Prelude> :reload
[1 of 1] Compiling Main             ( promedio.hs, interpreted )

promedio.hs:3:14:
    No instance for (Fractional Int)
      arising from a use of `/' at promedio.hs:3:14-68
    Possible fix: add an instance declaration for (Fractional Int)
    In the expression:
          fromIntegral (sum (x : xs)) / fromIntegral (length (x : xs))
    In the definition of `prom':
        prom (x : xs)
               = fromIntegral (sum (x : xs)) / fromIntegral (length (x : xs))
Failed, modules loaded: none.