--
def inc (r : IO.Ref Nat) : IO Unit := do
let v ← r.get;
r.set (v+1);
IO.println (">> " ++ toString v)

def initArray (r : IO.Ref (Array Nat)) (n : Nat) : IO Unit :=
n.forM $ fun i => do
  r.modify $ fun a => a.push (2*i)

def showArrayRef (r : IO.Ref (Array Nat)) : IO Unit := do
let a ← r.swap ∅;
a.size.forM (fun i => IO.println ("[" ++ toString i ++ "]: " ++ toString (a.get! i)));
discard $ r.swap a;
pure ()

def tst (n : Nat) : IO Unit := do
let r₁ ← IO.mkRef 0;
n.forM fun _ => inc r₁;
let r₂ ← IO.mkRef (∅ : Array Nat);
initArray r₂ n;
showArrayRef r₂

#eval tst 10