[Haskell-beginners] Function to compute the mean

[Joe King]

Greeetings I am new here and pretty new to Haskell.

I was wondering what are the relative advanatges/disadvatnages of specifying a mean function in these two ways:

mean :: [Double] -> Double
mean xs = sum xs / fromIntegral (length xs)

and

mean1 :: (Real a, Fractional b) => [a] -> b
mean1 xs = realToFrac (sum xs) / genericLength xs

I understand that mean1 has the advantage that it can be called with lists of any Real type, so would work with things like 

foo :: [Int]
foo = [1,2,3]

mean foo
-- type mismatch error

mean1 foo
-- no error

But suppose that I know I will only ever use lists of Double, is there still any advantage (or disadvantage of using mean1). For example is there any performance benefit by using mean in that case since mean1 has additional function evaluation. 

Are there any other considerations ?

Thanks in advance
JK