get on a String is refused as at is, a dyn get's trap names get, a kept else-less form beside a number is blamed at itself in arithmetic and comparisons, and a kept if-let chain with no final else gives an Option.

TODO.org

@@ -19,7 +19,8 @@ Waits on the dyn char lane and the literal inference lane.
 ** DONE if let
 CLOSED: [2026-09-26]
 =(if-let [P v] then else)= in paren syntax; an elif chain is the else. With no else it
-is a statement, not an Option. Rules out a plain name or =_= as the pattern (use =let=).
+is a statement unless kept, when it is an Option as =when= is. Rules out a plain name or
+=_= as the pattern (use =let=).
 ** 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

lib/check.ml

@@ -8709,8 +8709,8 @@ and kept_open ~used want (e : Ast.expr) =
   in
   let rec open_ (e : Ast.expr) =
     match e.Ast.e with
-    | Ast.Do [] | Ast.If (_, _, None) -> true
+    | Ast.Do [] | Ast.If (_, _, None) | Ast.IfLet (_, _, None) -> true
-    | Ast.If (_, _, Some e') -> open_ e'
+    | Ast.If (_, _, Some e') | Ast.IfLet (_, _, Some e') -> open_ e'
     | _ -> false
   in
   kept && open_ e
@@ -9763,12 +9763,18 @@ and check_array_gen ctx ~want loc dims f =
     (array_build ctx loc ns elem ~pre:[ (fs, f) ]
        ~element:(fun idxs -> mk loc elem (Tast.CallPtr (fv, idxs))))
 
-and check_match ctx ?(tail = false) ?(used = false) ?(stmt = false) ?want loc
+and check_match ctx ?(tail = false) ?(used = false) ?(stmt = false) ?(opt = false)
-    scrutinee arms =
+    ?want loc scrutinee arms =
   (* [stmt] is an [if let] with no else: a statement, Unit whatever its arm
-     answers, as a one-armed [if] is when nothing keeps it. *)
+     answers, as a one-armed [if] is when nothing keeps it. [opt] is one that
+     is kept: its arm answers [Some], and the arm with no body [None]. *)
   let used = used && not stmt in
-  let want = if stmt then None else want in
+  let arms_ast_for_opt = arms in
+  let want =
+    if stmt then None
+    else if opt then (match want with Some (Types.Option i) -> Some i | _ -> None)
+    else want
+  in
   let s = check ctx scrutinee in
   (* What the arms are alternatives over. An [Option] is a two-case data type
      wearing a special coat, so the two shapes below are the same shape: a set
@@ -10173,7 +10179,9 @@ and check_match ctx ?(tail = false) ?(used = false) ?(stmt = false) ?want loc
            ctx.tail <- tail;
            ctx.used <- used;
            let arm = (a, ctor, binds) in
+           let empty = opt && a.Ast.body = [] in
            let body =
+             if empty then unit_at a.Ast.aloc else
              if free && !want <> None && not (literal_arm arm) then
                (* As an [if]'s else arm: at the join so far first, on its
                   own terms only when that is refused as a mismatch. *)
@@ -10223,7 +10231,7 @@ and check_match ctx ?(tail = false) ?(used = false) ?(stmt = false) ?want loc
              then mk body.Tast.loc Types.Unit (Tast.Do [ body; unit_at body.Tast.loc ])
              else body
            in
-           (if body.Tast.ty <> Types.Never && not stmt then
+           (if body.Tast.ty <> Types.Never && not stmt && not empty then
               match !want with
               | None -> want := Some body.Tast.ty
               | Some w when free ->
@@ -10255,8 +10263,11 @@ and check_match ctx ?(tail = false) ?(used = false) ?(stmt = false) ?want loc
          is said, as each would be checked at it. *)
       List.map
         (fun (i, (arm : Tast.arm)) ->
+           let empty =
+             opt && (let (a : Ast.arm), _, _ = List.nth resolved i in a.Ast.body = [])
+           in
            match arm.Tast.abody with
-           | [ b ] when not (Types.equal b.Tast.ty j || b.Tast.ty = Types.Never) ->
+           | [ b ] when not (empty || Types.equal b.Tast.ty j || b.Tast.ty = Types.Never) ->
              let at = value_loc i in
              let b =
                try expect ctx at ~want:(Some j) b
@@ -10270,6 +10281,20 @@ and check_match ctx ?(tail = false) ?(used = false) ?(stmt = false) ?want loc
   let arms =
     List.map snd (List.sort (fun (i, _) (j, _) -> compare i j) checked)
   in
+  let opt_ty = Option.map (fun t -> Types.Option t) !want in
+  let arms =
+    match opt, opt_ty with
+    | true, Some oty ->
+      List.map2
+        (fun (a : Ast.arm) (arm : Tast.arm) ->
+           match a.Ast.body, arm.Tast.abody with
+           | [], _ -> { arm with Tast.abody = [ mk a.Ast.aloc oty Tast.None_ ] }
+           | _, [ b ] when b.Tast.ty <> Types.Never ->
+             { arm with Tast.abody = [ mk b.Tast.loc oty (Tast.Some_ b) ] }
+           | _ -> arm)
+        arms_ast_for_opt arms
+    | _ -> arms
+  in
   (* Exhaustiveness is refused, not defaulted. A match that silently fell
      through would have to produce a value of the match's type out of nothing,
      and there is no such value for most types; and the case a data type grows
@@ -10319,6 +10344,7 @@ and check_match ctx ?(tail = false) ?(used = false) ?(stmt = false) ?want loc
       (if List.length missing = 1 then "it" else "them");
   let ty =
     if stmt then Types.Unit
+    else if opt then (match opt_ty with Some t -> t | None -> Types.Never)
     else match !want with Some t -> t | None -> Types.Never
   in
   match subject with
@@ -10392,7 +10418,39 @@ and check_if_let ctx ~tail ~used ?want loc scrutinee (arm : Ast.arm) els =
    | Ast.Pctor (n, []) when not (is_case n) -> irrefutable (Some n)
    | _ -> ());
   let wild body = { Ast.pat = Ast.Pwild; body; aloc = loc } in
+  (* Kept with no else at the end of its chain, it is a [when] over a
+     pattern: [Some] of the arm that ran and [None] when none did — or, where
+     a dyn is wanted, the value or nil. *)
+  let open_end =
+    match els with None -> true | Some e -> kept_open ~used:true None e
+  in
+  let kept =
+    match want with
+    | Some (Types.Unit | Types.Never) -> false
+    | Some _ -> true
+    | None -> used
+  in
+  let at (x : Ast.expr) e = { Ast.e; loc = x.Ast.loc } in
   match els with
+  | _ when kept && open_end ->
+    let rest = match els with Some e -> e | None -> at scrutinee (Ast.Var "nil") in
+    (match want with
+     | Some Types.Dyn ->
+       check_match ctx ~tail ~used:true ?want loc scrutinee [ arm; wild [ rest ] ]
+     | _ ->
+       match els with
+       | None ->
+         expect ctx loc ~want
+           (check_match ctx ~tail ~used:true ~opt:true ?want loc scrutinee
+              [ arm; wild [] ])
+       | Some rest ->
+         let some =
+           List.map
+             (fun (b : Ast.expr) -> at b (Ast.Call (at b (Ast.Var "Some"), [ b ])))
+             arm.Ast.body
+         in
+         check_match ctx ~tail ~used:true ?want loc scrutinee
+           [ { arm with Ast.body = some }; wild [ rest ] ])
   | Some e ->
     check_match ctx ~tail ~used ?want loc scrutinee [ arm; wild [ e ] ]
   | None ->
@@ -11168,27 +11226,56 @@ and fold_left_prim ctx ~want loc name p ~needs ok what args =
   expect ctx loc ~want acc
   end
 
-(* An operand is kept, so a [when] written as one answers an Option, which
+(* An operand is kept, so a form with no else at its end — a [when], a
-   no arithmetic takes. Said at the [when], before the operands are checked
+   [cond] or an [if]/[if let] chain with no final else, or a [do] or [let]
-   against each other — where the literal beside it would be blamed instead.
+   ending in one — answers an Option there. Beside a number that is refused
-   A [when] over a dyn answers a dyn, and that is left to the fold. *)
+   at the form itself, before the operands are checked against each other,
+   where the number beside it would be blamed instead. One over a dyn answers
+   a dyn, and that is left to the operator. *)
 and refuse_kept_when ctx name (args : Ast.expr list) =
+  (* The form with no else, found at the end of [a]. *)
+  let rec else_less (a : Ast.expr) =
+    match a.Ast.e with
+    | Ast.If (_, _, None) | Ast.IfLet (_, _, None) -> Some a
+    | Ast.If (_, _, Some e) | Ast.IfLet (_, _, Some e) ->
+      if open_tail e then Some a else None
+    | Ast.Do (_ :: _ as xs) | Ast.Let (_, (_ :: _ as xs)) ->
+      else_less (List.nth xs (List.length xs - 1))
+    | _ -> None
+  and open_tail (e : Ast.expr) =
+    match e.Ast.e with
+    | Ast.Do [] -> true
+    | _ -> else_less e <> None
+  in
+  let ty (a : Ast.expr) = probe ctx a.Ast.loc (fun () -> (check ctx a).Tast.ty) in
+  let number (a : Ast.expr) =
+    match a.Ast.e with
+    | Ast.Int _ | Ast.UInt _ | Ast.Float _ | Ast.Byte _ -> true
+    | _ when else_less a <> None -> false
+    | _ -> (match ty a with Some t -> Types.is_numeric t | None -> false)
+  in
   List.iter
     (fun (a : Ast.expr) ->
-       match a.Ast.e with
+       match else_less a with
-       | Ast.If (_, _, None) ->
+       | Some form ->
-         (match probe ctx a.Ast.loc (fun () -> (check ctx a).Tast.ty) with
+         (match ty a with
-          | Some (Types.Option _ as t) ->
+          | Some (Types.Option _ as t)
-            let fln = fln_source a.Ast.loc in
+            when List.exists (fun b -> b != a && number b) args ->
-            Loc.failk "check/kept-when" a.Ast.loc
+            let fln = fln_source form.Ast.loc in
+            let what =
+              match form.Ast.e with
+              | Ast.If (_, _, None) -> if fln then "if without an else" else "when"
+              | Ast.IfLet _ -> "if let without an else"
+              | _ -> if fln then "if chain without an else" else "chain without an else"
+            in
+            Loc.failk "check/kept-when" form.Ast.loc
               "this %s is an operand of %s, so its value is kept, and there it \
-               gives %s: Some of its value when the test holds, None when it \
+               gives %s: Some of its value when a test holds, None when none \
-               does not. %s takes numbers. Give it an else, or unwrap what it \
+               does. The other side is a number. Give it an else, or unwrap \
-               gives with match"
+               what it gives with match"
-              (if fln then "if without an else" else "when") name
+              what name (tyname form.Ast.loc t)
-              (tyname a.Ast.loc t) name
           | _ -> ())
-       | _ -> ())
+       | None -> ())
     args
 
 (* The dyn lowering of a fold: one call per operator application, left to
@@ -12604,6 +12691,7 @@ and named_call ?(qualified = false) ctx ~want loc name args =
     let x, y, rest =
       match args with x :: y :: rest -> x, y, rest | _ -> assert false
     in
+    refuse_kept_when ctx name args;
     if List.exists (fun a -> peeks_string ctx a) args then
       string_compare ctx ~want loc name p args
     else
@@ -13732,6 +13820,8 @@ and named_call ?(qualified = false) ctx ~want loc name args =
         | Types.Map _ ->
           fail loc "a map's get takes one key, as (get m k), and this has %d"
             (List.length idx)
+        | Types.Named "String" ->
+          (ignore (refuse_string_index (List.hd idx).Ast.loc ~store:false); assert false)
         | Types.Dyn -> dyn_get ctx ~want loc target idx
         | _ -> checked_get ctx ~want loc target idx)
      | [ target; k ] ->
@@ -13739,6 +13829,8 @@ and named_call ?(qualified = false) ctx ~want loc name args =
        (match target.Tast.ty with
         | Types.Array _ | Types.Slice _ | Types.String | Types.Vec _ ->
           checked_get ctx ~want loc target [ k ]
+        (* The same refusal [at] gives: a String is not indexed. *)
+        | Types.Named "String" -> (ignore (refuse_string_index k.Ast.loc ~store:false); assert false)
         | Types.Dyn -> dyn_get ctx ~want loc target [ k ]
         | _ ->
        (* A dyn map's absence is nil, not None: the typed map can promise an

runtime/flan_dyn.c

@@ -4896,22 +4896,22 @@ static int64_t need_index(const uint8_t *loc, int64_t loclen, const char *op,
  * count the same way. [soft] is [get]'s — an index out of range is nil
  * rather than a trap. */
 static flan_dyn at_core(flan_dyn v, flan_dyn i, const uint8_t *loc,
-                        int64_t loclen, int soft) {
+                        int64_t loclen, int soft, const char *op) {
   int64_t k;
   flan_obj *o;
   /* m[:k] on a map is (get m :k), whatever the key: get's rule, nil when
    * absent. */
   if (is_map(v)) return flan_dyn_get(v, i, loc, loclen);
   if (!is_text(v) && !is_vec(v))
-    trap2(loc, loclen, TYPE_TRAP, "at", "only a text, a vec or a map is indexed",
+    trap2(loc, loclen, TYPE_TRAP, op, "only a text, a vec or a map is indexed",
           v, i);
-  k = need_index(loc, loclen, "at", v, i);
+  k = need_index(loc, loclen, op, v, i);
   o = dyn_obj(v);
   if (o->kind == OBJ_VIEW) {
-    int64_t len = view_len(loc, loclen, "at", o);
+    int64_t len = view_len(loc, loclen, op, o);
     if (k < 0 || k >= len) {
       if (soft) return flan_dyn_nil();
-      trap_range(loc, loclen, "at", v, k, len);
+      trap_range(loc, loclen, op, v, k, len);
     }
     {
       uint8_t *p = view_elem_at(o, k);
@@ -4919,7 +4919,7 @@ static flan_dyn at_core(flan_dyn v, flan_dyn i, const uint8_t *loc,
       case VIEW_FAST_I64: { int64_t x; memcpy(&x, p, 8); return flan_dyn_from_i64(x); }
       case VIEW_FAST_F64: { double x; memcpy(&x, p, 8); return flan_dyn_from_f64(x); }
       case VIEW_FAST_BOOL: return flan_dyn_from_bool(*p ? 1 : 0);
-      default: return view_read(loc, loclen, "at", o, o->u.view.desc, p);
+      default: return view_read(loc, loclen, op, o, o->u.view.desc, p);
       }
     }
   }
@@ -4928,7 +4928,7 @@ static flan_dyn at_core(flan_dyn v, flan_dyn i, const uint8_t *loc,
     int w;
     if (k < 0 || k >= o->u.i) {
       if (soft) return flan_dyn_nil();
-      trap_range(loc, loclen, "at", v, k, o->u.i);
+      trap_range(loc, loclen, op, v, k, o->u.i);
     }
     off = text_offset(o, k);
     return dyn_make(BOX_CHAR,
@@ -4936,14 +4936,14 @@ static flan_dyn at_core(flan_dyn v, flan_dyn i, const uint8_t *loc,
   }
   if (k < 0 || k >= o->len) {
     if (soft) return flan_dyn_nil();
-    trap_range(loc, loclen, "at", v, k, o->len);
+    trap_range(loc, loclen, op, v, k, o->len);
   }
   return o->u.v.items[k];
 }
 
 flan_dyn flan_dyn_at(flan_dyn v, flan_dyn i, const uint8_t *loc,
                      int64_t loclen) {
-  return at_core(v, i, loc, loclen, 0);
+  return at_core(v, i, loc, loclen, 0, "at");
 }
 
 /* (slice s lo) and (slice s lo hi) over a text; nil for [hi] is the length.
@@ -5192,7 +5192,7 @@ flan_dyn flan_dyn_get_at(flan_dyn v, flan_dyn i, const uint8_t *loc,
   /* Through [at]'s own body, so however [at] counts a text — by code point —
    * [get] counts the same. */
   if (!is_text(v) && !is_vec(v)) return flan_dyn_get(v, i, loc, loclen);
-  return at_core(v, i, loc, loclen, 1);
+  return at_core(v, i, loc, loclen, 1, "get");
 }
 
 static flan_dyn contains_walk(flan_dyn m, flan_dyn k) {

spec-syntax.md

@@ -267,6 +267,7 @@ Each item: the proposal, then the reason in one line.
 - **`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 pattern that cannot fail, a
   plain name or `_`, is refused toward `let`. **Built.**

test/programs/get-trap.flan

@@ -0,0 +1,5 @@
+;;;; A dyn get with an index that is not an int traps, in get's own words:
+;;;; the message names get and shows the get call, not at.
+
+(defn main [] ()
+  (println (get (the dyn "héllo") 1.5)))

test/programs/if-let-kept.fln

@@ -0,0 +1,38 @@
+;; A kept if let with no else at the end of its chain gives an Option: Some of
+;; the arm that ran, None when none did.
+
+fn pick(a: Option(i32), k: i32) -> Option(i32)
+  if let Some(x) = a then x
+  elif k > 0 then k
+
+fn only(a: Option(i32)) -> Option(i32)
+  if let Some(x) = a then x * 2
+
+fn lead(k: i32, b: Option(i32)) -> Option(i32)
+  if k > 5
+    k
+  elif let Some(y) = b
+    y
+
+fn dyn_only(a: Option(i32)) -> dyn
+  if let Some(x) = a then x
+
+fn show(o: Option(i32))
+  match o
+    Some(v) -> println(v)
+    None -> println("none")
+
+fn main()
+  show(pick(Some(1), 0))
+  show(pick(None, 4))
+  show(pick(None, 0))
+  show(only(Some(3)))
+  show(only(None))
+  show(lead(9, None))
+  show(lead(1, Some(2)))
+  show(lead(1, None))
+  println(dyn_only(Some(5)))
+  println(dyn_only(None))
+  ;; As a statement it is unchanged.
+  if let Some(x) = Some(7)
+    println(x)

test/test_acceptance.ml

@@ -2245,7 +2245,21 @@ let () =
     let if_let_out =
       "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\n5\nnil\n7\n" in
+    outputs "a kept if let chain" "programs/if-let-kept.fln" if_let_kept_out;
+    outputs ~opt:"-O0" "a kept if let chain, -O0" "programs/if-let-kept.fln" if_let_kept_out;
+    outputs ~x86:true "a kept if let chain, --x86" "programs/if-let-kept.fln" if_let_kept_out;
     outputs "if let" "programs/if-let.fln" if_let_out;
+    (let exe = compile "programs/get-trap.flan" in
+     let code, text = run exe None in
+     if code <> 134 || not (contains text "dyn get:")
+        || not (contains text "(get \"h\195\169llo\" 1.5)")
+     then begin
+       incr failures;
+       Printf.printf "FAIL  a dyn get's trap names get\n        got: %S (exit %d)\n"
+         text code
+     end;
+     (try Sys.remove exe with Sys_error _ -> ()));
     outputs ~opt:"-O0" "if let, -O0" "programs/if-let.fln" if_let_out;
     outputs ~x86:true "if let, --x86" "programs/if-let.fln" if_let_out;
     (* format-f64, the first number formatter a caller can steer. The three

test/test_flan.ml

@@ -8344,6 +8344,24 @@ let () =
   rejects_check "on either side"
     ~needle:"this when is an operand of +, so its value is kept"
     "(defn main [] () (println (+ (when true 5) 1)))";
+  (* Any form with no else at its end, beside a number, is blamed itself. *)
+  List.iter
+    (fun (what, e, needle) ->
+       rejects_check ("a kept " ^ what ^ " beside a number is blamed itself")
+         ~needle
+         ("(defn main [] () (let [c true] (println " ^ e ^ ")))"))
+    [ ("when in <", "(< 1 (when c 5))", "this when is an operand of <, so its value is kept");
+      ("when in =", "(= 1 (when c 5))", "this when is an operand of =, so its value is kept");
+      ("cond", "(+ 1 (cond c 5))", "this chain without an else is an operand of +");
+      ("if ending in a when", "(+ 1 (if c 5 (when c 6)))",
+       "this chain without an else is an operand of +");
+      ("do ending in a when", "(+ 1 (do (when c 5)))", "this when is an operand of +");
+      ("if-let", "(+ 1 (if-let [(Some x) (Some c)] 5))",
+       "this if let without an else is an operand of +") ];
+  (* get on a String is refused as at is. *)
+  rejects_check "get on a String is refused as at is"
+    ~needle:"a String is not indexed"
+    "(defn main [] () (let [s (string-new \"abc\")] (println (get s 1))))";
   accepts "a lambda's when takes the Option its position wants"
     "(defn call-it [f (Fn [] (Option i32))] () (println (f)))\n\
      (defn main [] () (call-it (fn [] (when true 6))))";

test/test_syntax.ml

@@ -1161,6 +1161,9 @@ let () =
   reads "elif let reads as a nested if-let"
     "if let Some(x) = a\n  f(x)\nelif let None = b\n  g()\nelse\n  h()"
     "(if-let [(Some x) a] (f x) (if-let [None b] (g) (h)))";
+  round "a kept if let chain with no else"
+    "(defn f [a (Option i32) k i32] (Option i32) (if-let [(Some x) a] (do (g) x) (cond (> k 0) k)))"
+    "  if let Some(x) = a\n    g()\n    x\n  elif k > 0\n    k";
   round "a kept when" "(defn f [] () (let [w (when (> a 1) 2)] (g w)))"
     "  let w = when a > 1 then 2";
   reads "when with a block" "when a\n  b()\n  c()" "(when a (b) (c))";