haskell: Phase 7 string=[Char] — O(1) string-view head/tail + chr/ord/toUpper/toLower/++ (+35 tests, 810/810)

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

lib/haskell/runtime.sx

@@ -128,3 +128,48 @@
 (hk-register-con! "LT" 0 "Ordering")
 (hk-register-con! "EQ" 0 "Ordering")
 (hk-register-con! "GT" 0 "Ordering")
+
+(define
+  hk-str?
+  (fn (v) (or (string? v) (and (dict? v) (has-key? v "hk-str")))))
+
+(define
+  hk-str-head
+  (fn
+    (v)
+    (if
+      (string? v)
+      (char-code (char-at v 0))
+      (char-code (char-at (get v "hk-str") (get v "hk-off"))))))
+
+(define
+  hk-str-tail
+  (fn
+    (v)
+    (let
+      ((buf (if (string? v) v (get v "hk-str")))
+        (off (if (string? v) 1 (+ (get v "hk-off") 1))))
+      (if (>= off (string-length buf)) (list "[]") {:hk-off off :hk-str buf}))))
+
+(define
+  hk-str-null?
+  (fn
+    (v)
+    (if
+      (string? v)
+      (= (string-length v) 0)
+      (>= (get v "hk-off") (string-length (get v "hk-str"))))))
+
+(define
+  hk-str-to-native
+  (fn
+    (v)
+    (if
+      (string? v)
+      v
+      (let
+        ((buf (get v "hk-str")) (off (get v "hk-off")))
+        (reduce
+          (fn (acc i) (str acc (char-at buf i)))
+          ""
+          (range off (string-length buf)))))))