Iter 15g-aux — symmetric $u early-return in unify_for_subst

Fix the codegen asymmetry that 15g surfaced. unify_for_subst had an
arg-side-only early-return for $u-prefixed synth wildcards. The
function's prev-binding recursion can swap a $u from arg into param
position when re-unifying a previously-bound type against a fresh
arg. Reduced repro: `length [Left 1, Right 10]` — `a` first binds
to `Either<Int, $u>`, then the recursive unification against
`Either<$u, Int>` lands $u in param-pos[1] and falls through to
the catch-all error.

Fix is three lines: add a symmetric $u early-return for param side.
$u is a synth-only wildcard regardless of which side it ends up on
after the prev-binding swap. Doc comment expanded to record the
origin and justification.

std_either_list_demo refactored: mkleft/mkright workaround helpers
removed; the list is now constructed inline by mixing
(term-ctor std_either.Either Left 1) and (... Right 10) directly.
Same expected output (2, 3, 2, 3); the demo doubles as the 15g-aux
regression fixture.

Tests: 94/94, unchanged. The fix expanded what compiles without
changing observable behaviour for any prior fixture.

crates/ailang-codegen/src/lib.rs

@@ -2687,18 +2687,36 @@ fn unify_for_subst(
     vars: &BTreeSet<&str>,
     subst: &mut BTreeMap<String, Type>,
 ) -> Result<()> {
-    // Iter 14a fix: an arg-side `$u`-prefixed var is a synth-only
+    // Iter 14a fix, extended in 15g-aux: a `$u`-prefixed var is a
-    // wildcard produced by `synth_arg_type` for nullary ctors of a
+    // synth-only wildcard produced by `synth_arg_type` for nullary
-    // parameterised ADT (e.g. `Nil : List<$u>`). It carries no real
+    // ctors of a parameterised ADT (e.g. `Nil : List<$u>`). It
-    // constraint — accept without binding so a sibling arg can pin
+    // carries no real constraint — accept without binding so a
-    // the type var instead. Without this, `Cons(Int, Nil)` synth
+    // sibling arg can pin the type var instead. Without this,
-    // would unify `a = Int` (from head) and then `a = $u` (from
+    // `Cons(Int, Nil)` synth would unify `a = Int` (from head) and
-    // tail's recursive `List<a>` slot) and falsely error.
+    // then `a = $u` (from tail's recursive `List<a>` slot) and
+    // falsely error.
+    //
+    // 15g-aux: the early-return must accept `$u` on **either** side.
+    // `$u` enters in arg position from synth, but the prev-binding
+    // recursion below (`unify_for_subst(&prev, arg, ...)`) can swap
+    // a `$u` onto the param side when a previously-bound type is
+    // unified against a fresher arg whose roles differ. Reduced
+    // repro: `length [Left 1, Right 10]` — `a` first binds to
+    // `Either<Int, $u>` from `Left 1`, then a recursive unification
+    // against `Either<$u, Int>` from `Right 10` lands `$u` in the
+    // param-pos[1] slot. Symmetric early-return is correct because
+    // `$u` is a synth-only wildcard regardless of which side carries
+    // it after the prev-binding swap.
     if let Type::Var { name } = arg {
         if name.starts_with("$u") {
             return Ok(());
         }
     }
+    if let Type::Var { name } = param {
+        if name.starts_with("$u") {
+            return Ok(());
+        }
+    }
     match (param, arg) {
         (Type::Var { name }, _) if vars.contains(name.as_str()) => {
             if let Some(prev) = subst.get(name).cloned() {

docs/JOURNAL.md

@@ -2991,3 +2991,54 @@ under `ailang-codegen/src/lib.rs`.
 acceptance — small, one-line in `unify_for_subst`); 16b (local
 recursive `let`); 16c (Lit-in-Ctor patterns); 17a (per-fn arena,
 gated on user discussion).
+
+---
+
+## Iter 15g-aux — symmetric `$u` early-return in `unify_for_subst`
+
+**Goal.** Fix the codegen asymmetry that 15g surfaced: inline list
+literals mixing `Left n` and `Right n` ctor calls of the same
+`Either<e, a>` errored at synth time even though both elements have
+fully-determined concrete-after-pinning types.
+
+**Diagnosis.** `unify_for_subst` (`crates/ailang-codegen/src/lib.rs`)
+had an arg-side-only early-return for `$u`-prefixed synth wildcards.
+The function has a prev-binding recursion path that re-invokes
+itself with the previously-bound type as the new param and the
+fresh arg, which can swap a `$u` from arg position into param
+position. Concretely, `length [Left 1, Right 10]` synths the list
+elements as `Either<Int, $u>` and `Either<$u, Int>`. The first
+element pins `a = Either<Int, $u>` for `Cons`'s parameter `a`. The
+second element triggers a recursive unification of the previously-
+bound `Either<Int, $u>` against `Either<$u, Int>`, walking
+pairwise: `Int` vs `$u` (arg-side `$u`, ok) and `$u` vs `Int`
+(param-side `$u`, **falls through** to the catch-all error).
+
+**Fix.** Three lines in `unify_for_subst`: add a symmetric early-
+return for param-side `$u`. Doc comment expanded to record the
+asymmetry's origin and the symmetric extension's justification
+(`$u` is a synth-only wildcard regardless of which side it ends
+up on after the prev-binding swap).
+
+**Demo refactor.** `examples/std_either_list_demo.ailx` no longer
+uses the `mkleft`/`mkright` monomorphic helpers introduced as the
+15g workaround. The list is now constructed inline by mixing
+`(term-ctor std_either.Either Left 1)` and `(term-ctor
+std_either.Either Right 10)` directly. Same expected output
+(2, 3, 2, 3); the demo doubles as the 15g-aux regression fixture.
+
+**Tests: 94/94, unchanged.** The fix expanded what compiles, did
+not change observable behaviour for any prior fixture.
+
+**Cumulative state, post-15g-aux.**
+
+- Stdlib unchanged (5 modules, 27 combinators).
+- One latent codegen bug retired. The bug count in the dogfood
+  audit since 14a stays at 4 surfaced + fixed (this is a fresh
+  surface from 15g, so 5 surfaced / 5 fixed).
+- The std_either_list_demo workaround is gone, leaving the inline
+  cross-module mixed-ctor list as the canonical idiom.
+
+**Queue update.** 15g-aux done. Unchanged: 16b (local recursive
+let), 16c (Lit-in-Ctor patterns), 17a (per-fn arena, gated on
+user discussion of memory management).

examples/std_either_list_demo.ailx

@@ -1,20 +1,16 @@
-; Iter 15g — fifth consumer demo. First demo to import four stdlib
+; Iter 15g (refactored in 15g-aux) — fifth consumer demo. Imports four
-; modules (std_list, std_either, std_pair, std_either_list); first to
+; stdlib modules (std_list, std_either, std_pair, std_either_list);
-; thread a List<Either<Int, Int>> through three combinators that
+; threads a List<Either<Int, Int>> through three combinators that
 ; together exercise every monomorphisation site introduced by 15g.
 ;
-; The list is constructed via the monomorphic helpers `mkleft` and
+; The list is constructed inline by mixing `(term-ctor std_either.Either
-; `mkright` rather than bare `(term-ctor std_either.Either Left ...)` /
+; Left ...)` and `(... Right ...)`. Iter 15g surfaced a `unify_for_subst`
-; `(term-ctor std_either.Either Right ...)`. This sidesteps a codegen
+; asymmetry that would have rejected this construction at synth time;
-; bug that 15g surfaced: when an inline list literal mixes
+; 15g-aux fixed it by accepting `$u` synth-wildcards on either side of
-; `Either Left n` (pins e, leaves a as `$u`) and `Either Right n` (pins
+; the param/arg unification (see ailang-codegen/src/lib.rs's
-; a, leaves e as `$u`), the synth-time unification of the outer
+; `unify_for_subst`). This demo is the regression fixture: if the
-; `List<a>`'s parameter against the `$u`-bearing element types fails
+; symmetric early-return is reverted, the inline form below errors out
-; on the param side because `unify_for_subst` only accepts arg-side
+; before reaching codegen.
-; `$u` wildcards. The helpers fully pin both type vars at the call
-; site, so each list element arrives with concrete `Either<Int, Int>`
-; and the bug is dodged. Logged as known-debt under 15g; the fix is
-; a one-line symmetric extension of the `$u` early-return.
 
 (module std_either_list_demo
 
@@ -23,24 +19,12 @@
   (import std_pair)
   (import std_either_list)
 
-  (fn mkleft
-    (doc "Monomorphic Left-injector at Either<Int, Int>. Pins both type vars at the call site so the codegen synth doesn't leave a `$u` wildcard on the second var.")
-    (type (fn-type (params (con Int)) (ret (con std_either.Either (con Int) (con Int)))))
-    (params x)
-    (body (term-ctor std_either.Either Left x)))
-
-  (fn mkright
-    (doc "Monomorphic Right-injector at Either<Int, Int>. Symmetric to mkleft.")
-    (type (fn-type (params (con Int)) (ret (con std_either.Either (con Int) (con Int)))))
-    (params x)
-    (body (term-ctor std_either.Either Right x)))
-
   (fn main
     (doc "Drive lefts, rights, and partition_eithers on the same five-element List<Either<Int, Int>> (Left 1, Right 10, Left 2, Right 20, Right 30). Expected output (one per line): 2, 3, 2, 3.")
     (type (fn-type (params) (ret (con Unit)) (effects IO)))
     (params)
     (body
-      (seq (do io/print_int (app std_list.length (app std_either_list.lefts (term-ctor std_list.List Cons (app mkleft 1) (term-ctor std_list.List Cons (app mkright 10) (term-ctor std_list.List Cons (app mkleft 2) (term-ctor std_list.List Cons (app mkright 20) (term-ctor std_list.List Cons (app mkright 30) (term-ctor std_list.List Nil)))))))))
+      (seq (do io/print_int (app std_list.length (app std_either_list.lefts (term-ctor std_list.List Cons (term-ctor std_either.Either Left 1) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 10) (term-ctor std_list.List Cons (term-ctor std_either.Either Left 2) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 20) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 30) (term-ctor std_list.List Nil)))))))))
-      (seq (do io/print_int (app std_list.length (app std_either_list.rights (term-ctor std_list.List Cons (app mkleft 1) (term-ctor std_list.List Cons (app mkright 10) (term-ctor std_list.List Cons (app mkleft 2) (term-ctor std_list.List Cons (app mkright 20) (term-ctor std_list.List Cons (app mkright 30) (term-ctor std_list.List Nil)))))))))
+      (seq (do io/print_int (app std_list.length (app std_either_list.rights (term-ctor std_list.List Cons (term-ctor std_either.Either Left 1) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 10) (term-ctor std_list.List Cons (term-ctor std_either.Either Left 2) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 20) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 30) (term-ctor std_list.List Nil)))))))))
-      (seq (do io/print_int (app std_list.length (app std_pair.fst (app std_either_list.partition_eithers (term-ctor std_list.List Cons (app mkleft 1) (term-ctor std_list.List Cons (app mkright 10) (term-ctor std_list.List Cons (app mkleft 2) (term-ctor std_list.List Cons (app mkright 20) (term-ctor std_list.List Cons (app mkright 30) (term-ctor std_list.List Nil))))))))))
+      (seq (do io/print_int (app std_list.length (app std_pair.fst (app std_either_list.partition_eithers (term-ctor std_list.List Cons (term-ctor std_either.Either Left 1) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 10) (term-ctor std_list.List Cons (term-ctor std_either.Either Left 2) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 20) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 30) (term-ctor std_list.List Nil))))))))))
-           (do io/print_int (app std_list.length (app std_pair.snd (app std_either_list.partition_eithers (term-ctor std_list.List Cons (app mkleft 1) (term-ctor std_list.List Cons (app mkright 10) (term-ctor std_list.List Cons (app mkleft 2) (term-ctor std_list.List Cons (app mkright 20) (term-ctor std_list.List Cons (app mkright 30) (term-ctor std_list.List Nil))))))))))))))))
+           (do io/print_int (app std_list.length (app std_pair.snd (app std_either_list.partition_eithers (term-ctor std_list.List Cons (term-ctor std_either.Either Left 1) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 10) (term-ctor std_list.List Cons (term-ctor std_either.Either Left 2) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 20) (term-ctor std_list.List Cons (term-ctor std_either.Either Right 30) (term-ctor std_list.List Nil))))))))))))))))