the block of the if, elif, while \
+            or when it is a test of, so it stands in that condition's and \
+            chain and not inside parentheses. Write it without them: if %s as \
+            g and ..."
+           (text_of e)
        | COMMA ->
          failk "tuple" (peek p).loc
            "parentheses group one value, and this comma starts a second. \
@@ -1487,12 +1494,7 @@ and if_expr p =
   let t = advance p in
   let word = match t.tok with NAME w -> w | _ -> "if" in
   let letp = if word = "if" then if_let_head p else None in
-  let c = match letp with Some m -> m | None -> fst (binary p 1) in
+  let c = match letp with Some m -> m | None -> cond_head p in
-  let letp, c =
-    match letp with
-    | Some _ -> (letp, c)
-    | None -> (match as_head p c with Some m when word = "if" -> (Some m, m) | _ -> (None, c))
-  in
   (match (peek p).tok with
    | NAME "then" -> ignore (advance p)
    | _ ->
@@ -1533,53 +1535,89 @@ and if_let_head p =
        failk "if-let-name" pat.loc
          "if let %s = %s has no pattern to test. To test that %s holds a \
           value, write if %s?, and in the block it is what it holds; to name \
-          what it holds, write if %s? as %s"
+          what it holds, write if %s as %s"
          g (text_of v) (text_of v) (text_of v) (text_of v) g
      | _ -> ());
     Some (mk p lt.loc (Form.Vec [ pat; v ]))
   | _ -> None
 
-(* [e? as g]: after a test [e?], the name what [e] holds is bound to, as
+(* A condition, where [e as g] may stand as a test of a top-level [and]
-   the head [[g e]] an [if let] over a plain name stands as (decision 133). *)
+   chain (decisions 133, 136): [e] holds a value, and [g] names it for the
-and as_head p (c : Form.t) =
+   rest of the chain and the block. [e? as g] is the same test. Each binding
+   reads [(as g e)] in its test's place, in one flat [(and ...)]; the checker
+   gives [g] its scope. It cannot stand under [or] or [not], where the test
+   holding would not mean [e] held anything. *)
+and cond_head p =
+  let x, lvl = binary p 1 in
   match (peek p).tok with
-  | NAME "as" ->
+  | NAME "as" -> as_chain p x lvl
-    let at = advance p in
+  | _ -> x
-    (match c.v with
+
-     | Form.List [ { v = Form.Sym "?"; _ }; e ] ->
+and as_chain p (x : Form.t) lvl =
-       let g =
+  let at = advance p in
-         match (peek p).tok with
+  let refuse_or loc =
-         | NAME g when g <> "" && g.[0] <> '.' ->
+    failk "as-or" loc
-           let gt = advance p in
+      "as names what a test found, for the rest of an and chain and the \
-           check_name gt g;
+       block. With or, the block can run when that test did not hold, and \
-           sym gt.loc g
+       there would be nothing to name. Bind with as in an if of its own, and \
-         | tk ->
+       test the rest inside it"
-           failk "as-name" (where_ p) "as takes the name to bind, and found %s" (show tk)
+  in
-       in
+  let items (f : Form.t) =
-       Some (Form.make (Form.Vec [ g; e ]) c.loc)
+    match f.v with
-     | _ ->
+    | Form.List ({ v = Form.Sym "and"; _ } :: (_ :: _ as xs)) -> xs
-       (* [a |> f? as g] would test [f] alone: a compound test is bracketed. *)
+    | _ -> [ f ]
-       let t = text_of c in
+  in
-       let compound =
+  (* Level 1 is an [or], or a pipe, which is loosest: [a |> f(1) as v]
-         let depth = ref 0 and quoted = ref false and hit = ref false in
+     names what the pipe answers. *)
-         String.iteri
+  let is_or (f : Form.t) =
-           (fun i ch ->
+    match f.v with Form.List ({ v = Form.Sym "or"; _ } :: _) -> true | _ -> false
-              if !quoted then
+  in
-                (if ch = '"' && (i = 0 || t.[i - 1] <> '\\') then quoted := false)
+  if lvl = 1 && is_or x then refuse_or x.loc;
-              else
+  let before, last =
-                match ch with
+    match List.rev (if lvl = 2 then items x else [ x ]) with
-                | '"' -> quoted := true
+    | last :: rb -> (List.rev rb, last)
-                | '(' | '[' | '{' -> incr depth
+    | [] -> ([], x)
-                | ')' | ']' | '}' -> decr depth
+  in
-                | ' ' when !depth = 0 -> hit := true
+  (match last.v with
-                | _ -> ())
+   | Form.List ({ v = Form.Sym "not"; _ } :: _) ->
-           t;
+     failk "as-not" at.loc
-         !hit
+       "as names what a test found, and not turns the test around: the \
-       in
+        block runs when %s holds nothing, so there is nothing to name. Bind \
-       failk "as-test" at.loc
+        with as, and put what runs when it is absent in the else"
-         "as names what a test found, and %s is not one. Write %s? as name"
+       (text_of last)
-         t (if compound then "(" ^ t ^ ")" else t))
+   | Form.List ({ v = Form.Sym "or"; _ } :: _) -> refuse_or last.loc
-  | _ -> None
+   | _ -> ());
+  let e = match last.v with Form.List [ { v = Form.Sym "?"; _ }; e ] -> e | _ -> last in
+  let g =
+    match (peek p).tok with
+    | NAME g when g <> "" && g.[0] >= 'A' && g.[0] <= 'Z' ->
+      failk "as-name" (where_ p)
+        "as binds a name to what %s holds, and %s is a case, not a name. \
+         Match a pattern with if let, as in if let %s(v) = %s"
+        (text_of e) g g (text_of e)
+    | NAME g when g <> "" && g.[0] <> '.' && not (is_op_word g) ->
+      let gt = advance p in
+      check_name gt g;
+      sym gt.loc g
+    | tk -> failk "as-name" (where_ p) "as takes the name to bind, and found %s" (show tk)
+  in
+  let bound = mk p last.loc (Form.List [ sym at.loc "as"; g; e ]) in
+  let rest =
+    match (peek p).tok with
+    | NAME "and" ->
+      ignore (advance p);
+      let y, ylvl = binary p 1 in
+      (match (peek p).tok with
+       | NAME "as" -> items (as_chain p y ylvl)
+       | _ ->
+         if ylvl = 1 && is_or y then refuse_or y.loc;
+         if ylvl = 2 then items y else [ y ])
+    | NAME "or" -> refuse_or (peek p).loc
+    | _ -> []
+  in
+  match before @ (bound :: rest) with
+  | [ one ] -> one
+  | xs -> mk p x.loc (Form.List (sym x.loc "and" :: xs))
 
 (* The if an [if let] head was read into, rewritten to (if-let [P v] then
    else): [(if [P v] a b)], [(when [P v] body ...)] and an elif chain's
@@ -2855,12 +2893,7 @@ and header (s : st) w : Form.t =
     form [ alias; path ]
   | "if" | "when" ->
     let letp = if w = "if" then if_let_head p else None in
-    let c = match letp with Some m -> m | None -> fst (binary p 1) in
+    let c = match letp with Some m -> m | None -> cond_head p in
-    let letp, c =
-      match letp with
-      | Some _ -> (letp, c)
-      | None -> (match as_head p c with Some m when w = "if" -> (Some m, m) | _ -> (None, c))
-    in
     (* The elif and else clauses at the if's column, then the whole form.
        [oneline] when the if was [if c then a]: its clauses may then be
        one-line too, [elif c then x] and [else y], or take blocks. *)
@@ -2877,11 +2910,7 @@ and header (s : st) w : Form.t =
           let c =
             match if_let_head p with
             | Some m -> elif_lets := m :: !elif_lets; m
-            | None ->
+            | None -> cond_head p
-              let c = fst (binary p 1) in
-              (match as_head p c with
-               | Some m -> elif_lets := m :: !elif_lets; m
-               | None -> c)
           in
           (match (peek p).tok with
            | NAME "then" when oneline ->
@@ -2993,21 +3022,30 @@ and header (s : st) w : Form.t =
       | KW k, n when n <> NEWLINE -> let kt = advance p in [ Form.make (Form.Kw k) kt.loc ]
       | _ -> []
     in
-    let c, _ = expr p in
+    let c = cond_head p in
-    (match (if w = "while" then as_head p c else None) with
+    let binds =
-    (* [while e? as g]: [(while true (if-let [g e] (do body) (break)))]. A
+      let is_as (f : Form.t) =
-       break or continue in the body is this loop's. *)
+        match f.v with Form.List ({ v = Form.Sym "as"; _ } :: _) -> true | _ -> false
-    | Some m ->
+      in
-      expect_line_end p ~after:(w ^ " " ^ text_of c ^ " as ...");
+      match c.v with
-      let body = block s ~after:w in
+      | Form.List ({ v = Form.Sym "and"; _ } :: xs) -> List.exists is_as xs
+      | _ -> is_as c
+    in
+    if binds && w = "until" then
+      failk "as-until" c.Form.loc
+        "until runs while its test does not hold, so as would name what a \
+         test found when it found nothing. Write while, with the test the \
+         other way round";
+    expect_line_end p ~after:(w ^ " " ^ text_of c);
+    let body = block s ~after:w in
+    if binds then
+      (* [while c]: [(while true (if c (do body) (break)))], so what [c]
+         binds reaches the body. A break or continue in the body is this
+         loop's, and a continue tests [c] again. *)
       let at = c.Form.loc in
       let f items = Form.make (Form.List items) at in
-      form (label @ [ sym at "true";
+      form (label @ [ sym at "true"; f [ sym at "if"; c; f (sym at "do" :: body); f [ sym at "break" ] ] ])
-                      f [ sym at "if-let"; m; f (sym at "do" :: body); f [ sym at "break" ] ] ])
+    else form (label @ (c :: body))
-    | None ->
-      expect_line_end p ~after:(w ^ " " ^ text_of c);
-      let body = block s ~after:w in
-      form (label @ (c :: body)))
   | "for" ->
     let label =
       match (peek p).tok with

lib/load.ml

@@ -255,7 +255,9 @@ let rec rename_expr owned alias bound (e : Ast.expr) : Ast.expr =
           (bound, []) bs
       in
       Ast.Let (List.rev bs, List.map (rename_expr owned alias bound) body)
-    | Ast.If (c, t, e') -> Ast.If (go c, go t, Option.map go e')
+    (* What an [as] in the condition binds is bound in the block. *)
+    | Ast.If (c, t, e') ->
+      Ast.If (go c, rename_expr owned alias (Ast.as_binds c @ bound) t, Option.map go e')
     | Ast.While (l, c, body) -> Ast.While (l, go c, gos body)
     (* A loop's names are its own and are never imported; its initial
        values and its body are ordinary expressions. *)
@@ -295,6 +297,7 @@ let rec rename_expr owned alias bound (e : Ast.expr) : Ast.expr =
     | Ast.Chain (n, v, b) ->
       Ast.Chain (n, go v, rename_expr owned alias (n :: bound) b)
     | Ast.Narrow (ns, b) -> Ast.Narrow (ns, go b)
+    | Ast.Alias (ps, b) -> Ast.Alias (ps, go b)
     (* A quoted symbol naming something the package declares.
        [(Form.Sym {.s "Cursor"})] is what a quasiquote desugars to, and it is
        the one place a package's name survives into a *string* — which is
@@ -858,7 +861,7 @@ let rec expr_uses acc (e : Ast.expr) =
     go sc; List.iter (fun (a : Ast.arm) -> gos a.Ast.body) arms
   | Ast.IfLet (sc, a, e') -> go sc; gos a.Ast.body; Option.iter go e'
   | Ast.Chain (_, v, b) -> go v; go b
-  | Ast.Narrow (_, b) -> go b
+  | Ast.Narrow (_, b) | Ast.Alias (_, b) -> go b
   | Ast.Struct (n, kvs) ->
     acc := (n, e.Ast.loc) :: !acc;
     List.iter (fun (_, v) -> go v) kvs

lib/parse.ml

@@ -508,6 +508,8 @@ and form f mk (head : Form.t) (args : Form.t list) : Ast.expr =
      | _ -> fail f "?. is (?. [name value] body)")
 
   (* Short-circuiting, so they cannot be ordinary calls. *)
+  | Sym "and" when List.exists is_as args -> as_chain f args
+  | Sym "as" -> as_chain f [ f ]
   | Sym "and" -> shortcircuit f args ~is_and:true
   | Sym "or" -> shortcircuit f args ~is_and:false
 
@@ -1303,6 +1305,33 @@ and cond f (args : Form.t list) : Ast.expr =
    actually fix it is check_if preferring the arm that is not a compiler temp
    when it reports, which is check.ml's call. Written up in TODO.org, "and's
    last operand gets a misdirected caret". *)
+(* [(as g e)]: the test that [e] holds a value, naming it [g] (decision
+   136). The reader writes one only as a condition or a test of the [and]
+   chain that is one. Each test of that chain is an [If] whose else is
+   [false], and each [as] an [IfLet] over the plain name [g] whose body is
+   the rest of the chain and whose else is [false]; [Check.as_cond] reads
+   that shape and gives [g] to the block the condition guards as well. *)
+and is_as (f : Form.t) =
+  match f.v with List ({ v = Sym "as"; _ } :: _) -> true | _ -> false
+
+and as_chain f (args : Form.t list) : Ast.expr =
+  (* At no position, so a pause mark cannot land on the chain's own
+     [false]: it has none in the source. *)
+  let no (_ : Form.t) = { Ast.e = Ast.Var "false"; loc = Loc.unknown } in
+  let rec go = function
+    | [] -> { Ast.e = Ast.Var "true"; loc = f.loc }
+    | [ x ] when not (is_as x) -> expr x
+    | ({ v = List [ { v = Sym "as"; _ }; { v = Sym g; _ }; e ]; _ } as x) :: rest ->
+      let arm = { Ast.pat = Ast.Pctor (g, []); body = [ go rest ]; aloc = x.loc } in
+      { Ast.e = Ast.IfLet (expr e, arm, Some (no x)); loc = x.loc }
+    | x :: _ when is_as x -> fail x "as is (as name value)"
+    | x :: rest -> { Ast.e = Ast.If (expr x, go rest, Some (no x)); loc = x.loc }
+  in
+  (* The chain's own node is at the [and], so a pause mark there has a form
+     to stop before. *)
+  let top = go args in
+  { top with Ast.loc = f.loc }
+
 and shortcircuit f (args : Form.t list) ~is_and : Ast.expr =
   let mk e = { Ast.e; loc = f.loc } in
   let rec go = function

lib/session.ml

@@ -1481,7 +1481,7 @@ let eval ?(origin = "<eval>") ?base ?forms ?pause ?(step = false) ?(running = tr
         { Tast.name = Printf.sprintf "install/%d" t.thunks;
           params = []; ret = Types.Unit; body;
           fdefers = []; fenv = None; fparent = None; floc = loc;
-          slots = [||]; snames = [||] }
+          slots = [||]; snames = [||]; as_slots = [] }
   in
   let ir =
     match run_thunk with
@@ -2232,7 +2232,8 @@ let write_slot ?(origin = "<set>") t ~frame ~(fn : Tast.fn) ~slot ~path
                       scratch and have none to keep. *)
                    snames =
                      Array.append bnames
-                       (Array.make (List.length !extra) None) }
+                       (Array.make (List.length !extra) None);
+                   as_slots = [] }
                in
                (* A struct copy the values named first, laid out in this
                   module and kept, as [eval_expr] keeps one. *)
@@ -2368,7 +2369,8 @@ let arm_restart ?(origin = "<restart>") t ~index ~(params : Types.t list)
           @ [ nullary "flan/dev-end" ];
         fdefers = []; fenv = None; fparent = None; floc = loc;
         slots = Array.append base (Array.of_list (List.rev !extra));
-        snames = Array.append bnames (Array.make (List.length !extra) None) }
+        snames = Array.append bnames (Array.make (List.length !extra) None);
+      as_slots = [] }
     in
     let copies = Check.fresh_copies t.env t.program.Tast.structs in
     let program =
@@ -2420,7 +2422,10 @@ let in_frame t ~frame:(index, (fn : Tast.fn), bound) (parsed : Ast.expr) =
     @ List.filter (fun (i, _) -> List.mem i bound) named
   in
   let scope =
-    List.map (fun (i, name) -> (name, fn.Tast.slots.(i), i >= nparams)) order
+    List.map
+      (fun (i, name) ->
+         (name, fn.Tast.slots.(i), i >= nparams, List.mem i fn.Tast.as_slots))
+      order
   in
   let checked, base, bnames, syn = Check.expression_in_scope t.env ~scope parsed in
   let table = List.map2 (fun (i, name) (_, j) -> (j, (i, name))) order syn in
@@ -2546,7 +2551,8 @@ let eval_expr ?(origin = "<eval>") ?(pause = false) ?frame t src : change =
       slots = Array.append base (Array.of_list (List.rev !extra));
       (* The expression's own [let]s keep their names; the slots [render] added
          behind them are the walk's own scratch and have none to keep. *)
-      snames = Array.append bnames (Array.make (List.length !extra) None) }
+      snames = Array.append bnames (Array.make (List.length !extra) None);
+      as_slots = [] }
   in
   (* Built against the program but never spliced into it: an evaluation is not
      a declaration, and adding one would leave the session carrying an eval/N

lib/tast.ml

@@ -354,6 +354,10 @@ type fn = {
      backend is free to ignore it entirely -- nothing is *resolved* through it,
      and a slot is still only ever referred to by index. *)
   snames  : string option array;
+  (* The slots an [as] bound (decision 136): named, and read-only, so
+     evaluating in a stopped frame may read them and may not assign them,
+     as the program itself may not. *)
+  as_slots : int list;
   ret     : Types.t;
   body    : expr list;
   (* The defers again, innermost first. [body] already has them spliced onto

runtime/flan_dyn.c

@@ -1909,6 +1909,26 @@ flan_dyn flan_dyn_from_i64(int64_t x) {
   return dyn_make(BOX_OBJ, (uint64_t)(uintptr_t)o);
 }
 
+/* A u64 has a dyn int to become only up to the largest i64; past it the trap
+ * names the number, which read as an i64 would be a different one. [what] is
+ * "u64" for a scalar crossing and "u64 element" for one read through a view,
+ * so both say the same sentence. */
+static flan_dyn u64_to_dyn(const uint8_t *loc, int64_t loclen, const char *op,
+                           const char *what, uint64_t x) {
+  if (x > (uint64_t)INT64_MAX) {
+    flan_say(loc, loclen,
+             "dyn%s%s: this %s is %llu, above the largest dyn int "
+             "(9223372036854775807), so it has no dyn value",
+             op[0] ? " " : "", op, what, (unsigned long long)x);
+    dyn_trap((const uint8_t *)"DynRange", 8);
+  }
+  return flan_dyn_from_i64((int64_t)x);
+}
+
+flan_dyn flan_dyn_from_u64(uint64_t x, const uint8_t *loc, int64_t loclen) {
+  return u64_to_dyn(loc, loclen, "", "u64", x);
+}
+
 flan_dyn flan_dyn_from_f64(double x) {
   flan_dyn v;
   /* Every NaN becomes the one positive quiet NaN, which is what keeps a
@@ -2797,11 +2817,14 @@ int64_t flan_dyn_need_i64(flan_dyn v) {
  * makes rather than a surprise somebody meets. Softening the check here is the
  * option not to take: this function sees a tag and nothing else, so it could
  * not tell (g 1) from (g (length xs)). */
-double flan_dyn_need_f64(flan_dyn v) {
+/* The _at forms trap at the site the checker hands over; the plain ones are
+ * for C callers that have none. */
+double flan_dyn_need_f64_at(flan_dyn v, const uint8_t *loc, int64_t loclen) {
   if (flan_dyn_tag(v) != FLAN_DYN_TAG_FLOAT)
-    trap1(NULL, 0, TYPE_TRAP, "f64", "a float was wanted", v);
+    trap1(loc, loclen, TYPE_TRAP, "f64", "a float was wanted", v);
   return dyn_num_value(v);
 }
+double flan_dyn_need_f64(flan_dyn v) { return flan_dyn_need_f64_at(v, NULL, 0); }
 
 static const char *an(const char *w);   /* forward: "a" or "an" */
 
@@ -2894,11 +2917,12 @@ uint32_t flan_dyn_need_char(flan_dyn v, const uint8_t *loc, int64_t loclen) {
   flan_trap((const uint8_t *)"DynType", 7);
 }
 
-uint8_t flan_dyn_need_bool(flan_dyn v) {
+uint8_t flan_dyn_need_bool_at(flan_dyn v, const uint8_t *loc, int64_t loclen) {
   if (flan_dyn_tag(v) != FLAN_DYN_TAG_BOOL)
-    trap1(NULL, 0, TYPE_TRAP, "bool", "a bool was wanted", v);
+    trap1(loc, loclen, TYPE_TRAP, "bool", "a bool was wanted", v);
   return (uint8_t)(dyn_payload(v) ? 1 : 0);
 }
+uint8_t flan_dyn_need_bool(flan_dyn v) { return flan_dyn_need_bool_at(v, NULL, 0); }
 
 /* ── A numeric cast opening a box ───────────────────────────────────────
  *
@@ -4046,14 +4070,7 @@ static flan_dyn view_read(const uint8_t *loc, int64_t loclen, const char *op,
   case 'L': {
     uint64_t x;
     memcpy(&x, p, 8);
-    if (x > (uint64_t)INT64_MAX) {
+    return u64_to_dyn(loc, loclen, op, "u64 element", x);
-      flan_say(loc, loclen,
-               "dyn %s: this u64 element is %llu, above the largest dyn int "
-               "(9223372036854775807), so it has no dyn value",
-               op, (unsigned long long)x);
-      dyn_trap((const uint8_t *)"DynRange", 8);
-    }
-    return flan_dyn_from_i64((int64_t)x);
   }
   case 'f': { float x;  memcpy(&x, p, 4); return flan_dyn_from_f64((double)x); }
   case 'd': { double x; memcpy(&x, p, 8); return flan_dyn_from_f64(x); }

runtime/flan_dyn.h

@@ -86,6 +86,8 @@ typedef struct flan_desc {
 
 flan_dyn flan_dyn_nil(void);
 flan_dyn flan_dyn_from_i64(int64_t x);
+/* Traps at [loc] on a u64 above the largest i64, which no dyn int holds. */
+flan_dyn flan_dyn_from_u64(uint64_t x, const uint8_t *loc, int64_t loclen);
 flan_dyn flan_dyn_from_f64(double x);
 flan_dyn flan_dyn_from_bool(uint8_t b);
 
@@ -304,6 +306,9 @@ void flan_dyn_emit_watch(flan_dyn v);
 int64_t flan_dyn_need_i64(flan_dyn v);
 double  flan_dyn_need_f64(flan_dyn v);
 uint8_t flan_dyn_need_bool(flan_dyn v);
+/* The same, trapping at the site [loc] names. */
+double  flan_dyn_need_f64_at(flan_dyn v, const uint8_t *loc, int64_t loclen);
+uint8_t flan_dyn_need_bool_at(flan_dyn v, const uint8_t *loc, int64_t loclen);
 /* For a typed integer of width [kind], 0..7 for i8 u8 i16 u16 i32 u32 i64
  * u64: an int in range, or a char's code point where it fits (ASCII only
  * into a byte), as an i64 the caller narrows. Anything else traps at [loc].

spec-syntax.md

@@ -244,6 +244,16 @@ Each item: the proposal, then the reason in one line.
     holds — `a?.f(x)`, `a?[i]`, `a?.b.c`. A result that is already an Option
     is not wrapped again, so `a?.b?.c` is one Option. A rest with no value
     makes the whole a statement. `~o1` is a fresh name no reader produces.
+  - A `T` where a `T?` is wanted is `Some` of it (decision 138): an
+    assignment, a `let` with a type, an argument, a return, a struct field, an
+    array or `Vec` element, and an `if` or `match` arm beside an Option arm.
+    A literal is built at `T` first, so `s = -1` over an `i64?` is `Some(-1)`
+    at i64. One level at a time: a `T` into a `T??` is `Some(Some(t))`, a `T?`
+    into a `T??` is `Some` of it. A `$t` meeting `$u?` binds `$u` to `T`.
+    Never inside a container (`Vec(i32)` is not a `Vec(i32?)`), and never the
+    other way: a `T?` where a `T` is wanted still needs `!`, `??`, `x?` or
+    `as`. A kept chain whose arms are a `T` and a `T?` is a `T?` (decision
+    140, under `when c`). **Built.**
 - **Casts and type-taking builtins are calls:** `i32(x)`, `vec-new(u8)`,
   `max-value(u8)`, `the([3 f32], [1 2 3.5])`. A pointer cast is the type
   called: `Ptr(Color)(p)` reads `((Ptr Color) p)`. **Built.**
@@ -307,14 +317,20 @@ Each item: the proposal, then the reason in one line.
   A `when` whose value is kept (a `let`'s value, an argument, a return) gives
   `Some(a)` when `c` holds and `None` when it does not; where a `dyn` is
   wanted, `a` or `nil`. As a statement it gives nothing. An `if`/`elif` chain
-  with no `else` is the same when kept: `None` when no test holds. **Built.**
+  with no `else` is the same when kept: `None` when no test holds. When `a`
+  is already an Option it is not wrapped again (decision 140): `when c then
+  o` over an `i32?` is an `i32?`, `None` when `c` fails or `o` is `None`, and
+  a chain mixing `T` and `T?` arms is a `T?`. One level only: an arm that is
+  a `T??` gives a `T??`. Where an Option of the arm's type is wanted, as a
+  `T??` over a `T?` arm, the arm is `Some` of its value and a failed test is
+  the outer `None`. `if let` with no `else` follows the same rule. **Built.**
 - **`if let P = v`** plus a block reads as `(if-let [P v] then)`; `elif` and
   `else` follow as for `if`, the rest of the chain being the `if-let`'s else.
   `elif let P = v` is a further `if-let` nested in that else.
   Kept with no `else` at the end of its chain, it gives an Option as `when` does.
   `P` is any `match` pattern, and its names are bound in the block only. One
   line: `if let Some(g) = o then g else 0`. A plain name, `if let g = o`, is
-  refused toward `if o?` and `if o? as g` below; `_` is refused toward `let`.
+  refused toward `if o?` and `if o as g` below; `_` is refused toward `let`.
   **Built.**
 - **`x?` tests that a value is present** (decision 133): a bool, true when an
   Option is `Some` and when a dyn is not `nil`. It reads `(? x)`. In `if x?`,
@@ -326,15 +342,33 @@ Each item: the proposal, then the reason in one line.
   present; giving it an Option is refused, and a parameter or a captured copy
   is no more assignable than outside it. A local whose address is taken, or
   that a `fn` assigns, anywhere in the function is not narrowed (something
-  else could clear it); `if x? as g` copies what it holds instead. A `?` after
+  else could clear it); `if x as g` copies what it holds instead. A `?` after
   a chain tests the whole chain: `o?.i?`. A capitalised name before `?` is
   read as a type, so a local tested this way needs a lowercase name.
   **Built.**
-- **`e? as g`** names what a test found, for an `e` that is not a plain name:
+- **`e as g`** tests that `e` holds a value and names it `g` (decisions 133,
-  `if get(grid, r, c)? as cell` reads `(if-let [cell (get grid r c)] …)`, an
+  136): an Option that is `Some` binds its payload, a dyn that is not `nil`
-  `if-let` over a plain name, which binds what an Option holds or a dyn that
+  binds itself. `e? as g` is the same test. Over any other type it is
-  is not `nil`. It works after `if`, `elif` and `while`; `while e? as g` plus
+  refused; `as` is never a conversion, which is written `i32(x)`. It stands
-  a block reads `(while true (if-let [g e] (do …) (break)))`. **Built.**
+  after `if`, `elif`, `while` and a one-line `if … then` or kept `when`, as
+  the whole condition or as any test of an `and` chain, and `g` is bound for
+  the rest of that chain and for the block: Swift's `if let g = e, c`.
+
+  ```
+  if get(grid, r + 1, c - 1) as g and is-empty-cell(g)
+    move(g)
+  if a as x and b as y and x < y
+    println(x, y)
+  let left = when get(grid, r + 1, c - 1) as g and is-empty-cell(g) then g
+  ```
+
+  The tests run left to right and stop at the first that fails, so each
+  value is found once. `g` is not bound in the `else`, in an `elif` or after
+  the block. It is refused under `or` and `not`, and after `until`, where the
+  block could run with nothing found. `x?` on a plain local narrows beside it
+  in the same chain. Reads `(as g e)` in the test's place, `(when (and a (as
+  g e) (f g)) …)`; `while c` with one reads `(while true (if c (do …)
+  (break)))`. **Built.**
 - **`while c`, `until c`**, optional label first: `while :outer c`. **Built.**
 - **`for i in range(n)`**, `range(a, b)`, `range(a, b, step)` read as
   `dotimes`. `range` here is syntax, not a function. `..` is avoided because

test/programs/as-chain-dyn.fln

@@ -0,0 +1,31 @@
+;; e as g over a dyn inside an and chain (decision 136): g is bound when e
+;; is not nil, for the rest of the chain and the block.
+
+fn pet-name(m)
+  if m.pet as pet and pet != "cat"
+    pet
+  elif m.name as who and who != "bo"
+    who
+  else
+    "nobody"
+
+fn first-big(xs, lo)
+  let found = when get(xs, 0) as x and x > lo then x
+  found
+
+fn run(m, xs, a, b)
+  println(pet-name(m), pet-name({:pet "cat" :name "bo"}), pet-name({:name "ann"}))
+  println(first-big(xs, 0), first-big(xs, 5), first-big([], 0))
+  let i = 0
+  let total = 0
+  while get(xs, i) as x and x > 0
+    total += x
+    i += 1
+  println(total, i)
+  if a? and get(xs, 1) as y and a + y > 4
+    println(a + y)
+  let v = if b as z and z > 1 then z else -1
+  println(v)
+
+fn main()
+  run({:pet "dog" :name "ann"}, [4, 2, 0, 7], 3, nil)

test/programs/as-chain.fln

@@ -0,0 +1,92 @@
+;; e as g inside an and chain (decision 136): g is bound for the rest of the
+;; chain and for the block, and not in the else, an elif or after the block.
+;; e? as g is the same test.
+
+struct Grain
+  color-idx: i32
+
+let calls: i32 = 0
+
+fn get-cell(grid: [4 i32], i: i32) -> Grain?
+  calls += 1
+  if i < 0 or i >= 4
+    return None
+  Some(Grain{.color-idx grid[i]})
+
+fn is-empty-cell(g: Grain) -> bool
+  g.color-idx < 0
+
+fn tick(n: i32) -> i32
+  calls += 1
+  print(n, "")
+  n
+
+fn half(n: i32) -> i32?
+  calls += 1
+  if n % 2 == 0 then Some(n / 2) else None
+
+fn classify(grid: [4 i32], i: i32) -> str
+  if get-cell(grid, i) as g and is-empty-cell(g)
+    "empty"
+  elif get-cell(grid, i) as g and g.color-idx > 1
+    "big"
+  elif half(i) as h and h > 0
+    "half"
+  else
+    "other"
+
+fn main()
+  let grid = [1, -1, 2, -3]
+  ;; A block if, with an else that does not see g.
+  let g = 100
+  if get-cell(grid, 1) as g and is-empty-cell(g)
+    println("empty", g.color-idx)
+  if get-cell(grid, 0) as g and is-empty-cell(g)
+    println("empty", g.color-idx)
+  else
+    println("else sees the outer g", g)
+  println("after", g)
+  ;; A kept when gives a Grain?.
+  let left = when get-cell(grid, 3) as g and is-empty-cell(g) then g
+  println(left!.color-idx)
+  let none: Grain? = when get-cell(grid, 0)? as g and is-empty-cell(g) then g
+  println(none?)
+  ;; Two bindings, and a test over both.
+  let a: i32? = Some(3)
+  let b: i32? = Some(5)
+  if a as x and b as y and x < y
+    println(x, y)
+  if a as x and b as y and x > y
+    println(x, y)
+  else
+    println("not less")
+  ;; One line, in a let.
+  let v = if half(8) as h and h > 3 then h * 10 else -1
+  let w = if half(6) as h and h > 3 then h * 10 else -1
+  println(v, w)
+  ;; An elif chain.
+  println(classify(grid, 1), classify(grid, 2), classify(grid, 0), classify(grid, 4), classify(grid, 5))
+  ;; Each value is found once, left to right, and a failed test stops the chain.
+  calls = 0
+  if tick(1) > 0 and half(tick(2)) as h and tick(3) + h > 0
+    println("ran", h)
+  println(calls)
+  calls = 0
+  if tick(1) > 0 and half(tick(3)) as h and tick(5) + h > 0
+    println("ran", h)
+  else
+    println("stopped")
+  println(calls)
+  ;; x? narrows beside as in one chain.
+  let n: i32? = Some(40)
+  if n? and half(n) as h and n + h > 50
+    println(n + h)
+  if half(6) as h and n? and n + h > 40
+    println(n + h)
+  ;; while: pop while the next cell is empty.
+  let i = 1
+  let seen = 0
+  while get-cell(grid, i) as c and is-empty-cell(c)
+    seen += c.color-idx
+    i += 2
+  println(seen, i)

test/programs/autowrap.fln

@@ -0,0 +1,148 @@
+;; A T where a T? is wanted is Some of it (decision 138): each position,
+;; a literal built at the payload's type, nested Options, generics, and the
+;; arms of an if, a match and a kept chain.
+
+struct P
+  a: i64?
+  b: i32?
+
+fn show(o: i32?) -> i32 = o ?? -9
+
+fn back(b: bool, x: i32) -> i32?
+  if b
+    return x
+  None
+
+fn pick(b: bool, x: i32) -> i32?
+  if b then x else None
+
+fn pick2(b: bool, x: i32) -> i32?
+  if b then None else x
+
+fn two(o: Option(i32?)) -> str
+  match o
+    Some(i) -> if i? then "some some" else "some none"
+    None -> "none"
+
+fn first(o: $u?) -> $u = o!
+
+fn wrap(x: $t) -> $t? = x
+
+;; A generic body's own $t handed to a $u? parameter.
+fn through(x: $t) -> $t = first(x)
+
+;; A None arm first takes its type from the arms after it.
+fn arms(k: i32, x: i32) -> i32?
+  match k
+    5 -> None
+    4 -> x
+    _ -> 0
+
+fn big(x: i64?) -> i64 = x ?? 0
+
+;; A T?? is wanted, so each arm is Some of its value and the chain's None is
+;; the outer one.
+fn chain(a: bool, b: bool, opt: i32?) -> Option(i32?)
+  if a
+    1
+  elif b
+    opt
+
+;; Nothing wanted: a T arm beside a T? arm makes the chain a T?, flattened
+;; one level (decision 140).
+fn flat(a: bool, b: bool, opt: i32?) -> i32?
+  let r =
+    if a
+      1
+    elif b
+      opt
+  r
+
+fn main()
+  ;; assignment, and a literal at the payload's width
+  let s: i64? = None
+  s = -1
+  println(s ?? 0)
+  ;; a let with an annotation, from a literal, a name and arithmetic
+  let x: i32 = 4
+  let a: i32? = x + 1
+  let w: i64? = x
+  let f: f64? = 2
+  println(a ?? 0, w ?? 0, f ?? 0.0)
+  ;; an argument and a return value
+  println(show(7), back(true, 8) ?? -1, back(false, 8) ?? -1)
+  ;; a struct field
+  let p = P{.a 3 .b x}
+  println(p.a ?? 0, p.b ?? 0)
+  p.b = 7
+  println(p.b ?? 0)
+  ;; an array and a Vec element
+  let xs: [3 i32?] = [1, None, x]
+  println(xs[0] ?? 0, xs[1] ?? 0, xs[2] ?? 0)
+  xs[1] = 5
+  println(xs[1] ?? 0)
+  let v: Vec(i32?) = vec-new(i32?)
+  push(v, 6)
+  push(v, None)
+  println(length(v), v[0] ?? 0, v[1] ?? 0)
+  ;; the arms of an if and a match
+  println(pick(true, 2) ?? -1, pick(false, 2) ?? -1, pick2(true, 2) ?? -1, pick2(false, 2) ?? -1)
+  let m = match x
+    4 -> x
+    _ -> None
+  println(m ?? 0)
+  println(arms(5, 3) ?? -1, arms(4, 3) ?? -1, arms(1, 3) ?? -1)
+  ;; nested: a T into a T?? is Some(Some(t)), a T? is Some of it
+  let nn: Option(i32?) = 5
+  let none: i32? = None
+  let nn2: Option(i32?) = none
+  println(two(nn), two(nn2))
+  ;; kept chains over T and T? arms
+  println(two(chain(true, false, none)), two(chain(false, true, none)), two(chain(false, false, none)))
+  println(flat(true, false, none) ?? -1, flat(false, true, Some(6)) ?? -1, flat(false, true, none) ?? -1, flat(false, false, none) ?? -1)
+  let kk =
+    if x > 9
+      none
+    elif x > 1
+      1
+  println(kk ?? -1)
+  ;; generics
+  println(first(9), first(Some(8)), wrap(3) ?? 0, through(5))
+  ;; a literal local takes the payload's type from an Option use, as it
+  ;; takes T from a T use: each pair prints the same
+  let la = 4
+  let wa: i64? = la
+  let lb = 4
+  let wb: i64 = lb
+  println(la * 1000000000, lb * 1000000000, wa ?? 0, wb)
+  let lc = 4
+  println(big(lc), lc * 1000000000)
+  let lf = 7
+  let wf: f64? = lf
+  let lg = 7
+  let wg: f64 = lg
+  println(lf / 2, lg / 2, wf ?? 0.0, wg)
+  let lu = 200
+  let wu: u8? = lu
+  let lv = 200
+  let wv: u8 = lv
+  println(wu ?? 0, wv)
+  ;; and from a typed array's element, with or without an Option
+  let la2 = 3
+  let xa: [1 i64] = [la2]
+  let lb2 = 3
+  let xb: [2 i64?] = [lb2, None]
+  println(la2 * 1000000000, lb2 * 1000000000, xa[0], xb[0] ?? 0)
+  ;; None first in an if, as in a match
+  let e1 = if x > 1 then None else 5
+  let e2 = if x > 1 then 5 else None
+  println(e1 ?? -1, e2 ?? -1)
+  ;; an Option around an array of Options
+  let ao: [2 i32?]? = [1, None]
+  let ai = ao!
+  println(ai[0] ?? 0, ai[1] ?? 0)
+  ;; a narrowed name still takes a payload value
+  let o: i32? = Some(1)
+  if o?
+    o = 10
+    println(o)

test/programs/dev-as.fln

@@ -0,0 +1,26 @@
+;; A program that stops inside an if o as g block (decision 136): the break
+;; loop's locals list g, with the value the test found, on both backends.
+import agent "vendor:agent"
+
+struct Boom
+  why: i32
+
+let ticks: i64 = 0
+
+fn look(o: i32?, d) -> i64
+  if o as g and g > 1 and d as e
+    restart-case
+      error(Boom{.why g})
+      0
+    restart carry-on()
+      5
+  else
+    0
+
+fn main() -> i32
+  agent/start("/tmp/flan-dev-as-fallback.sock")
+  println(look(Some(41), "hi"))
+  for i in range(4000)
+    agent/wait(5)
+    ticks += 1
+  0

test/programs/dyn-crossing.flan

@@ -0,0 +1,94 @@
+;;;; Typed values crossing into dyn beside a dyn operand: a u64, a dyn nil past
+;;;; a fold's pair (a bare nil is refused, see test_flan), and an is-numeric $t.
+;;;; With an argument, the program runs the trap that argument names instead.
+
+(defn add-dyn [x $t] dyn
+  {:where (is-numeric $t)}
+  (+ x (the dyn 1)))
+
+(defn add-late [x $t d dyn] dyn
+  {:where (is-numeric $t)}
+  (+ x 1 d))
+
+(defn dyn-first [x $t d dyn] dyn
+  {:where (is-numeric $t)}
+  (* d x 2))
+
+(defn less-late [x $t d dyn] bool
+  {:where (is-numeric $t)}
+  (< x 10 d))
+
+(defn biggest [x $t d dyn] dyn
+  {:where (is-numeric $t)}
+  (max x d 0))
+
+(defn low-bits [x $t d dyn] dyn
+  {:where (is-integer $t)}
+  (bit-and x 7 d))
+
+(defn boxed [x $t] dyn
+  {:where (is-numeric $t)}
+  (the dyn x))
+
+(defn main [args [str]] i32
+  (let [z (the dyn 0)
+        n (the dyn 4)
+        small (the u64 9223372036854775807)
+        big (the u64 18446744073709551615)
+        nothing (the dyn nil)]
+    ;; A u64 up to the largest i64 crosses as its value.
+    (println (+ z small) (max small 0 z) (the dyn small) (= small (+ z small)))
+    ;; A $t at i32, f64 and u64, beside a dyn in any position.
+    (println (add-dyn 2) (add-dyn 2.5) (add-dyn (the u64 7)))
+    (println (add-late 2 n) (add-late 2.5 n) (dyn-first 3 n) (dyn-first 1.5 n))
+    (println (less-late 1 n) (less-late 1 (the dyn 20)) (less-late 0.5 (the dyn 10.5)))
+    (println (biggest 3 n) (biggest -2.5 (the dyn -1)) (low-bits 13 n) (low-bits (the u8 255) n))
+    (println (boxed 7) (boxed 0.25) (boxed small))
+    ;; A nil past the pair is compared, not refused.
+    (println (= 1 1 nothing) (!= 1 2 nothing) (= 1 1 (the dyn nil)))
+    (when (> (length args) 1)
+      (let [a (at args 1)]
+        (when (= a "u64") (println (+ z big)))
+        (when (= a "u64-max") (println (max big 0 z)))
+        (when (= a "u64-generic") (println (boxed big)))
+        (when (= a "nil-first") (println (+ (the dyn nil) 1 2)))
+        (when (= a "nil-last") (println (+ 1 2 (the dyn nil))))
+        (when (= a "nil-less") (println (< 1 2 (the dyn nil))))
+        (when (= a "nil-min") (println (min 1 2 nothing)))
+        (when (= a "nil-bits") (println (bit-or 1 2 nothing)))
+        ;; At a typed want too: the dyn nil traps in any position.
+        (let [r (the i32 0)]
+          (when (= a "nil-want-first") (set r (+ (the dyn nil) 1 2)))
+          (when (= a "nil-want-last") (set r (+ 1 2 (the dyn nil))))
+          (set r (+ r (at-i32 a))) (the-literal a)
+          (println r)))))
+  0)
+
+;; A dyn beside typed operands at an i32 want: the whole form is dyn, and only
+;; its answer is opened at i32, so every position wraps nowhere and traps alike.
+(defn at-i32 [a str] i32
+  (let [big (the dyn 2147483647)
+        none (the dyn nil)]
+    (cond
+      (= a "big-first") (+ big 1 1)
+      (= a "big-mid") (+ 1 big 1)
+      (= a "big-last") (+ 1 1 big)
+      (= a "big-pair") (+ 1 big)
+      (= a "big-shift") (<< big 1)
+      (= a "i32-nil-first") (+ none 1 1)
+      (= a "i32-nil-mid") (+ 1 none 1)
+      (= a "i32-nil-last") (+ 1 1 none)
+      :else 0)))
+
+;; (the dyn <literal>) is a dyn like any other: beside nil it traps at run
+;; time, as a dyn bound to a name does. And opening one at f64 or bool names
+;; its site.
+(defn the-literal [a str] ()
+  (let [x (the f64 0.0)
+        b false]
+    (when (= a "lit-int") (println (+ (the dyn 1) nil)))
+    (when (= a "lit-float") (println (+ (the dyn 1.5) nil)))
+    (when (= a "lit-bool") (println (+ (the dyn true) nil)))
+    (when (= a "lit-let") (let [d (the dyn 1)] (println (+ d nil))))
+    (when (= a "want-f64") (set x (the dyn 3)) (println x))
+    (when (= a "want-bool") (set b (the dyn 3)) (println b))))

test/programs/if-let-kept.fln

@@ -21,6 +21,28 @@ fn early(a: Option(i32)) -> Option(i32)
   elif true
     3
 
+;; An arm that is already an Option is the whole, flattened (decision 140).
+fn flat(a: Option(i32), o: Option(i32)) -> Option(i32)
+  if let Some(x) = a then o
+
+;; An Option of it wanted: the arm is Some of its value, and no match is the
+;; outer None.
+fn nest(a: Option(i32), o: Option(i32)) -> Option(Option(i32))
+  if let Some(x) = a then o
+
+fn level(oo: Option(Option(i32)))
+  match oo
+    Some(o) -> if o? then println(o) else println("some none")
+    None -> println("none")
+
+fn flat_chain(a: Option(i32), k: i32)
+  let r =
+    if let Some(x) = a
+      x
+    elif k > 0
+      None
+  show(r)
+
 fn dyn_only(a: Option(i32)) -> dyn
   if let Some(x) = a then x
 
@@ -40,6 +62,15 @@ fn main()
   show(lead(1, None))
   show(early(None))
   show(early(Some(1)))
+  show(flat(Some(1), Some(4)))
+  show(flat(Some(1), None))
+  show(flat(None, Some(4)))
+  level(nest(Some(1), Some(4)))
+  level(nest(Some(1), None))
+  level(nest(None, Some(4)))
+  flat_chain(Some(3), 0)
+  flat_chain(None, 1)
+  flat_chain(None, 0)
   println(dyn_only(Some(5)))
   println(dyn_only(None))
   ;; As a statement it is unchanged.

test/programs/when-value.flan

@@ -1,7 +1,8 @@
 ;;;; A when whose value is kept answers an Option: Some of its body when the
 ;;;; test holds, None when it does not. As a statement it answers nothing.
 ;;;; Where a dyn is wanted it answers the body or nil, since dyn has no
-;;;; Option. A body that is already an Option is not flattened.
+;;;; Option. A body that is already an Option is that Option, flattened one
+;;;; level (decision 140), unless an Option of it is what is wanted.
 
 (defn show [o (Option i32)] ()
   (match o (Some v) (println v) None (println "none")))
@@ -9,10 +10,13 @@
 ;; Returned: the return type is the want.
 (defn half [n i32] (Option i32) (when (= 0 (% n 2)) (/ n 2)))
 
-;; Nested, as Rust's bool::then: None from the body stays apart from a
+;; An (Option (Option i32)) is wanted, so the body is Some of it and the
-;; failed test.
+;; failed test is the outer None.
 (defn wrap [c bool o (Option i32)] (Option (Option i32)) (when c o))
 
+;; Flattened: None from the body and a failed test are one answer.
+(defn flat [c bool o (Option i32)] (Option i32) (when c o))
+
 (defn level [oo (Option (Option i32))] ()
   (match oo
     (Some o) (match o (Some v) (println v) None (println "some none"))
@@ -45,6 +49,12 @@
   (level (wrap true (Some 1)))
   (level (wrap true None))
   (level (wrap false (Some 1)))
+  (show (flat true (Some 4)))
+  (show (flat true None))
+  (show (flat false (Some 4)))
+  (let [o (the (Option i32) (Some 8))
+        f (when true o)]
+    (show f))
   (println (dyn-when true))
   (println (dyn-when nil))
   (show (early None))

test/test_acceptance.ml

@@ -2226,8 +2226,8 @@ let () =
       dyn_if_truthy_out;
     (* A kept when is an Option; get is a checked lookup; if let. *)
     let when_value_out =
-      "5\nnone\n42\nnone\n9\n1\nsome none\nnone\n5\nnil\n3\nnone\n6\nnone\n20\nnone\n2\n\
+      "5\nnone\n42\nnone\n9\n1\nsome none\nnone\n4\nnone\nnone\n8\n5\nnil\n3\nnone\n6\nnone\n\
-       a\nnil\ncond stmt\nran\nend\n"
+       20\nnone\n2\na\nnil\ncond stmt\nran\nend\n"
     in
     outputs "when as a value" "programs/when-value.flan" when_value_out;
     outputs ~opt:"-O0" "when as a value, -O0" "programs/when-value.flan" when_value_out;
@@ -2246,7 +2246,8 @@ let () =
       "5\n100\n0\n9\n1\n20\n100\n0\n1\n7\nabsent\nnorth\n6\n-1\n-2\n14\nwhen block\n"
     in
     let if_let_kept_out =
-      "1\n4\nnone\n6\nnone\n9\n2\nnone\n3\nnone\n5\nnil\n7\n"
+      "1\n4\nnone\n6\nnone\n9\n2\nnone\n3\nnone\n4\nnone\nnone\n4\nsome none\nnone\n\
+       3\nnone\nnone\n5\nnil\n7\n"
     in
     (* One fn, several arities, picked by the number of arguments. *)
     let versions_out =
@@ -2282,8 +2283,18 @@ let () =
          outputs ~opt:"-O0" (path ^ ", -O0") ("programs/" ^ path) want;
          outputs ~x86:true (path ^ ", --x86") ("programs/" ^ path) want)
       [ ("optionals.fln", optionals_out); ("optionals-dyn.fln", optionals_dyn_out);
+        (* A T where a T? is wanted is Some of it (decision 138). *)
+        ("autowrap.fln",
+         "-1\n5 4 2\n7 8 -1\n3 4\n7\n1 0 4\n5\n2 6 0\n2 -1 -1 2\n4\n-1 3 0\n\
+          some some some none\nsome some some none none\n1 6 -1 -1\n1\n9 8 3 5\n\
+          4000000000 4000000000 4 4\n4 4000000000\n3.5 3.5 7 7\n200 200\n3000000000 3000000000 3 3\n-1 5\n1 0\n10\n");
         (* x? tests and narrows, e? as g names what it found (decision 133). *)
-        ("presence.fln", "true false true\n6\n-1\n3\n101 209 0\n11\n42\n2\nabsent\n6\nfalse true\n3\n6\n15\n"); ("presence-dyn.fln", "true false\n103 209 0\nno pet\nann\n3 2\n") ];
+        ("presence.fln", "true false true\n6\n-1\n3\n101 209 0\n11\n42\n2\nabsent\n6\nfalse true\n3\n6\n15\n"); ("presence-dyn.fln", "true false\n103 209 0\nno pet\nann\n3 2\n");
+        (* e as g inside an and chain, typed and dyn (decision 136). *)
+        ("as-chain.fln",
+         "empty -1\nelse sees the outer g 100\nafter 100\n-3\nfalse\n3 5\nnot less\n40 -1\n\
+          empty big other half other\n1 2 3 ran 1\n4\n1 3 stopped\n3\n60\n43\n-4 5\n");
+        ("as-chain-dyn.fln", "dog nobody ann\n4 nil nil\n6 2\n5\n-1\n") ];
     (* The pipe (decision 137): chains, multi-line, qualified, dyn, and the
        left side run before the other arguments (the last 1 2 3). *)
     (let want = "7\n6\n6\n60\n10\n8\nfalse\n30\n7\n7 -1\n6\n7\n1\n2\n3\n-4\n" in
@@ -5679,6 +5690,68 @@ level "1"
              (if x86 then ", --x86" else "") text code
          end)
       [ false; true ];
+    (* A u64, a dyn nil past the pair and an is-numeric $t, each beside a
+       dyn: the typed value crosses, and what cannot cross traps at its site. *)
+    let crossing_out =
+      "9223372036854775807 9223372036854775807 9223372036854775807 true\n\
+       3 3.5 8\n7 7.5 24 12\nfalse true true\n4 0 4 4\n\
+       7 0.25 9223372036854775807\nfalse true false\n"
+    in
+    outputs "dyn: typed values crossing beside a dyn" "programs/dyn-crossing.flan"
+      crossing_out;
+    outputs ~opt:"-O0" "dyn: typed values crossing beside a dyn, -O0"
+      "programs/dyn-crossing.flan" crossing_out;
+    outputs ~x86:true "dyn: typed values crossing beside a dyn, --x86"
+      "programs/dyn-crossing.flan" crossing_out;
+    let too_big = "dyn: this u64 is 18446744073709551615, above the largest \
+                   dyn int (9223372036854775807), so it has no dyn value" in
+    let too_wide n =
+      "dyn: an i32 is wanted here, and the int " ^ n ^ " is outside an i32's range" in
+    let crossing_traps =
+      [ "u64", "51:36", too_big;
+        "u64-max", "52:40", too_big;
+        "u64-generic", "31:12", too_big;
+        "nil-first", "54:42", "dyn +: nil and int, and it takes two numbers — (+ nil 1)";
+        "nil-last", "55:41", "dyn +: int and nil, and it takes two numbers — (+ 3 nil)";
+        "nil-less", "56:41", "dyn <: int and nil";
+        "nil-min", "57:40", "dyn min: int and nil";
+        "nil-bits", "58:41", "dyn bit-or: int and nil";
+        "nil-want-first", "61:47", "dyn +: nil and int, and it takes two numbers — (+ nil 1)";
+        "nil-want-last", "62:46", "dyn +: int and nil, and it takes two numbers — (+ 3 nil)";
+        (* At an i32 want the form is dyn in every position and only its
+           answer is opened: no position wraps. *)
+        "big-first", "73:25", too_wide "2147483649";
+        "big-mid", "74:23", too_wide "2147483649";
+        "big-last", "75:24", too_wide "2147483649";
+        "big-pair", "76:24", too_wide "2147483648";
+        "big-shift", "77:25", too_wide "4294967294";
+        "i32-nil-first", "78:29", "dyn +: nil and int, and it takes two numbers — (+ nil 1)";
+        "i32-nil-mid", "79:27", "dyn +: int and nil, and it takes two numbers — (+ 1 nil)";
+        "i32-nil-last", "80:28", "dyn +: int and nil, and it takes two numbers — (+ 2 nil)";
+        (* (the dyn <literal>) beside nil traps as a named dyn does, and an
+           f64 or bool opening names its site. *)
+        "lit-int", "89:36", "dyn +: int and nil, and it takes two numbers — (+ 1 nil)";
+        "lit-float", "90:38", "dyn +: float and nil, and it takes two numbers — (+ 1.5 nil)";
+        "lit-bool", "91:37", "dyn +: bool and nil, and it takes two numbers — (+ true nil)";
+        "lit-let", "92:57", "dyn +: int and nil, and it takes two numbers — (+ 1 nil)";
+        "want-f64", "93:35", "dyn f64: int, and a float was wanted — (f64 3)";
+        "want-bool", "94:36", "dyn bool: int, and a bool was wanted — (bool 3)" ]
+    in
+    List.iter
+      (fun x86 ->
+         let exe = compile ~x86 "programs/dyn-crossing.flan" in
+         List.iter
+           (fun (arg, at, msg) ->
+              let code, text = run exe (Some arg) in
+              let want = "programs/dyn-crossing.flan:" ^ at ^ ": " ^ msg in
+              if code <> 134 || not (contains text want) then begin
+                incr failures;
+                Printf.printf "FAIL  dyn: crossing trap %s%s\n        \
+                               got: %S (exit %d)\n"
+                  arg (if x86 then ", --x86" else "") text code
+              end)
+           crossing_traps)
+      [ false; true ];
     List.iter
       (fun x86 ->
          let exe = compile ~x86 "programs/char-arith.flan" in

test/test_dev.ml

@@ -2483,6 +2483,104 @@ let () =
     null_park "--llvm";
     null_park "--x86";
 
+    (* A stop inside an [if o as g and ... and d as e] block (decision 136):
+       each name an [as] binds is a slot of its own under its name, so the
+       locals list it with what the test found. *)
+    let as_locals backend =
+      let asock = tmp ("as" ^ backend ^ ".sock") and aout = tmp ("as" ^ backend ^ ".out") in
+      (try Sys.remove asock with Sys_error _ -> ());
+      let afd = Unix.openfile aout [ Unix.O_WRONLY; Unix.O_CREAT; Unix.O_TRUNC ] 0o600 in
+      let apid =
+        Unix.create_process flan
+          [| flan; "dev"; "programs/dev-as.fln"; "-s"; asock; backend |]
+          Unix.stdin afd Unix.stderr
+      in
+      Unix.close afd;
+      if not (listening ~pid:apid asock) then begin
+        fail "the as daemon (%s) %s" backend !listen_why;
+        (try Unix.kill apid Sys.sigkill with Unix.Unix_error _ -> ())
+      end
+      else begin
+        let c = connect asock in
+        let ask sexp = Wire.parse (Wire.send c sexp; Wire.recv c) in
+        let stopped r =
+          match Wire.field r "stopped" with
+          | Some { Form.v = Form.Sym "t"; _ } -> true
+          | _ -> false
+        in
+        if not (await (fun () -> stopped (ask "(:op \"describe\")"))) then
+          fail "the as program never stopped (%s)" backend
+        else begin
+          let r = ask "(:op \"locals\" :frame 0)" in
+          let rows =
+            match Wire.field r "locals" with
+            | Some { Form.v = Form.List l; _ } ->
+              List.filter_map
+                (fun (e : Form.t) ->
+                   match e.Form.v with
+                   | Form.List ({ Form.v = Form.Str n; _ } :: { Form.v = Form.Str ty; _ }
+                                :: { Form.v = Form.Str v; _ } :: _) -> Some (n, (ty, v))
+                   | _ -> None)
+                l
+            | _ -> []
+          in
+          (match List.assoc_opt "g" rows with
+           | Some ("i32", "41") -> ()
+           | Some (ty, v) -> fail "locals show g as %s %s (%s)" ty v backend
+           | None ->
+             fail "locals do not show g (%s): %s" backend
+               (String.concat " " (List.map fst rows)));
+          (match List.assoc_opt "e" rows with
+           | Some ("dyn", v) when Test_support.contains v "hi" -> ()
+           | Some (ty, v) -> fail "locals show e as %s %s (%s)" ty v backend
+           | None -> fail "locals do not show e (%s)" backend);
+          (* Eval-in-frame reads them and, as the program may not, cannot
+             assign them. *)
+          let eval code =
+            ask
+              (Printf.sprintf "(:op \"eval-expr\" :frame 0 :code %s :syntax \"indented\")"
+                 (Wire.quote code))
+          in
+          let r = eval "g + 1" in
+          if Wire.string_field r "value" <> Some "42" then
+            fail "eval-in-frame g + 1 (%s): %s" backend
+              (Option.value ~default:(status r) (Wire.string_field r "message"));
+          List.iter
+            (fun (code, name) ->
+               let r = eval code in
+               if status r = "ok" then
+                 fail "eval-in-frame %s was accepted (%s)" code backend
+               else if
+                 not
+                   (Test_support.contains
+                      (Option.value ~default:"" (Wire.string_field r "message"))
+                      (name ^ " names what an as test found"))
+               then
+                 fail "eval-in-frame %s (%s) said: %s" code backend
+                   (Option.value ~default:"" (Wire.string_field r "message")))
+            [ ("g = 7", "g"); ("e = 1", "e") ];
+          let r = eval "g" in
+          if Wire.string_field r "value" <> Some "41" then
+            fail "g changed after a refused assignment (%s): %s" backend
+              (Option.value ~default:(status r) (Wire.string_field r "value"))
+        end;
+        ignore (ask "(:op \"close\")");
+        Unix.close c;
+        if not
+             (await ~ms:5000 (fun () ->
+                match Unix.waitpid [ Unix.WNOHANG ] apid with
+                | 0, _ -> false
+                | _ -> true
+                | exception Unix.Unix_error _ -> true))
+        then begin
+          (try Unix.kill apid Sys.sigkill with Unix.Unix_error _ -> ());
+          (try ignore (Unix.waitpid [] apid) with Unix.Unix_error _ -> ())
+        end
+      end
+    in
+    as_locals "--llvm";
+    as_locals "--x86";
+
     (* ── The locals of a stopped frame ─────────────────────────────── *)
 
     (* A third daemon, over a program that stops with something worth looking

test/test_flan.ml

@@ -1563,6 +1563,15 @@ let () =
   refuses_all ~fln:true "an arity takes no rest parameter"
     "fn f\n  (a: i32) -> i32 = a\n  (a: i32, & xs) -> i32 = a\n"
     "& (a rest parameter) is not one";
+  (* A name an as bound is read-only, and says so as itself, not as a
+     parameter (decision 136). *)
+  refuses_all ~fln:true "assigning an as name"
+    "fn f(o: i32?, d) -> i32\n  if o as g and d as e\n    g = 5\n    g\n  else\n    0\n"
+    "g names what an as test found, and it cannot be given a new value. To \
+     change it, copy it into a local first: let g2 = g";
+  refuses_all ~fln:true "assigning a dyn as name"
+    "fn f(o: i32?, d) -> i32\n  if o as g and d as e\n    e = 5\n    g\n  else\n    0\n"
+    "e names what an as test found";
   rejects_check "popcount of a float"
     "(defn f [a f64] f64 (popcount a))" ~needle:"popcount takes integers, found f64";
   rejects_check "a rotation's count does not widen the value"
@@ -2122,6 +2131,86 @@ let () =
   rejects_check "nil at a bare T is refused at compile time"
     "(defn take [n i32] i32 n)\n(defn main [] i32 (take nil))"
     ~needle:"nil has no None to become";
+  (* A bare nil in an arithmetic, bitwise or ordering operator with nothing
+     else dyn is refused in every position, with or without a want; a dyn that
+     holds nil is left to trap at run time (dyn-crossing.flan). *)
+  (* The 1-based column of [sub] in [form], placed at column [base]. *)
+  let col_of base form sub =
+    let n = String.length sub in
+    let rec at i = if String.sub form i n = sub then i else at (i + 1) in
+    base + at 0
+  in
+  List.iter
+    (fun form ->
+       List.iter
+         (fun (what, base, src) ->
+            let col = col_of base form "nil" in
+            match
+              (try ignore (checked src); None with Loc.Error d -> Some d)
+            with
+            | Some d when contains d.Loc.dmsg "nil has no None to become at i32"
+                          && d.Loc.dloc.Loc.col = col -> ()
+            | Some d ->
+              incr failures;
+              Printf.printf "FAIL  a bare nil in %s, %s: got %d: %s\n" form what
+                d.Loc.dloc.Loc.col d.Loc.dmsg
+            | None ->
+              incr failures;
+              Printf.printf "FAIL  a bare nil in %s, %s: accepted\n" form what)
+         [ "at a want", 16,
+           Printf.sprintf "(defn f [] i32 %s)\n(defn main [] i32 (f))" form;
+           "with none", 25,
+           Printf.sprintf "(defn f [] i32 (println %s) 0)\n(defn main [] i32 (f))" form ])
+    [ "(+ nil 1 2)"; "(+ 1 nil 2)"; "(+ 1 2 nil)"; "(+ 1 nil)";
+      "(< nil 1 2)"; "(< 1 2 nil)"; "(min 1 2 nil)"; "(bit-or nil 1 2)";
+      "(bit-or 1 2 nil)" ];
+  accepts "a dyn holding nil in a fold is left to the run time"
+    "(defn f [p dyn] i32 (+ 1 2 p (the dyn nil)))\n\
+     (defn g [] i32 (let [r (the i32 0)] (set r (+ (the dyn nil) 1 2)) r))\n\
+     (defn main [] i32 (println (< 1 2 (the dyn nil))) 0)";
+  accepts "nil beside (the dyn <literal>) is left to the run time"
+    "(defn main [] i32\n\
+     (println (+ (the dyn 1) nil) (+ (the dyn 1.5) nil) (+ (the dyn true) nil)\n\
+     (< (the dyn 1) nil) (min 2 (the dyn 1.5) nil)) 0)";
+  accepts "nil beside a real dyn in a fold is left to the run time"
+    "(defn f [p dyn] dyn (+ 1 2 p nil))\n(defn main [] i32 0)";
+  (* An operand past the pair asked on its own terms records nothing: the
+     float y is blamed, not the int x it would have merged with. *)
+  List.iter
+    (fun form ->
+       let col = col_of 45 form "y)" in
+       match
+         (try ignore (checked ("(defn main [] i32 (let [x 5 y 2.5] (println "
+                               ^ form ^ ")) 0)")); None
+          with Loc.Error d -> Some d)
+       with
+       | Some d when d.Loc.dloc.Loc.col = col -> ()
+       | Some d ->
+         incr failures;
+         Printf.printf "FAIL  %s blames col %d, not y at %d: %s\n" form
+           d.Loc.dloc.Loc.col col d.Loc.dmsg
+       | None -> incr failures; Printf.printf "FAIL  %s: accepted\n" form)
+    [ "(+ (the i64 1) 2 (+ x y))"; "(* (the i64 1) 2 (* x y))";
+      "(max (the i64 1) 2 (max x y))"; "(< (the i64 1) 2 (+ x y))" ];
+  rejects_check "a wide literal at dyn with nothing to retype"
+    "(defn main [] i32 (println (the dyn 0xFFFFFFFFFFFFFFFF)) 0)"
+    ~needle:"0xFFFFFFFFFFFFFFFF, which is above the largest dyn int \
+             (9223372036854775807), so it has no dyn value";
+  rejects_check "a wide literal beside a dyn"
+    "(defn main [] i32 (println (+ (the dyn 0) 0xFFFFFFFFFFFFFFFF)) 0)"
+    ~needle:"so it has no dyn value";
+  (* A $t crosses into dyn only under a bound every type of which has a dyn
+     value; an unbounded one could be a pointer, refused at no call site. *)
+  rejects_check "an unbounded $t does not cross into dyn"
+    "(defn f [x $t] dyn (+ x (the dyn 1)))\n\
+     (defn main [] i32 (println (f 2)) 0)"
+    ~needle:"$t may be a type with no dyn value, such as a pointer or an \
+             enum, so it crosses into dyn only as a number. Write \
+             {:where (is-numeric $t)}";
+  rejects_check "an is-ordered $t does not cross into dyn"
+    "(defn f [x $t] dyn {:where (is-ordered $t)} (the dyn x))\n\
+     (defn main [] i32 (println (f 2)) 0)"
+    ~needle:"crosses into dyn only as a number";
   rejects_check "nil at a bare T is refused at compile time, return position"
     "(defn f [] i64 nil)\n(defn main [] i32 0)"
     ~needle:"nil has no None to become";
@@ -8088,11 +8177,11 @@ let () =
   parse_rejects "a wide enum member is refused for its range"
     "(defenum E [A 0xFFFFFFFFFFFFFFFF B])"
     ~needle:"the member A of E is 0xFFFFFFFFFFFFFFFF, which does not fit i32";
-  rejects_check "a wide literal in a dyn global names the u64 cast"
+  rejects_check "a wide literal in a dyn global names the type u64"
     "(defonce big 0xFFFFFFFFFFFFFFFF)"
-    ~needle:"Write (u64 0xFFFFFFFFFFFFFFFF) for the u64";
+    ~needle:"so it has no dyn value. Give big the type u64";
-  accepts "the cast the dyn refusal names compiles"
+  accepts "the type the dyn refusal names compiles"
-    "(defonce big (u64 0xFFFFFFFFFFFFFFFF))";
+    "(defonce big u64 0xFFFFFFFFFFFFFFFF)";
   (* A macro's Form has one integer case; the literal comes back wide all the
      same, and is refused where it would have been refused unexpanded. *)
   rejects_check "a wide literal through a macro is still wide"
@@ -8472,6 +8561,26 @@ let () =
   parse_rejects "_ in a defgeneric's return slot"
     ~needle:"defgeneric's methods each have their own"
     "(defgeneric area [s] _)";
+  (* Decision 138: a T is wrapped where a T? is wanted, and never the other
+     way. The program half is programs/autowrap.fln. *)
+  accepts "a T is Some of it at a T?" "(defn f [] (Option i64) -1)\n(defn main [] ())";
+  rejects_check "a T? is not unwrapped at a T" ~needle:"expected i32, found (Option i32)"
+    "(defn f [o (Option i32)] i32 o)\n(defn main [] ())";
+  rejects_check "a T? argument is not unwrapped" ~needle:"expected i32, found (Option i32)"
+    "(defn g [x i32] i32 x)\n(defn f [o (Option i32)] i32 (g o))\n(defn main [] ())";
+  rejects_check "a payload that does not fit is refused at the Option"
+    ~needle:"expected (Option i32), found str"
+    "(defn f [] (Option i32) \"no\")\n(defn main [] ())";
+  rejects_check "a literal that does not fit is refused at the Option"
+    ~needle:"expected (Option str), found the integer literal 5"
+    "(defn f [] (Option str) 5)\n(defn main [] ())";
+  rejects_check "a narrowing is not wrapped" ~needle:"expected (Option i32), found i64"
+    "(defn f [x i64] (Option i32) x)\n(defn main [] ())";
+  rejects_check "no wrap inside a container" ~needle:"expected (Vec (Option i32)), found (Vec i32)"
+    "(defn f [v (Vec i32)] (Vec (Option i32)) v)\n(defn main [] ())";
+  rejects_check "a narrowed name still refuses an Option"
+    ~needle:"it cannot be given an Option here"
+    "(defn main [] () (let [o (the (Option i32) (Some 1))] (when (? o) (set o (Some 2)))))";
   (* A plain name binds what an Option or a dyn holds; over anything else
      it cannot fail, and is refused toward let. The program half is
      programs/if-let.flan and programs/optionals.fln. *)

test/test_session.ml

@@ -1947,7 +1947,7 @@ let () =
      { Tast.name = "f"; params = []; ret = Types.Unit; body = [];
        fdefers = []; fenv = None; fparent = None; floc = Loc.unknown;
        slots = Array.make (Array.length snames) (Types.Int Types.I32);
-       snames }
+       snames; as_slots = [] }
    in
    (match Session.shown_names (fn [| Some "k~2"; None |]) with
     | [| Some "k"; None |] -> ()
@@ -2180,4 +2180,59 @@ let () =
     | exception Loc.Error _ -> ()
     | exception Loc.Errors _ -> fail "one error came as a list"));
 
+  (* A pause mark at any column of a condition leaves what it binds bound:
+     an [as] (decision 136), and an [x?] narrowing through [and] (133).
+     [mark_pause] wraps a test of the chain in [(do (pause) test)]. *)
+  (let t, _ = Session.create ~file:"programs/reload.flan" () in
+   let marks name origin src line =
+     let text = List.nth (String.split_on_char '\n' src) (line - 1) in
+     let hits = ref 0 in
+     for col = 1 to String.length text do
+       let syntax = if Filename.check_suffix origin ".fln" then Source.Indented else Source.Paren in
+       match
+         Source.with_code ~syntax ~at:None (fun () ->
+             Session.eval ~origin ~pause:(line, col) t src)
+       with
+       | _ -> incr hits
+       | exception Loc.Error d when has d.Loc.dmsg "nothing to pause" -> ()
+       | exception Loc.Error d ->
+         fail "%s, a mark at %d:%d: %s" name line col d.Loc.dmsg
+       | exception e -> fail "%s, a mark at %d:%d: %s" name line col (Printexc.to_string e)
+     done;
+     if !hits < 3 then fail "%s: only %d columns took a mark" name !hits
+   in
+   marks "if o? as g" "mark-a.fln" "fn pa(o: i32?) -> i32\n  if o? as g then g else 0\n" 2;
+   marks "if o as g and" "mark-b.fln" "fn pb(o: i32?) -> i32\n  if o as g and g > 1 then g else 0\n" 2;
+   marks "if o? and" "mark-c.fln" "fn pc(o: i32?) -> i32\n  if o? and o > 1 then o else 0\n" 2;
+   let paren = "(defn pd [o (Option i32)] i32 (if (and (as g o) (> g 1)) g 0))" in
+   marks "(and (as g o) ...)" "<eval>" paren 1;
+   (* The chain has a node of its own at the [(and]. *)
+   let col =
+     let rec find i = if String.sub paren i 4 = "(and" then i + 1 else find (i + 1) in
+     find 0
+   in
+   (match Session.eval ~pause:(1, col) t paren with
+    | _ -> ()
+    | exception Loc.Error d -> fail "a mark at (and: %s" d.Loc.dmsg);
+   (* What an as finds is copied once, into g's own named slot, which the
+      rest of the chain and the block read: two bindings, the Option held
+      and g, where another copy into the block's would be three. *)
+   ignore
+     (Source.with_code ~syntax:Source.Indented ~at:None (fun () ->
+          Session.eval ~origin:"copies.fln" t
+            "fn pe(o: i32?) -> i32\n  if o? as g and g > 1 then g + 1 else 0\n"));
+   match
+     List.find_opt (fun (f : Tast.fn) -> f.Tast.name = "pe") t.Session.program.Tast.fns
+   with
+   | None -> fail "pe was not installed"
+   | Some f ->
+     let n = ref 0 in
+     List.iter
+       (Tast.walk (fun (e : Tast.expr) ->
+            match e.Tast.e with Tast.Let (bs, _) -> n := !n + List.length bs | _ -> ()))
+       f.Tast.body;
+     if !n <> 2 then fail "if o? as g binds %d slots, wanted 2" !n;
+     if not (Array.exists (fun x -> x = Some "g") f.Tast.snames) then
+       fail "if o? as g has no slot named g");
+
   Test_support.report ~label:"session" ()

test/test_syntax.ml

@@ -1409,14 +1409,46 @@ let () =
      found; if let over a plain name is refused toward those. *)
   refuses "if let over a plain name" "if let g = x\n  g" "indent/if-let-name"
     "write if x?, and in the block it is what it holds; to name what it holds, \
-     write if x? as g";
+     write if x as g";
   reads "x? is a test" "y = f(x)? and not z.w?" "(set y (and (? (f x)) (not (? (.w z)))))";
   reads "e? as g" "if f(x)? as g\n  g\nelif y? as h\n  h\nelse\n  0"
-    "(if-let [g (f x)] g (if-let [h y] h 0))";
+    "(cond (as g (f x)) g (as h y) h :else 0)";
-  reads "one-line e? as g" "v = if y? as h then h else 0" "(set v (if-let [h y] h 0))";
+  reads "one-line e? as g" "v = if y? as h then h else 0" "(set v (if (as h y) h 0))";
   reads "while e? as g" "while pop(s)? as x\n  f(x)"
-    "(while true (if-let [x (pop s)] (do (f x)) (break)))";
+    "(while true (if (as x (pop s)) (do (f x)) (break)))";
-  refuses "as after no test" "if x as y\n  y" "indent/as-test" "Write x? as name";
+  (* Decision 136: e as g without the ?, and inside an and chain. *)
+  reads "e as g" "if f(x) as g\n  g" "(when (as g (f x)) g)";
+  reads "as in an and chain" "if a and f(x) as g and g > 1 and b\n  g"
+    "(when (and a (as g (f x)) (> g 1) b) g)";
+  reads "two as in one chain" "v = if a as x and b? as y and x < y then x else y"
+    "(set v (if (and (as x a) (as y b) (< x y)) x y))";
+  reads "a kept when with as" "left = when get(grid, r + 1, c - 1) as g and is-empty-cell(g) then g"
+    "(set left (when (and (as g (get grid (+ r 1) (- c 1))) (is-empty-cell g)) g))";
+  reads "while as and" "while pop(s) as x and x > 0\n  f(x)"
+    "(while true (if (and (as x (pop s)) (> x 0)) (do (f x)) (break)))";
+  reads "elif as and" "if a\n  1\nelif f(x) as g and g > 1\n  g"
+    "(cond a 1 (and (as g (f x)) (> g 1)) g)";
+  refuses "as under or" "if a or f(x) as g\n  g" "indent/as-or" "Bind with as in an if of its own";
+  refuses "or after as" "if f(x) as g or b\n  g" "indent/as-or" "nothing to name";
+  refuses "or later in the chain" "if f(x) as g and a or b\n  g" "indent/as-or" "nothing to name";
+  refuses "as under not" "if not f(x) as g\n  g" "indent/as-not" "not turns the test around";
+  refuses "as in parentheses under not" "if not (a as g)\n  g" "indent/as-paren"
+    "not inside parentheses";
+  refuses "as in a bracketed chain" "if (a as g and g > 1) and b\n  g" "indent/as-paren"
+    "if a as g and";
+  refuses "as after until" "until f(x) as g\n  g" "indent/as-until" "Write while";
+  refused "as-not-optional.fln" "fn main()\n  let n = 5\n  if n as g and g > 1\n    println(g)\n"
+    [ "n is i32, which always holds a value, so as has nothing to test";
+      "It is not a conversion" ];
+  refused "as-not-in-else.fln"
+    "fn f(o: i32?) -> i32\n  if o as g and g > 1\n    g\n  else\n    g\n\nfn main()\n  println(f(None))\n"
+    [ "unknown name g" ];
+  refused "as-not-in-elif.fln"
+    "fn f(o: i32?) -> i32\n  if o as g and g > 1\n    g\n  elif g > 0\n    1\n  else\n    0\n\nfn main()\n  println(f(None))\n"
+    [ "unknown name g" ];
+  refused "as-not-after.fln"
+    "fn f(o: i32?) -> i32\n  if o as g and g > 1\n    println(g)\n  g\n\nfn main()\n  println(f(None))\n"
+    [ "unknown name g" ];
   (* Decision 137: x |> f(a) is f(x, a), below or. *)
   reads "|> into a call" "y = x |> f(1, 2)" "(set y (f x 1 2))";
   reads "|> into an empty call" "y = x |> f()" "(set y (f x))";
@@ -1456,11 +1488,10 @@ let () =
   refuses "|> before a test" "y = a |> f?" "indent/pipe-target" "\n    (a |> f)?";
   refuses "|> chained before ==" "y = a |> b |> f(1) == 2" "indent/pipe-target"
     "\n    (a |> b |> f(1)) == 2";
-  refuses "|> before as" "if a |> f(1) as v\n  v" "indent/as-test" "Write (a |> f(1))? as name";
+  reads "as names what a pipe answers" "if a |> f(1) as v and v > 0\n  v"
-  refuses "|> before as over a pattern" "if a |> f as Some(v)\n  v" "indent/as-test"
+    "(when (and (as v (f a 1)) (> v 0)) v)";
-    "Write (a |> f)? as name";
+  refuses "as before a pattern" "if a |> f as Some(v)\n  v" "indent/as-name"
-  refuses "a call before as is not bracketed" "if f(x) as v\n  v" "indent/as-test"
+    "if let Some(v) = a |> f";
-    "Write f(x)? as name";
   reads "|> into a macro passes the place" "y = x |> set(5)" "(set y (set x 5))";
   refuses "|> into parentheses" "y = x |> (f)" "indent/pipe-target" "(f) is neither";
   refuses "|> unspaced on the right" "y = x |>[f]" "indent/unspaced-operator" "x |> f(a)";
@@ -1478,7 +1509,7 @@ let () =
   refused "narrowed-set.fln"
     "fn main()\n  let x: i32? = Some(1)\n  if x?\n    x = None\n  println(x ?? 0)\n"
     [ "x is tested with x? above, so in this block it is i32";
-      "while x? as item" ];
+      "while x as item" ];
   refused "not-narrowed-in-else.fln"
     "fn main()\n  let x: i32? = None\n  if x?\n    println(x + 1)\n  else\n    println(x + 1)\n"
     [ "Option(i32)" ];
@@ -1512,7 +1543,7 @@ let () =
   reads "a trailing ? tests the whole chain" "y = o?.i?"
     "(set y (? (?. [~o1 o] (.i ~o1))))";
   reads "and ? then as binds the chain's result" "if d?.k? as k\n  k"
-    "(if-let [k (?. [~o1 d] (.k ~o1))] k)";
+    "(when (as k (?. [~o1 d] (.k ~o1))) k)";
   refused "narrowed-param.fln"
     "fn f(x: i32?)\n  if x?\n    x += 100\n\nfn main()\n  f(Some(1))\n"
     [ "x is a parameter, and a parameter is not assignable" ];
@@ -1520,7 +1551,7 @@ let () =
     "fn main()\n  let x: i32? = Some(1)\n  if x?\n    x.n = 1\n"
     [ "so here it is what the Option holds, i32, and i32 has no fields" ];
   refuses "a chain is no place, and the fix is a test" "q?.x = 5" "indent/chain-assign"
-    "Test it first: if q?, and in the block q is what it holds, or if q? as g";
+    "Test it first: if q?, and in the block q is what it holds, or if q as g";
   refused "lowercase-type-arg.fln"
     "struct grain\n  w: i32\n\nfn main()\n  let v = vec-new(grain?)\n"
     [ "grain? here is the test that a value is present, and grain is a type";
@@ -1528,7 +1559,7 @@ let () =
   refused "addr-taken.fln"
     "fn clear(p: Ptr(i32?))\n  deref(p) = None\n\nfn main()\n  let x: i32? = Some(1)\n\
      \  let p = addr(x)\n  if x?\n    clear(p)\n    println(x + 1)\n"
-    [ "expected Option(i32)" ];
+    [ "+ takes numbers, found Option(i32)" ];
   refused "capital-local.fln" "fn main()\n  let X: i32? = Some(1)\n  println(X?)\n"
     [ "To test the local X, give it a lowercase name, as in x?" ];
   checks "addr-taken-test.fln"
@@ -1881,9 +1912,9 @@ let () =
   | _ -> fail "addr-taken-note.fln checked"
   | exception (Loc.Error d | Loc.Errors (d :: _)) ->
     if not (List.exists
-              (fun (n : Loc.note) -> Test_support.contains n.Loc.nmsg "Write if x? as g")
+              (fun (n : Loc.note) -> Test_support.contains n.Loc.nmsg "Write if x as g")
               d.Loc.notes)
-    then fail "addr-taken-note.fln: no note naming if x? as g on: %s" d.Loc.dmsg
+    then fail "addr-taken-note.fln: no note naming if x as g on: %s" d.Loc.dmsg
   | exception e -> fail "addr-taken-note.fln: %s" (diag_text e)
 
 let () = Test_support.report ~label:"syntax" ()