Go back to a `sum` in prelude

src/projectEuler/question1.hs

@@ -5,9 +5,6 @@ The sum of these multiples is 23.
 
 Find the sum of all the multiples of 3 or 5 below 1000.
 -}
-{-# OPTIONS_GHC -Wno-unrecognised-pragmas #-}
-{-# HLINT ignore "Use sum" #-}
-
 module Main where
 
 import           Data.List (union)
@@ -16,26 +13,13 @@ main :: IO ()
 main = print ans
 
 ans :: Integer
-ans = sum $ allNumbersUnder 1000
-
-sum' :: Num a => [a] -> a
-sum' = foldl (+) 0
-
-isMultipleOf :: Integral a => a -> a -> Bool
-isMultipleOf n x = x `rem` n == 0
-
-all3sUnder :: Integral a => a -> [a]
-all3sUnder n = filter (isMultipleOf 3) [1..n-1]
-all5sUnder :: Integral a => a -> [a]
-all5sUnder n = filter (isMultipleOf 5) [1..n-1]
-
-allXsUnderN :: Integral a => a -> a -> [a]
-allXsUnderN x n = filter (isMultipleOf x) [1..n-1]
-
-allNumbersUnder :: Integral a => a -> [a]
-allNumbersUnder l = allXsUnderN 3 l <> allXsUnderN 5 l
-
+ans = sum (_3s <> _5s)
 
+_3s :: [Integer]
 _3s = [3,6..999]
+
+_5s :: [Integer]
 _5s = [5,10..999]
+
+solution :: Integer
 solution = sum $ union _3s _5s