recursion - How to calculate how many recursive calls happened in this Haskell function?

Question

I've been trying to figure this out for a couple of hours now. I need to calculate how many recursive calls happened using a certain function:

maximum' :: (Ord a) => [a] -> a  
maximum' [] = error "maximum of empty list"  
maximum' [x] = x  
maximum' (x:xs)   
    | x > maxTail = x  
    | otherwise = maxTail  
    where maxTail = maximum' xs  

Many thanks in advance

Answer 1

You can rewrite your function to carry information about recursion depth in an extra argument, and return it as second (or first, if you so prefer) element of a tuple:

maximum' :: (Ord a) => [a] -> Int -> (a, Int)
maximum' [] _ = error "maximum of empty list"  
maximum' [x] n = (x, n)
maximum' (x:xs) n   
    | x > fst maxTail = (x, snd maxTail)
    | otherwise = maxTail  
    where maxTail = maximum' xs (n + 1)

Then you can call the function with maximum' lst 0 or maximum' lst 1 depending on whether or not you want the first call to count as a recursion level.

It can be done with any recursive function, but it's probably not necessary in your case. As chepner wrote, the answer is known without extra computations.