Also the + operator is typed with Num; from a Haskell repl: λ> :t (+) (+) :: Num a => a -> a -> a In case of average we need /: λ> :t (/) (/) :: Frac

Also the + operator is typed with Num; from a Haskell repl: λ> :t (+) (+) :: Num a => a -> a -> a In case of average we need /: λ> :t (/) (/) :: Fractional a => a -> a -> a So our type is going to be constrained by Fractional instead of Num). So an implementation could be (from here [1]): import Data.List average :: (Real a, Fractional b) => [a] -> b average xs = realToFrac (sum xs) / genericLength xs [1]: https://stackoverflow.com/questions/2376981 https://stackoverflow.com/questions/2376981