Подсчет количества вхождений каждого символа в строку

import Data.List
 
add :: Eq a => a -> [(a, Int)] -> [(a, Int)]
add x [] = [(x, 1)]
add x ((y, n):rest) = if x == y
    then (y, n+1) : rest
    else (y, n) : add x rest
 
count :: Eq a => [a] -> [(a, Int)]
count = sortBy f . foldr add [] where
    f (_, x) (_, y) = compare y x
 
main = print $ count "Hello"

import Data.List
 
countCharsMy::String->[(Char,Int)]
countCharsMy = sortBy (\b a->compare (snd a) (snd b)).countCharsMy'
  where
    countCharsMy'::String->[(Char,Int)]
    countCharsMy' sp@(x:sx) = (x,(length.filter (x==)) sp):countCharsMy' (filter (x/=) sx)
    countCharsMy' _ = []
 
λ> countCharsMy "hello"
[('l',2),('h',1),('e',1),('o',1)]

countCharsMy :: String -> [(Char,Int)]
countCharsMy ""     = []
countCharsMy (s:ss) =  (s,n) : countCharsMy ww   
                       where ww = filter (\ q -> (q /= s)) ss
                             n  = (length ss)-(length ww)+1
 
Main> countCharsMy "abrakadabra"
[('a',5),('b',2),('r',2),('k',1),('d',1)]

λ> countCharsMy "hello"
[('h',1),('e',1),('l',2),('o',1)]

countCharsMy :: String -> [(Char,Int)]
countCharsMy ""     = []
countCharsMy (s:ss) =  (s,n) : countCharsMy ww   
                       where ww = filter (\ q -> (q /= s)) ss
                             n  = (length ss)-(length ww)+1
                             
-- Сортировка вставками
 
sortBy :: [(Char,Int)] -> [(Char,Int)]
sortBy [] = []
sortBy x = mix : sortBy xx
           where mix = foldl (\ a p -> if (fst a) > (fst p) then p else a) (head x) x
                 xx  = filter (\ a -> a /= mix) x
 
Main> sortBy $ countCharsMy "Hello world"
[(' ',1),('H',1),('d',1),('e',1),('l',3),('o',2),('r',1),('w',1)]

import Data.List(nub, sortBy)
import Data.Function(on)
 
count' :: String -> Char -> Int
count' = foldr (\ e f x -> if x == e then succ (f x) else f x) (const 0)
 
count :: String -> [(Char,Int)]
count str = sort' $ nub $ zip str $ count' str `map` str
  where
    sort' = reverse . sortBy (compare `on` snd)

sortBy :: [(Char,Int)] -> [(Char,Int)]
sortBy [] = []
sortBy x = mix : sortBy xx
           where mix = foldl (\ a p -> if (snd a) > (snd p) then p else a) (head x) x
                 xx  = filter (\ a -> a /= mix) x