function memoize(fn)
  local t = {}
  return function(x)
    local y = t[x]
    if y == nil then y = fn(x); t[x] = y end
    return y
  end
end

-- And here a very slow function:
function fib (n)
  if n == 1 or n == 2
    then return 1
    else return fib(n-1) + fib(n-2)
  end
end

-- Take the time:
function take_time(s)
  t1 = os.clock();
  f = loadstring(s);
  f();
  t2 = os.clock();
  print("Time = " .. t2 - t1 .. " secs");
end

-- How to use it:
take_time("fib(31)")
take_time("fib(32)")
mf = memoize(fib)     
take_time("mf(31)")
take_time("mf(31)")