haskell: thunks + force, app args become lazy (+6 tests, 333/333)

lib/haskell/tests/eval.sx

@@ -6,7 +6,7 @@
   hk-prog-val
   (fn
     (src name)
-    (get (hk-eval-program (hk-core src)) name)))
+    (hk-deep-force (get (hk-eval-program (hk-core src)) name))))
 
 ;; ── Literals ──
 (hk-test "int literal" (hk-eval-expr-source "42") 42)
@@ -230,6 +230,46 @@
     "if True then 1 else error \"unreachable\"")
   1)
 
+;; ── Laziness: app args evaluate only when forced ──
+(hk-test
+  "second arg never forced"
+  (hk-eval-expr-source
+    "(\\x y -> x) 1 (error \"never\")")
+  1)
+
+(hk-test
+  "first arg never forced"
+  (hk-eval-expr-source
+    "(\\x y -> y) (error \"never\") 99")
+  99)
+
+(hk-test
+  "constructor argument is lazy under wildcard pattern"
+  (hk-eval-expr-source
+    "case Just (error \"deeply\") of Just _ -> 7 ; Nothing -> 0")
+  7)
+
+(hk-test
+  "lazy: const drops its second argument"
+  (hk-prog-val
+    "const x y = x\nresult = const 5 (error \"boom\")"
+    "result")
+  5)
+
+(hk-test
+  "lazy: head ignores tail"
+  (hk-prog-val
+    "myHead (x:_) = x\nresult = myHead (1 : (error \"tail\") : [])"
+    "result")
+  1)
+
+(hk-test
+  "lazy: Just on undefined evaluates only on force"
+  (hk-prog-val
+    "wrapped = Just (error \"oh no\")\nresult = case wrapped of Just _ -> True ; Nothing -> False"
+    "result")
+  (list "True"))
+
 ;; ── not / id built-ins ──
 (hk-test "not True" (hk-eval-expr-source "not True") (list "False"))
 (hk-test "not False" (hk-eval-expr-source "not False") (list "True"))