Euler ex. 1 Haskell.hs

-- Alex's simple Haskell version
foldl (+) 0 [x | x <- [1..999], x `mod` 3 == 0 || x `mod` 5 == 0]

-- List comprehensions, naturally lazy
let multipleOf3Or5 x = (mod x 3 == 0) || (mod x 5 == 0)
foldl (+) 0  $  takeWhile (<1000) [x | x <- [1..], multipleOf3Or5 x]

-- Sets
-- Prelude> :m + Data.Set
let multiplesOf3 = Data.Set.fromList [0,3..1000]
let multiplesOf5 = Data.Set.fromList [0,5..995]
Data.Set.fold (+) 0  $ Data.Set.union multiplesOf3 multiplesOf5

-- or more briefly:
Data.Set.fold (+) 0  $ Data.Set.union (Data.Set.fromList [0,3..1000])  (Data.Set.fromList [0,5..995])