Computing the mean of a list efficiently in Haskell

Question

I\'ve designed a function to compute the mean of a list. Although it works fine, but I think it may not be the best solution due to it takes two functions rather than one. Is it

Answer 1

When I saw your question, I immediately thought "you want a fold there!"

And sure enough, a similar question has been asked before on StackOverflow, and this answer has a very performant solution, which you can test in an interactive environment like GHCi:

import Data.List

let avg l = let (t,n) = foldl' (\(b,c) a -> (a+b,c+1)) (0,0) l 
            in realToFrac(t)/realToFrac(n)

avg ([1,2,3,4]::[Int])
2.5
avg ([1,2,3,4]::[Double])
2.5