Solve day 1, part 2

Main.hs

@@ -6,7 +6,9 @@ day1 :: IO ()
 day1 = do
   r <- readFile "./input/day1"
   putStr "[Day 1-1] fix: "
-  print . Day1.fixExpenseReport . fmap read . lines $ r
+  print . Day1.fixExpenseReport 2020 2 . fmap read . lines $ r
+  putStr "[Day 1-2] fix: "
+  print . Day1.fixExpenseReport 2020 3 . fmap read . lines $ r
 
 
 main :: IO ()

src/Day1.hs

@@ -3,17 +3,23 @@ module Day1 (fixExpenseReport) where
 import qualified Data.IntSet as DIS
 import Data.Maybe (mapMaybe)
 
-yearToFind :: Int
+type Target = Int
-yearToFind = 2020
+type Expenses = [Int]
+type Expenses' = DIS.IntSet
 
 -- Turn the list into an IntSet. For each element in the set, look up the
 -- number in the set to make it 2020. If found, return the multiplication of
 -- the two numbers.
-fixExpenseReport :: [Int] -> Int
+fixExpenseReport :: Target -> Int -> Expenses -> Maybe Int
-fixExpenseReport expenses =
+fixExpenseReport t c es
-    let s = DIS.fromList expenses in
+    | c <= 0 = error "unsupported"
-        head $ mapMaybe (`findOther` s) expenses
+    | otherwise = fixExpenseReport' t c (DIS.fromList es)
 
-findOther :: Int -> DIS.IntSet -> Maybe Int
+fixExpenseReport' :: Target -> Int -> Expenses' -> Maybe Int
-findOther n s = let n' = yearToFind - n in
+fixExpenseReport' t 1 es = if DIS.member t es then Just t else Nothing
-    if DIS.member n' s then Just (n*n') else Nothing
+fixExpenseReport' t c es =
+    let foo x = (*x) <$> fixExpenseReport' (t-x) (c-1) (DIS.delete x es)
+        bar = mapMaybe foo (DIS.toList es) in
+    case bar of
+        [] -> Nothing
+        (x:_) -> Just x