haskell: inference unit tests — 55+ expressions, Phase 4 complete (+16 tests, 492/492)

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

plans/haskell-on-sx.md

@@ -96,7 +96,7 @@ Key mappings:
 - [x] Reject untypeable programs that phase 3 was accepting
 - [x] Type-sig checking: user writes `f :: Int -> Int`; verify
 - [x] Let-polymorphism
-- [ ] Unit tests: inference for 50+ expressions
+- [x] Unit tests: inference for 50+ expressions
 
 ### Phase 5 — typeclasses (dictionary passing)
 - [ ] `class` / `instance` declarations
@@ -114,6 +114,14 @@ Key mappings:
 
 _Newest first._
 
+- **2026-05-05** — Phase 4 inference unit tests (50+ expressions). Added 16 new
+  `hk-t` expression tests to `tests/infer.sx`: nested application (`not(not True)`,
+  `negate(negate 1)`), bool/mixed lambdas (`\\x->\\y->x&&y`, `\\x->x==1`),
+  let variants (if-in-let, not-in-let, tuple-in-let, nested let, chain application),
+  more if expressions, 2-element tuples, and list operations on Bool lists.
+  infer.sx now has 75 tests covering 55+ distinct expression forms. Phase 4
+  complete. 492/492 green.
+
 - **2026-05-05** — Phase 4 let-polymorphism tests. `hk-w-let` already
   generalises let-bound types with `hk-generalise` before adding them to the
   env, so `id :: ∀a. a→a` is instantiated independently at each use site.