I'm looking to learn how to write efficient Haskell code. This snippet works, but the run time is slow. How can I write faster Haskell code?

input: maxPrice = 7

input: vendorsDelivery = [5, 4, 2, 3]

input: vendorsProducts = [[1, 1, 1],[3, -1, 3],[-1, 2, 2],[5, -1, -1]]

output: [1,2] (fastest delivery time for a given order)

import Data.List
import Data.List.Split
import Control.Monad

data Vendor a = Vendor { vendorNbr :: Int
                       , deliveryTime :: Int
                       , itemPrice :: Int
                       } deriving (Show)

populate d v = map (filter (\x -> itemPrice x /= (-1))) $ transpose $ map (\(x,y,z) -> map (\xs -> Vendor x y xs) z) $ zip3 [0..] d v 

minimalBasketPrice mP vD vP = nub . get1 $ foldr (\x xs -> if get2 x < (get2 xs) then x else xs) (possible !! 0) possible
    where combos = sequence (populate vD vP) 
          possible = filter (\(_,_,x) -> x <= mP) [((accu vendorNbr y), (total deliveryTime y), (total itemPrice y)) | y <- combos]

-- | Helper Functions 

get1 (x,_,_) = x
get2 (_,x,_) = x

total f = foldr (\x y -> (f x) + y) 0
accu f = foldr (\x y -> (f x) : y) []

I'm looking to learn how to write efficient Haskell code. This snippet works, but the run time is slow. How can I write faster Haskell code?

Input: maxPrice = 7

Input: vendorsDelivery = [5, 4, 2, 3]

Input: vendorsProducts = [[1, 1, 1],[3, -1, 3],[-1, 2, 2],[5, -1, -1]]

Output: [1,2] (fastest delivery time for a given order)

import Data.List
import Data.List.Split
import Control.Monad

data Vendor a = Vendor { vendorNbr :: Int
                       , deliveryTime :: Int
                       , itemPrice :: Int
                       } deriving (Show)

populate d v = map (filter (\x -> itemPrice x /= (-1))) $ transpose $ map (\(x,y,z) -> map (\xs -> Vendor x y xs) z) $ zip3 [0..] d v 

minimalBasketPrice mP vD vP = nub . get1 $ foldr (\x xs -> if get2 x < (get2 xs) then x else xs) (possible !! 0) possible
    where combos = sequence (populate vD vP) 
          possible = filter (\(_,_,x) -> x <= mP) [((accu vendorNbr y), (total deliveryTime y), (total itemPrice y)) | y <- combos]

-- | Helper Functions 

get1 (x,_,_) = x
get2 (_,x,_) = x

total f = foldr (\x y -> (f x) + y) 0
accu f = foldr (\x y -> (f x) : y) []