inductive Op : (inp out : List (Type u)) → Type (u+1) where
| push (a : α) : Op s (α::s)
| pop  : Op (α::s) s
| dup  : Op (α::s) (α::α::s)
| swap : Op (α::β::s) (β::α::s)
| call : Op ((α → β)::α::s) (β::s)
| seq  (a : Op i o) (b : Op o o') : Op i o'

abbrev Stack : List Type → Type
| [] => Unit
| t::ts => t × Stack ts

example : (α × Stack s) = Stack (α::s) := by simp

#reduce (types := true) Stack [Nat, Int, Int]

def Op.run : Op i o → Stack i → Stack o
| push x, t => (x, t)
| pop, (x, s) => s
| dup, (x, s) => (x, x, s)
| swap, (a, b, s) => (b, a, s)
| call, (f, a, s) => (f a, s)
| seq a b, s => b.run (a.run s)

def Op.runEmpty (op : Op [] o) : Stack o := op.run ()

#eval  (Op.push 1)
|>.seq (Op.push 2)
|>.seq (Op.push Nat.add)
|>.seq  Op.call
|>.seq  Op.call
|>.runEmpty

def main : IO Unit :=
  IO.println s!"Hello, world!"