pearson similarity

import Data.List (intersect)
import qualified Data.Map as Map

prefs = Map.fromList
        [("Lisa Rose", Map.fromList [("Lady in the Water", 2.5)
                                    , ("Snakes on a Plane", 3.5)
                                    , ("Just My Luck", 3.0)
                                    , ("Superman Returns", 3.5)
                                    , ("You, Me and Dupree", 2.5)
                                    , ("The Night Listener", 3.0)])
        , ("Gene Seymour", Map.fromList [("Lady in the Water", 3.0)
                                        , ("Snakes on a Plane", 3.5)
                                        , ("Just My Luck", 1.5)
                                        , ("Superman Returns", 5.0)
                                        , ("The Night Listener", 3.0)
                                        , ("You, Me and Dupree", 3.5)])
        ]

sim_pearson :: String -> String -> Double
sim_pearson prefs p1 p2 =
  if siLength > 0
  then si
  else []
  where p1Movies = snd (prefs !! p1)
        p2Movies = snd (prefs !! p2)
        si = Map.keys p1Movies `intersect` Map.keys p2Movies
        siLength = length si
        sum movies = foldl (\acc movie -> case (Map.lookup movie movies) of
                                           Just val -> acc + val
                                           Nothing  -> acc) 0 si
        sum1 = sum p1Movies
        sum2 = sum p2Movies
        sumsq movies = foldl (\acc movie -> case (Map.lookup movie movies) of
                                             Just val -> acc + (val**2)
                                             Nothing  -> acc) 0 si
        sum1sq = sumsq p1Movies
        sum2sq = sumsq p2Movies
        pSum = foldl (\acc movie -> acc + ((Map.lookup movie p1Movies) * (Map.lookup movie p2Movies))) 0 si
        num = pSum - ((sum1 * sum2) / siLength)