local A, F, I = {}, {}, {}

local function unwind(stack, sp)
  local x = stack[sp]
  if type(x) == 'table' then
    if x[1] == A then
      stack[sp + 1] = x[2]
      return unwind(stack, sp + 1)
    elseif x[1] == I then
      stack[sp] = x[2]
      return unwind(stack, sp)
    elseif x[1] == F then
      if sp - 1 >= x[3] then
        return x[2](stack, sp)
      else
        error("insufficient arguments for supercombinator " .. x[4])
      end
    end
  else
    return x, stack, sp
  end
end

local function repr(x)
  if type(x) == 'table' then
    if x[1] == A then
      return repr(x[2]) .. '(' .. repr(x[3])
    elseif x[1] == F then
      return x[4]
    elseif x[1] == I then
      return '&' .. repr(x[2])
    end
    return '<bad node>'
  else
    return tostring(x)
  end
end

local function eval(node)
  local stack, sp = { node }, 1
  return (unwind(stack, sp))
end

local function getchar(stack, sp)
  local k = stack[sp - 1][3]; sp = sp - 1
  local knil = stack[sp - 1][3]; sp = sp - 1
  local ch = io.read(1)
  if ch then
    stack[sp] = { A, k, ch:byte() }
  else
    stack[sp] = { A, knil, 0 }
  end
  return unwind(stack, sp)
end

local getchar_combinator = { F, getchar, 2 }

local function putchar()
  local ch = stack[sp - 1][3];
  local k = stack[sp - 2][3]; sp = sp - 1
  io.write(string.char(ch))
  stack[sp] = { A, k, ch }
  return unwind(stack, sp)
end

local putchar_combinator = { F, putchar, 2 }