cleaned up solution

day1.hs

@@ -13,15 +13,22 @@ main = do
        (Just x) -> putStrLn $ "day1b: " ++ (show x)
 
 solveA :: [Integer] -> Maybe Integer
-solveA [] = Nothing
-solveA xs = Just $ (\(x,y) -> x * y) lsa
-  where
-    lsa = if lsa' == [] then error "no answer" else head lsa'
-    lsa' = [ (x,y) | x <- xs, y <- xs, x + y == 2020 ]
+solveA = solveX (\(x,y) -> x * y)
+                (\xs -> [ (x,y) | x <- xs
+                                , y <- xs
+                                , x + y == 2020 ])
 
 solveB :: [Integer] -> Maybe Integer
-solveB [] = Nothing
-solveB xs = Just $ (\(x,y,z) -> x * y * z) lsb
+solveB = solveX (\(x,y,z) -> x * y * z)
+                (\xs -> [ (x,y,z) | x <- xs
+                                  , y <- xs
+                                  , z <- xs
+                                  , x + y + z == 2020 ])
+
+solveX :: Eq a => (a -> Integer) -> ([Integer] -> [a]) -> [Integer] -> Maybe Integer
+solveX _ _ [] = Nothing
+solveX f g l = if ls == []
+               then Nothing
+               else Just $ (f . head) ls
   where
-    lsb = if lsb' == [] then error "no answer" else head lsb'
-    lsb' = [ (x,y,z) | x <- xs, y <- xs, z <- xs, x + y + z == 2020 ]
+    ls = g l