FLOLAC 2018 Week 3 (2018-5-11 11:46) -- Goldbach's Conjecture

goldbach.hs

--  FLOLAC 2018 Week 3 -- Goldbach's conjecture

-- A simple, intuitive, straight fowward solution.
-- And may be not so efficient.  :-)   

goldbach :: Int -> Maybe (Int, Int)

goldbach nn 
    | nn < 3  = Nothing
    | odd nn  = Nothing
    | otherwise = 
        if null pairs 
            then  Nothing
            else  Just $ head pairs
          where  
             pairs = [ (m,n) | m <- tryRange,
                               n <- tryRange,
                               (m + n) == nn ]
             tryRange = takeWhile (\x -> x < nn) primeList 
             primeList = ppList [2 .. ] 
             ppList (n : ns) = n : ppList (filter (\x -> (mod x n) /= 0 ) ns)

-- Thanks to the call-by-need lazy evaluation,
-- The efficiency is really not bad.  :-)