haskell: do-notation desugar + stub IO monad (return/>>=/>>) (+14 tests, 382/382)

lib/haskell/eval.sx

@@ -614,6 +614,48 @@ plus a b = a + b
           "deepseq"
           (fn (a b) (do (hk-deep-force a) b))
           2))
+      ;; ── Stub IO monad ─────────────────────────────────────
+      ;; IO actions are tagged values `("IO" payload)`; `>>=` and
+      ;; `>>` chain them. Lazy in the action arguments so do-blocks
+      ;; can be deeply structured without forcing the whole chain
+      ;; up front.
+      (dict-set!
+        env
+        "return"
+        (hk-mk-lazy-builtin
+          "return"
+          (fn (x) (list "IO" x))
+          1))
+      (dict-set!
+        env
+        ">>="
+        (hk-mk-lazy-builtin
+          ">>="
+          (fn (m f)
+            (let ((io-val (hk-force m)))
+              (cond
+                ((and
+                    (list? io-val)
+                    (= (first io-val) "IO"))
+                  (hk-apply (hk-force f) (nth io-val 1)))
+                (:else
+                  (raise "(>>=): left side is not an IO action")))))
+          2))
+      (dict-set!
+        env
+        ">>"
+        (hk-mk-lazy-builtin
+          ">>"
+          (fn (m n)
+            (let ((io-val (hk-force m)))
+              (cond
+                ((and
+                    (list? io-val)
+                    (= (first io-val) "IO"))
+                  (hk-force n))
+                (:else
+                  (raise "(>>): left side is not an IO action")))))
+          2))
       ;; Operators as first-class values
       (dict-set! env "+" (hk-make-binop-builtin "+" "+"))
       (dict-set! env "-" (hk-make-binop-builtin "-" "-"))