Question

Here is my function. It checks for positive values, changes them to ones and sums them.

countPositive :: [Integer] -> Integer
countPositive xs = foldr (+) 0 $ map (^0) (filter (>0) xs)

Is there a better strategy to count positive values without using length but just foldr, map and filter?

Answer 1

Sure, just directly count them with foldr:

countPositive = foldr (\n count -> if n > 0 then count + 1 else count) 0

Or reimplement length with foldr:

countPositive = foldr (const succ) 0 . filter (>0)