haskell: Phase 7 conformance — runlength-str.hs + ++ thunk-tail fix (+9 tests, 9/9)

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

lib/haskell/conformance.sh

@@ -20,7 +20,7 @@ if [ ! -x "$SX_SERVER" ]; then
   fi
 fi
 
-PROGRAMS=(fib sieve quicksort nqueens calculator collatz palindrome maybe fizzbuzz anagram roman binary either primes zipwith matrix wordcount powers caesar)
+PROGRAMS=(fib sieve quicksort nqueens calculator collatz palindrome maybe fizzbuzz anagram roman binary either primes zipwith matrix wordcount powers caesar runlength-str)
 PASS_COUNTS=()
 FAIL_COUNTS=()
 

lib/haskell/eval.sx

@@ -432,12 +432,14 @@
   hk-list-append
   (fn
     (a b)
-    (cond
-      ((hk-str? a) (str (hk-str-to-native a) (hk-str-to-native b)))
-      ((and (list? a) (= (first a) "[]")) b)
-      ((and (list? a) (= (first a) ":"))
-        (hk-mk-cons (nth a 1) (hk-list-append (nth a 2) b)))
-      (:else (raise "++: not a list")))))
+    (let
+      ((a (hk-force a)))
+      (cond
+        ((hk-str? a) (str (hk-str-to-native a) (hk-str-to-native b)))
+        ((and (list? a) (= (first a) "[]")) b)
+        ((and (list? a) (= (first a) ":"))
+          (hk-mk-cons (nth a 1) (hk-list-append (nth a 2) b)))
+        (:else (raise "++: not a list"))))))
 
 ;; Eager finite-range spine — handles [from..to] and [from,next..to].
 ;; Step direction is governed by the sign of `step`; when step > 0 we

lib/haskell/tests/program-runlength-str.sx

@@ -0,0 +1,83 @@
+;; runlength-str.hs — run-length encoding on a String.
+;; Source: https://rosettacode.org/wiki/Run-length_encoding#Haskell (adapted).
+;;
+;; Exercises String pattern matching `(x:xs)`, `span` over a string view,
+;; tuple construction `(Int, Char)`, character equality, and tuple-in-cons
+;; patterns `((n, c) : rest)` — all enabled by Phase 7 string=[Char].
+
+(define
+  hk-prog-val
+  (fn
+    (src name)
+    (hk-deep-force (get (hk-eval-program (hk-core src)) name))))
+
+(define
+  hk-as-list
+  (fn
+    (xs)
+    (cond
+      ((and (list? xs) (= (first xs) "[]")) (list))
+      ((and (list? xs) (= (first xs) ":"))
+        (cons (nth xs 1) (hk-as-list (nth xs 2))))
+      (:else xs))))
+
+(define
+  hk-rle-source
+  "encodeRL []     = []\nencodeRL (x:xs) = let (same, rest) = span eqX xs\n                      eqX y         = y == x\n                  in (1 + length same, x) : encodeRL rest\n\nreplicateRL 0 _ = []\nreplicateRL n c = c : replicateRL (n - 1) c\n\ndecodeRL []                = []\ndecodeRL ((n, c) : rest)   = replicateRL n c ++ decodeRL rest\n")
+
+(hk-test
+  "rle.hs — encodeRL [] = []"
+  (hk-as-list (hk-prog-val (str hk-rle-source "r = encodeRL \"\"\n") "r"))
+  (list))
+
+(hk-test
+  "rle.hs — length (encodeRL \"aabbbcc\") = 3"
+  (hk-prog-val (str hk-rle-source "r = length (encodeRL \"aabbbcc\")\n") "r")
+  3)
+
+(hk-test
+  "rle.hs — map fst (encodeRL \"aabbbcc\") = [2,3,2]"
+  (hk-as-list
+    (hk-prog-val (str hk-rle-source "r = map fst (encodeRL \"aabbbcc\")\n") "r"))
+  (list 2 3 2))
+
+(hk-test
+  "rle.hs — map snd (encodeRL \"aabbbcc\") = [97,98,99]"
+  (hk-as-list
+    (hk-prog-val (str hk-rle-source "r = map snd (encodeRL \"aabbbcc\")\n") "r"))
+  (list 97 98 99))
+
+(hk-test
+  "rle.hs — counts of encodeRL \"aabbbccddddee\" = [2,3,2,4,2]"
+  (hk-as-list
+    (hk-prog-val
+      (str hk-rle-source "r = map fst (encodeRL \"aabbbccddddee\")\n")
+      "r"))
+  (list 2 3 2 4 2))
+
+(hk-test
+  "rle.hs — chars of encodeRL \"aabbbccddddee\" = [97,98,99,100,101]"
+  (hk-as-list
+    (hk-prog-val
+      (str hk-rle-source "r = map snd (encodeRL \"aabbbccddddee\")\n")
+      "r"))
+  (list 97 98 99 100 101))
+
+(hk-test
+  "rle.hs — singleton encodeRL \"x\""
+  (hk-as-list
+    (hk-prog-val (str hk-rle-source "r = map fst (encodeRL \"x\")\n") "r"))
+  (list 1))
+
+(hk-test
+  "rle.hs — decodeRL round-trip preserves \"aabbbcc\""
+  (hk-as-list
+    (hk-prog-val (str hk-rle-source "r = decodeRL (encodeRL \"aabbbcc\")\n") "r"))
+  (list 97 97 98 98 98 99 99))
+
+(hk-test
+  "rle.hs — replicateRL 4 65 = [65,65,65,65]"
+  (hk-as-list (hk-prog-val (str hk-rle-source "r = replicateRL 4 65\n") "r"))
+  (list 65 65 65 65))
+
+{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}

plans/haskell-completeness.md

@@ -89,7 +89,7 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 - [x] Tests in `lib/haskell/tests/string-char.sx` (≥ 15 tests: head/tail on
   string literal, map over string, filter chars, chr/ord roundtrip, toUpper,
   toLower, null/empty string view).
-- [ ] Conformance programs (WebFetch + adapt):
+- [x] Conformance programs (WebFetch + adapt):
   - `caesar.hs` — Caesar cipher. Exercises `map`, `chr`, `ord`, `toUpper`,
     `toLower` on characters.
   - `runlength-str.hs` — run-length encoding on a String. Exercises string
@@ -284,6 +284,19 @@ No OCaml changes are needed. The view type is fully representable as an SX dict.
 
 _Newest first._
 
+**2026-05-06** — Phase 7 conformance complete (runlength-str.hs) + `++` thunk fix:
+- New `lib/haskell/tests/program-runlength-str.sx` (9 tests). Exercises `(x:xs)`
+  pattern matching over Strings, `span` over a string view, tuple `(Int, Char)`
+  construction and `((n,c):rest)` destructuring, `++` between cons spines.
+- `runlength-str` added to `PROGRAMS` in `conformance.sh`.
+- `eval.sx`: `hk-list-append` now `(hk-force a)` on entry. Pre-existing latent
+  bug — when a cons's tail was a thunk (e.g. from the `:` operator inside a
+  recursive Haskell function like `replicateRL n c = c : replicateRL (n-1) c`),
+  the recursion `(hk-list-append (nth a 2) b)` saw a dict, not a list, and
+  raised `"++: not a list"`. Quicksort masked this by chaining `[x]` literals
+  whose tails are forced `("[]")` cells. Forcing in `hk-list-append` is
+  load-bearing for any `++` over a recursively-built spine.
+
 **2026-05-06** — Phase 7 conformance (caesar.hs):
 - New `lib/haskell/tests/program-caesar.sx` (8 tests). Caesar cipher exercising
   `chr`, `ord`, `isUpper`, `isLower`, `mod`, `map`, and `(x:xs)` pattern matching