haskell: Phase 17 — expression type annotations (x :: Int) (parse + desugar pass-through)

Parser hk-parse-parens gains a `::` arm after the first inner expression:
consume `::`, parse a type via the existing hk-parse-type, expect `)`,
emit (:type-ann EXPR TYPE). Sections, tuples, parenthesised expressions
and unit `()` are unchanged.

Desugar drops the annotation — :type-ann E _ → (hk-desugar E) — since
the existing eval path has no type-directed dispatch. Phase 20 will
extend infer.sx to consume the annotation and unify against the
inferred type.

tests/parse-extras.sx (12/12) covers literal, arithmetic, function arg,
string, bool, tuple, nested annotation, function-typed annotation, and
no-regression checks for plain parens / 3-tuples / left+right sections.
eval (66/0), exceptions (14/0), typecheck (15/0), records (14/0), ioref
(13/0) all still clean.

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

lib/haskell/desugar.sx

@@ -210,6 +210,7 @@
                 :op (nth node 1)
                 (hk-desugar (nth node 2))
                 (hk-desugar (nth node 3))))
+            ((= tag "type-ann") (hk-desugar (nth node 1)))
             ((= tag "neg") (list :neg (hk-desugar (nth node 1))))
             ((= tag "if")
               (list

lib/haskell/parser.sx

@@ -275,38 +275,47 @@
                             (list :sect-right op-name expr-e))))))
                   (:else
                     (let
-                      ((first-e (hk-parse-expr-inner))
-                        (items (list))
-                        (is-tuple false))
-                      (append! items first-e)
-                      (define
-                        hk-tup-loop
-                        (fn
-                          ()
-                          (when
-                            (hk-match? "comma" nil)
-                            (do
-                              (hk-advance!)
-                              (set! is-tuple true)
-                              (append! items (hk-parse-expr-inner))
-                              (hk-tup-loop)))))
-                      (hk-tup-loop)
+                      ((first-e (hk-parse-expr-inner)))
                       (cond
-                        ((hk-match? "rparen" nil)
+                        ((hk-match? "reservedop" "::")
                           (do
                             (hk-advance!)
-                            (if is-tuple (list :tuple items) first-e)))
+                            (let
+                              ((ann-type (hk-parse-type)))
+                              (hk-expect! "rparen" nil)
+                              (list :type-ann first-e ann-type))))
                         (:else
                           (let
-                            ((op-info2 (hk-section-op-info)))
+                            ((items (list)) (is-tuple false))
+                            (append! items first-e)
+                            (define
+                              hk-tup-loop
+                              (fn
+                                ()
+                                (when
+                                  (hk-match? "comma" nil)
+                                  (do
+                                    (hk-advance!)
+                                    (set! is-tuple true)
+                                    (append! items (hk-parse-expr-inner))
+                                    (hk-tup-loop)))))
+                            (hk-tup-loop)
                             (cond
-                              ((and (not (nil? op-info2)) (not is-tuple) (let ((after2 (hk-peek-at (get op-info2 "len")))) (and (not (nil? after2)) (= (get after2 "type") "rparen"))))
-                                (let
-                                  ((op-name (get op-info2 "name")))
-                                  (hk-consume-op!)
+                              ((hk-match? "rparen" nil)
+                                (do
                                   (hk-advance!)
-                                  (list :sect-left op-name first-e)))
-                              (:else (hk-err "expected ')' after expression"))))))))))))))
+                                  (if is-tuple (list :tuple items) first-e)))
+                              (:else
+                                (let
+                                  ((op-info2 (hk-section-op-info)))
+                                  (cond
+                                    ((and (not (nil? op-info2)) (not is-tuple) (let ((after2 (hk-peek-at (get op-info2 "len")))) (and (not (nil? after2)) (= (get after2 "type") "rparen"))))
+                                      (let
+                                        ((op-name (get op-info2 "name")))
+                                        (hk-consume-op!)
+                                        (hk-advance!)
+                                        (list :sect-left op-name first-e)))
+                                    (:else (hk-err "expected ')' after expression")))))))))))))))))
       (define
         hk-comp-qual-is-gen?
         (fn

lib/haskell/tests/parse-extras.sx

@@ -0,0 +1,63 @@
+;; Phase 17 — parser polish unit tests.
+
+(hk-test
+  "type-ann: literal int annotated"
+  (hk-deep-force (hk-run "main = (42 :: Int)"))
+  42)
+
+(hk-test
+  "type-ann: arithmetic annotated"
+  (hk-deep-force (hk-run "main = (1 + 2 :: Int)"))
+  3)
+
+(hk-test
+  "type-ann: function arg annotated"
+  (hk-deep-force
+    (hk-run "f x = x + 1\nmain = f (1 :: Int)"))
+  2)
+
+(hk-test
+  "type-ann: string annotated"
+  (hk-deep-force (hk-run "main = (\"hi\" :: String)"))
+  "hi")
+
+(hk-test
+  "type-ann: bool annotated"
+  (hk-deep-force (hk-run "main = (True :: Bool)"))
+  (list "True"))
+
+(hk-test
+  "type-ann: tuple annotated"
+  (hk-deep-force (hk-run "main = ((1, 2) :: (Int, Int))"))
+  (list "Tuple" 1 2))
+
+(hk-test
+  "type-ann: nested annotation in arithmetic"
+  (hk-deep-force (hk-run "main = (1 :: Int) + (2 :: Int)"))
+  3)
+
+(hk-test
+  "type-ann: function-typed annotation passes through eval"
+  (hk-deep-force
+    (hk-run "main = let f = ((\\x -> x + 1) :: Int -> Int) in f 5"))
+  6)
+
+(hk-test
+  "no regression: plain parens still work"
+  (hk-deep-force (hk-run "main = (5)"))
+  5)
+
+(hk-test
+  "no regression: 3-tuple still works"
+  (hk-deep-force (hk-run "main = (1, 2, 3)"))
+  (list "Tuple" 1 2 3))
+
+(hk-test
+  "no regression: section-left still works"
+  (hk-deep-force (hk-run "main = (3 +) 4"))
+  7)
+
+(hk-test
+  "no regression: section-right still works"
+  (hk-deep-force (hk-run "main = (+ 3) 4"))
+  7)