-- alist analogue of Data.Map.insertWith
insertWith :: Eq k => (a -> a -> a)
              -> k
              -> a
              -> [(k, a)]
              -> [(k, a)]
insertWith _ key val [] = [(key, val)]
insertWith f key val (x@(k, v):xs)
  = if k == key
    then (key, f val v) : xs
    else x : insertWith f key val xs
 
frequencies :: (Eq a) => [a] -> [(a, Int)]
frequencies = foldr updateCounts []
    where updateCounts c = insertWith (+) c 1
 
main = getContents >>= print . frequencies
 

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Hello, World!