added nicer error

day1.hs

@@ -16,10 +16,12 @@ solveA :: [Integer] -> Maybe Integer
 solveA [] = Nothing
 solveA xs = Just $ (\(x,y) -> x * y) lsa
   where
-    lsa = head [ (x,y) | x <- xs, y <- xs, x + y == 2020 ]
+    lsa = if lsa' == [] then error "no answer" else head lsa'
+    lsa' = [ (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
   where
-    lsb = head [ (x,y,z) | x <- xs, y <- xs, z <- xs, x + y + z == 2020 ]
+    lsb = if lsb' == [] then error "no answer" else head lsb'
+    lsb' = [ (x,y,z) | x <- xs, y <- xs, z <- xs, x + y + z == 2020 ]