What I want to do

I want to implement an example of banana-split law.

・(|f|),(|g|)・=(|・f”F(π1),g”F(π2)・|)

Script

I didn’t know how to ouptut tuple with foldr, I quited to implement it.
After following implementation, collecting foldr for only one makes the script be an example of banana-split law.

-- Before
sum' = foldr (\x xs -> x + xs) 0
length' = foldr (\x n -> 1 + n) 0
average ls = sum' ls / length' ls

-- After
average' ls = s / l
    where
        (s, l) = ((foldr (\x xs -> x + xs) 0) ls, (foldr (\x n -> 1 + n) 0) ls)

Result

After all, I’ll run the script.

$ghci
Prelude> :load banana-strip.hs
Main> average' [1,2,3,4,5]
3.0