actualiza :: [a] -> [(Int,a)] -> [a]

actualiza [3,5,2,4] [(2,1),(0,7)]  ==  [7,5,1,4]
sum (actualiza2 [1..10^4] [(i,2*i) | i <- [0..10^4-1]]) == 99990000

import qualified Data.Vector as V
import Data.Array

-- 1ª solución (por recursión)
-- ===========================

actualiza :: [a] -> [(Int,a)] -> [a]
actualiza xs []         = xs
actualiza xs ((n,y):ps) = actualiza (actualizaE xs (n,y)) ps

-- (actualizaE xs (n,y)) es la lista obtenida sustituyendo el  elemento
-- n-ésimo de xs por y. Por ejemplo,
--    actualiza [3,5,2,4] (2,1) ==  [3,5,1,4]
actualizaE :: [a] -> (Int,a) -> [a]
actualizaE xs (n,y) =
    take n xs ++ y : drop (n+1) xs

-- 2ª solución (por tablas)
-- ========================

actualiza2 :: [a] -> [(Int,a)] -> [a]
actualiza2 xs ps =
    elems (listArray (0,length xs - 1) xs // ps)

-- 3ª solución (por vectores)
-- ==========================

actualiza3 :: [a] -> [(Int,a)] -> [a]
actualiza3 xs ps =
    V.toList (V.fromList xs V.// ps)

-- Comparación de eficiencia
--    λ> let n = 10^2 in sum $ actualiza [1..n] [(i,2*i) | i <- [0..n-1]]
--    9900
--    (0.02 secs, 4668984 bytes)
--    λ> let n = 10^3 in sum $ actualiza [1..n] [(i,2*i) | i <- [0..n-1]]
--    999000
--    (0.28 secs, 77454496 bytes)
--    λ> let n = 10^4 in sum $ actualiza [1..n] [(i,2*i) | i <- [0..n-1]]
--    99990000
--    (63.46 secs, 8769501704 bytes)
--
--    λ> let n = 10^2 in sum $ actualiza2 [1..n] [(i,2*i) | i <- [0..n-1]]
--    9900
--    (0.02 secs, 4147304 bytes)
--    λ> let n = 10^3 in sum $ actualiza2 [1..n] [(i,2*i) | i <- [0..n-1]]
--    999000
--    (0.02 secs, 4694784 bytes)
--    λ> let n = 10^4 in sum $ actualiza2 [1..n] [(i,2*i) | i <- [0..n-1]]
--    99990000
--    (0.05 secs, 12276800 bytes)
--
--    λ> let n = 10^2 in sum $ actualiza3 [1..n] [(i,2*i) | i <- [0..n-1]]
--    9900
--    (0.01 secs, 4147304 bytes)
--    λ> let n = 10^3 in sum $ actualiza3 [1..n] [(i,2*i) | i <- [0..n-1]]
--    999000
--    (0.02 secs, 4682680 bytes)
--    λ> let n = 10^4 in sum $ actualiza3 [1..n] [(i,2*i) | i <- [0..n-1]]
--    99990000
--    (0.04 secs, 12595224 bytes)