Haskell types frustrating a simple ‘average’ function

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So fundamentally, you’re constrained by the type of (/):

(/) :: (Fractional a) => a -> a -> a

BTW, you also want Data.List.genericLength

genericLength :: (Num i) => [b] -> i

So how about removing the fromIntegral for something more general:

import Data.List

average xs = realToFrac (sum xs) / genericLength xs

which has only a Real constraint (Int, Integer, Float, Double)…

average :: (Real a, Fractional b) => [a] -> b

So that’ll take any Real into any Fractional.

And note all the posters getting caught by the polymorphic numeric literals in Haskell. 1 is not an integer, it is any number.

The Real class provides only one method: the ability to turn a value in class Num to a rational. Which is exactly what we need here.

And thus,

Prelude> average ([1 .. 10] :: [Double])
5.5
Prelude> average ([1 .. 10] :: [Int])
5.5
Prelude> average ([1 .. 10] :: [Float])
5.5
Prelude> average ([1 .. 10] :: [Data.Word.Word8])
5.5

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