VM upvalues + HO primitives + 40 tests (36 pass, 4 fail)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/lib/sx_primitives.ml

@@ -7,6 +7,12 @@ open Sx_types
 
 let primitives : (string, value list -> value) Hashtbl.t = Hashtbl.create 128
 
+(** Forward refs for calling SX functions from primitives (breaks cycle). *)
+let _sx_call_fn : (value -> value list -> value) ref =
+  ref (fun _ _ -> raise (Eval_error "sx_call not initialized"))
+let _sx_trampoline_fn : (value -> value) ref =
+  ref (fun v -> v)
+
 let register name fn = Hashtbl.replace primitives name fn
 
 let is_primitive name = Hashtbl.mem primitives name
@@ -590,4 +596,34 @@ let () =
       List.iter (fun (k, v) -> dict_set d k v) pairs;
       Dict d
     | _ -> raise (Eval_error "spread-attrs: 1 spread"));
+
+  (* Higher-order forms as callable primitives — used by the VM.
+     The CEK machine handles these as special forms with dedicated frames;
+     the VM needs them as plain callable values. *)
+  (* Call any SX callable — handles NativeFn, Lambda (via trampoline), VM closures *)
+  let call_any f args =
+    match f with
+    | NativeFn (_, fn) -> fn args
+    | _ -> !_sx_trampoline_fn (!_sx_call_fn f args)
+  in
+  register "map" (fun args ->
+    match args with
+    | [f; (List items | ListRef { contents = items })] ->
+      List (List.map (fun x -> call_any f [x]) items)
+    | _ -> raise (Eval_error "map: expected (fn list)"));
+  register "filter" (fun args ->
+    match args with
+    | [f; (List items | ListRef { contents = items })] ->
+      List (List.filter (fun x -> sx_truthy (call_any f [x])) items)
+    | _ -> raise (Eval_error "filter: expected (fn list)"));
+  register "for-each" (fun args ->
+    match args with
+    | [f; (List items | ListRef { contents = items })] ->
+      List.iter (fun x -> ignore (call_any f [x])) items; Nil
+    | _ -> raise (Eval_error "for-each: expected (fn list)"));
+  register "reduce" (fun args ->
+    match args with
+    | [f; init; (List items | ListRef { contents = items })] ->
+      List.fold_left (fun acc x -> call_any f [acc; x]) init items
+    | _ -> raise (Eval_error "reduce: expected (fn init list)"));
   ()