fix(typecheck): zero-arg thunk calls, tuple-destructuring let, RuntimeError catch — stdlib AOT 14/19 (#128) (#186)

Three compiler-level fixes surfaced by stdlib/testing.affine:

- ExprApp f []: a zero-arg call f() on an explicit () -> T thunk
  (record field / param / lambda) now consumes the unit and yields T.
  Zero-param fn *declarations* are bare-return-typed, so they are
  unaffected; curried/partial application never reaches this branch
  with an empty arg list, so peeling one unit arrow is sound.
- StmtLet: each destructured name is generalized at *its own*
  component type, not the whole value type. let x = e is unchanged;
  let (a, b) = e now types a and b at their element types.
- RuntimeError(String): registered as the interpreter's builtin
  exception variant (resolve seed + typecheck constructor_env) so
  try/catch arms in the honest stdlib resolve and type-check.

Plus stdlib/testing.affine: let mut found; tot / float(iterations).

Full suite 233/233, zero regression. Refs #128.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

lib/resolve.ml

@@ -88,7 +88,10 @@ let seed_builtins (symbols : Symbol.t) : unit =
   let defc name =
     let _ = Symbol.define symbols name SKConstructor Span.dummy Public in ()
   in
-  defc "Some"; defc "None"; defc "Ok"; defc "Err"
+  defc "Some"; defc "None"; defc "Ok"; defc "Err";
+  (* Interpreter builtin exception variant, pattern-matched in try/catch
+     arms by the honest stdlib (testing.affine, result.affine). *)
+  defc "RuntimeError"
 
 (** Create a new resolution context, pre-seeded with builtins. *)
 let create_context () : context =

lib/typecheck.ml

@@ -709,7 +709,24 @@ let rec synth (ctx : context) (expr : expr) : ty result =
   (* Function application *)
   | ExprApp (fn_expr, args) ->
     let* fn_ty = synth ctx fn_expr in
-    apply_args ctx fn_ty args
+    begin match args with
+      | [] ->
+        (* A zero-argument call [f()]. Zero-parameter function
+           *declarations* are typed as their bare return type (the param
+           fold in [check_fn_decl] is over []), so [apply_args fn_ty []]
+           correctly yields that type. But an explicit [() -> T] thunk
+           type — a record field, a parameter, a lambda — lowers to
+           [TArrow (Unit, _, T, _)]; invoking it must consume the unit and
+           yield [T]. Curried/partial application never reaches this branch
+           with [args = []] (it always carries a non-empty arg list at the
+           [ExprApp] site), so peeling a single unit arrow here is sound. *)
+        begin match repr fn_ty with
+          | TArrow (param_ty, _q, ret_ty, _eff)
+            when repr param_ty = ty_unit -> Ok ret_ty
+          | _ -> apply_args ctx fn_ty args
+        end
+      | _ -> apply_args ctx fn_ty args
+    end
 
   (* Tuple *)
   | ExprTuple exprs ->
@@ -1086,10 +1103,13 @@ and check_stmt (ctx : context) (stmt : stmt) : unit result =
         synth ctx sl_value
     end in
     exit_level ctx;
-    let sc = generalize ctx val_ty in
     let* bindings = check_pattern ctx sl_pat val_ty in
-    List.iter (fun (name, _ty) ->
-      bind_scheme ctx name sc
+    (* Bind each name to *its own* type. For a plain [let x = e] the sole
+       binding's type is [val_ty], so this generalizes exactly as before;
+       for a destructuring [let (a, b) = e] / record pattern each name is
+       generalized at its component type rather than the whole tuple. *)
+    List.iter (fun (name, ty) ->
+      bind_scheme ctx name (generalize ctx ty)
     ) bindings;
     Ok ()
   | StmtExpr e ->
@@ -1274,6 +1294,18 @@ let register_builtins (ctx : context) : unit =
     { sc_tyvars = [(v_erra, Types.KType); (v_errb, Types.KType)];
       sc_effvars = []; sc_rowvars = []; sc_body = err_ty };
   Hashtbl.replace ctx.constructor_env "Err" err_ty;
+  (* [RuntimeError(String)] is the interpreter's builtin exception variant
+     (see [Interp]: panics surface as [VVariant ("RuntimeError", VString
+     msg)]). The honest stdlib pattern-matches it in [try/catch] arms
+     (stdlib/testing.affine, stdlib/result.affine). The catch error type is
+     opaque (a fresh tyvar per [try]), so the result is left polymorphic and
+     unifies with whatever the arm expects. *)
+  let (v_rte, t_rte) = fresh_named () in
+  let rte_ty = TArrow (ty_string, QOmega, t_rte, EPure) in
+  bind_scheme ctx "RuntimeError"
+    { sc_tyvars = [(v_rte, Types.KType)]; sc_effvars = []; sc_rowvars = [];
+      sc_body = rte_ty };
+  Hashtbl.replace ctx.constructor_env "RuntimeError" rte_ty;
   bind_var ctx "string_get"
     (TArrow (ty_string, QOmega, TArrow (ty_int, QOmega, ty_char, EPure), EPure));
   bind_var ctx "string_sub"

stdlib/testing.affine

@@ -133,7 +133,7 @@ fn assert_length<T>(list: [T], expected_len: Int, message: String) -> () {
 
 /// Assert list contains a specific element
 fn assert_contains<T>(list: [T], element: T, message: String) -> () {
-  let found = false;
+  let mut found = false;
   for x in list {
     if x == element {
       found = true;
@@ -303,7 +303,7 @@ fn bench(f: () -> (), iterations: Int) -> BenchResult {
   {
     iterations: iterations,
     total_time: tot,
-    avg_time: tot / (iterations + 0.0)
+    avg_time: tot / float(iterations)
   }
 }