Add questions 1-3

src/projectEuler/question1.hs

@@ -0,0 +1,28 @@
+{-
+https://projecteuler.net/problem=1
+If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9.
+The sum of these multiples is 23.
+
+Find the sum of all the multiples of 3 or 5 below 1000.
+-}
+module Main where
+
+main :: IO ()
+main = print ans
+
+ans :: Integer
+ans = sum $ allNumbersUnder 1000
+
+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