haskell: Phase 13 — class default method registration + dispatch fallback

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plans/haskell-completeness.md

@@ -223,10 +223,10 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 - [x] Verify `where`-clauses in `instance` bodies desugar correctly. The
   `hk-bind-decls!` instance arm must call the same where-lifting logic as
   top-level function clauses. Write a targeted test to confirm.
-- [ ] Class declarations may include default method implementations. Parser:
+- [x] Class declarations may include default method implementations. Parser:
   `hk-parse-class` collects method decls; eval registers defaults under
   `"__default__ClassName_method"` in the class dict.
-- [ ] Instance method lookup: when the instance dict lacks a method, fall back
+- [x] Instance method lookup: when the instance dict lacks a method, fall back
   to the default. Wire this into the dictionary-passing dispatch.
 - [ ] `Eq` default: `(/=) x y = not (x == y)`. Verify it works without an
   explicit `/=` in every Eq instance.
@@ -307,6 +307,16 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-07** — Phase 13 default method implementations + dispatch fallback:
+- class-decl handler now also registers fun-clause method bodies under
+  `__default__ClassName_method` (paralleling the type-sig dispatcher pass).
+- Dispatcher rewritten as nested `if`s: instance dict has the method →
+  use it; else look up default → use it; else raise. Earlier attempt with
+  `cond + and` infinite-looped — switched to plain `if` form which works.
+- Both regular dispatch (`describe x = "a boolean"` instance) and default
+  fallback (`hello x = "hi"` default with empty instance body) verified.
+  No regressions in class/deriving/instance-where/eval suites.
+
 **2026-05-07** — Phase 13 `where`-clauses in `instance` bodies:
 - Bug discovered: `hk-desugar` didn't recurse into `instance-decl` method
   bodies, so a `where`-form in an instance method survived to eval and hit