reassoc.hs

{-# language TupleSections #-}

import Control.Arrow ((&&&))
import Data.Function (on)
import Data.List     (groupBy, sortBy)
import Data.Ord      (comparing)

f :: Ord k => [(v, [(k, x)])] -> [(k, [(v, x)])]
f = map (fst.head &&& map snd)         -- [[(k, (v, x))]] -> [(k, [(v, x)])]
  . groupBy ((==) `on` fst)            -- [(k, (v, x))]   -> [[(k, (v, x))]]
  . sortBy (comparing fst)             -- [(k, (v, x))]   -> [(k, (v, x))]
  . map (\(a, (b, c)) -> (b, (a, c)))  -- [(v, (k, x))]   -> [(k, (v, x))]
  . (uncurry (map . (,)) =<<)          -- [(v, [(k, x)])] -> [(v, (k, x))]