;; do-notation + stub IO monad. Desugaring is per Haskell 98 §3.14:
;;   do { e ; ss }      = e >> do { ss }
;;   do { p <- e ; ss } = e >>= \p -> do { ss }
;;   do { let ds ; ss } = let ds in do { ss }
;;   do { e }           = e
;; The IO type is just `("IO" payload)` for now — no real side
;; effects yet. `return`, `>>=`, `>>` are built-ins.

(define
  hk-prog-val
  (fn
    (src name)
    (hk-deep-force (get (hk-eval-program (hk-core src)) name))))

;; ── Single-statement do ──
(hk-test
  "do with a single expression"
  (hk-eval-expr-source "do { return 5 }")
  (list "IO" 5))

(hk-test
  "return wraps any expression"
  (hk-eval-expr-source "return (1 + 2 * 3)")
  (list "IO" 7))

;; ── Bind threads results ──
(hk-test
  "single bind"
  (hk-eval-expr-source
    "do { x <- return 5 ; return (x + 1) }")
  (list "IO" 6))

(hk-test
  "two binds"
  (hk-eval-expr-source
    "do\n  x <- return 5\n  y <- return 7\n  return (x + y)")
  (list "IO" 12))

(hk-test
  "three binds — accumulating"
  (hk-eval-expr-source
    "do\n  a <- return 1\n  b <- return 2\n  c <- return 3\n  return (a + b + c)")
  (list "IO" 6))

;; ── Mixing >> and >>= ──
(hk-test
  ">> sequencing — last wins"
  (hk-eval-expr-source
    "do\n  return 1\n  return 2\n  return 3")
  (list "IO" 3))

(hk-test
  ">> then >>= — last bind wins"
  (hk-eval-expr-source
    "do\n  return 99\n  x <- return 5\n  return x")
  (list "IO" 5))

;; ── do-let ──
(hk-test
  "do-let single binding"
  (hk-eval-expr-source
    "do\n  let x = 3\n  return (x * 2)")
  (list "IO" 6))

(hk-test
  "do-let multi-bind, used after"
  (hk-eval-expr-source
    "do\n  let x = 4\n      y = 5\n  return (x * y)")
  (list "IO" 20))

(hk-test
  "do-let interleaved with bind"
  (hk-eval-expr-source
    "do\n  x <- return 10\n  let y = x + 1\n  return (x * y)")
  (list "IO" 110))

;; ── Bind + pattern ──
(hk-test
  "bind to constructor pattern"
  (hk-eval-expr-source
    "do\n  Just x <- return (Just 7)\n  return (x + 100)")
  (list "IO" 107))

(hk-test
  "bind to tuple pattern"
  (hk-eval-expr-source
    "do\n  (a, b) <- return (3, 4)\n  return (a * b)")
  (list "IO" 12))

;; ── User-defined IO functions ──
(hk-test
  "do inside top-level fun"
  (hk-prog-val
    "addM x y = do\n  a <- return x\n  b <- return y\n  return (a + b)\nresult = addM 5 6"
    "result")
  (list "IO" 11))

(hk-test
  "nested do"
  (hk-eval-expr-source
    "do\n  x <- do { y <- return 3 ; return (y + 1) }\n  return (x * 2)")
  (list "IO" 8))

;; ── (>>=) and (>>) used directly as functions ──
(hk-test
  ">>= used directly"
  (hk-eval-expr-source
    "(return 4) >>= (\\x -> return (x + 100))")
  (list "IO" 104))

(hk-test
  ">> used directly"
  (hk-eval-expr-source
    "(return 1) >> (return 2)")
  (list "IO" 2))

{:fails hk-test-fails :pass hk-test-pass :fail hk-test-fail}