diff --git a/2015/Day21.hs b/2015/Day21.hs
index f2136b0..01f8482 100644
--- a/2015/Day21.hs
+++ b/2015/Day21.hs
@@ -2,16 +2,22 @@ module Main where
 
 import Utilities
 
+-- Input processing
+
+type Input = PlayerState
+
 data PlayerState = Player {
     hit_points :: Int,
     damage_score :: Int,
     armor_score :: Int
     }
 
-parse :: String -> PlayerState
-parse s = Player h d a
-  where
-    [h, d, a] = readNumbers s
+parse :: String -> Input
+parse s = case readNumbers s of
+    [h, d, a] -> Player h d a
+    _ -> error "bad input"
+
+-- Part One
 
 data Item = Item {
     name :: String,
diff --git a/2018/Day22.hs b/2018/Day22.hs
index f08f880..b1edd20 100644
--- a/2018/Day22.hs
+++ b/2018/Day22.hs
@@ -10,11 +10,12 @@ import Data.Array
 type Input = (Int, Position)
 
 parse :: String -> Input
-parse s = (runParser depth ld, runParser target lt)
+parse s = case lines s of
+    [ld, lt] -> (runParser depth ld, runParser target lt)
+    _ -> error "bad input"
   where
     depth = string "depth: " *> nat
     target = (,) <$ string "target: " <*> nat <* char ',' <*> nat
-    [ld, lt] = lines s
 
 type Position = (Int, Int)
 
diff --git a/2018/Day24.hs b/2018/Day24.hs
index f40a314..d745458 100644
--- a/2018/Day24.hs
+++ b/2018/Day24.hs
@@ -46,16 +46,16 @@ data AttackType = Fire | Cold | Slashing | Bludgeoning | Radiation
   deriving (Show, Eq, Enum, Bounded)
 
 parse :: String -> Input
-parse s =
-    State {
-        immuneSystem =
-            makeArmy (map (runParser unit_group) (tail immune_system_lines)),
-        infection =
-            makeArmy (map (runParser unit_group) (tail infection_lines))
-    }
+parse s = case paragraphs s of
+    [p1, p2] ->
+        State {
+            immuneSystem =
+                makeArmy (map (runParser unit_group) (tail (lines p1))),
+            infection =
+                makeArmy (map (runParser unit_group) (tail (lines p2)))
+        }
+    _ -> error "bad input"
   where
-    ls = lines s
-    (immune_system_lines, "":infection_lines) = span (not . null) ls
     unit_group =
         Group <$> nat <* string " units each with " <*> unit_type
     unit_type =
diff --git a/2019/Day03.hs b/2019/Day03.hs
index 3e5dd02..f41728c 100644
--- a/2019/Day03.hs
+++ b/2019/Day03.hs
@@ -19,9 +19,10 @@ data Direction = R | U | L | D
     deriving (Bounded, Enum, Show)
 
 parse :: String -> Input
-parse s = (w1, w2)
+parse s = case lines s of
+    [l1, l2] -> (runParser wire l1, runParser wire l2)
+    _ -> error "bad input"
   where
-    [w1, w2] = map (runParser wire) (lines s)
     wire = sepBy1 segment (char ',')
     segment = Segment <$> enumValue <*> nat
 
diff --git a/2020/Day16.hs b/2020/Day16.hs
index b218984..2d13446 100644
--- a/2020/Day16.hs
+++ b/2020/Day16.hs
@@ -27,17 +27,19 @@ data Range = Range Int Int
 type Ticket = [Int]
 
 parse :: String -> Input
-parse s = Input {
-    field_rules = map (runParser rule) p1,
-    your_ticket = runParser ticket (p2!!1),
-    others = map (runParser ticket) (tail p3)
-    }
+parse s = case map lines (paragraphs s) of
+    [p1, p2, p3] ->
+        Input {
+            field_rules = map (runParser rule) p1,
+            your_ticket = runParser ticket (p2!!1),
+            others = map (runParser ticket) (tail p3)
+        }
+    _ -> error "bad input"
   where
     rule = (,) <$> some (satisfy (/= ':')) <* string ": " <*> ranges
     ranges = (,) <$> range <* string " or " <*> range
     range = Range <$> nat <* char '-' <*> nat
     ticket = sepBy1 nat (char ',')
-    [p1, p2, p3] = map lines (paragraphs s)
 
 -- Part One
 
diff --git a/2020/Day20.hs b/2020/Day20.hs
index 0d1bad5..cd558b7 100644
--- a/2020/Day20.hs
+++ b/2020/Day20.hs
@@ -21,9 +21,10 @@ parse :: String -> Input
 parse = Map.fromList . map parseCamera . paragraphs
 
 parseCamera :: String -> (Int, Tile)
-parseCamera s = (runParser tileno header, parseTile rest)
+parseCamera s = case lines s of
+    header:rest -> (runParser tileno header, parseTile rest)
+    _ -> error "bad input"
   where
-    header:rest = lines s
     tileno = string "Tile " *> nat <* char ':'
 
 parseTile :: [String] -> Tile
diff --git a/2020/Day22.hs b/2020/Day22.hs
index 5eb78e2..7a69942 100644
--- a/2020/Day22.hs
+++ b/2020/Day22.hs
@@ -14,10 +14,11 @@ type Decks = (Deck, Deck)
 type Deck = Seq Int
 
 parse :: String -> Input
-parse s = (numbers p1, numbers p2)
+parse s = case paragraphs s of
+    [p1, p2] -> (numbers p1, numbers p2)
+    _ -> error "bad input"
   where
     numbers = Seq.fromList . map read . tail . lines
-    [p1, p2] = paragraphs s
 
 -- Part One
 
diff --git a/2020/Day25.hs b/2020/Day25.hs
index 536c683..e87b211 100644
--- a/2020/Day25.hs
+++ b/2020/Day25.hs
@@ -8,9 +8,9 @@ import Number
 type Input = (Int, Int)
 
 parse :: String -> Input
-parse s = (x, y)
-  where
-    x:y:_ = map read (lines s)
+parse s = case map read (lines s) of
+    [x, y] -> (x, y)
+    _ -> error "bad input"
 
 -- Part One
 
@@ -28,9 +28,9 @@ base = 7
 --
 -- find the value c ^ log_d = base ^ (log_d*log_c) = d ^ log_c.
 solve1 :: Input -> Int
-solve1 (d, c) = modularPower modulus c log_d
-  where
-    Just log_d = modularLogarithm modulus base d
+solve1 (d, c) = case modularLogarithm modulus base d of
+    Just log_d -> modularPower modulus c log_d
+    Nothing -> error "no solution"
 
 testInput :: String
 testInput = "\
diff --git a/2021/Day13.hs b/2021/Day13.hs
index 8f36a5f..c42eb9a 100644
--- a/2021/Day13.hs
+++ b/2021/Day13.hs
@@ -14,14 +14,15 @@ data Fold = FoldX Int | FoldY Int
 type Input = ([Position], [Fold])
 
 parse :: String -> Input
-parse s =
-    (map (runParser pos) (lines sp), map (runParser fold_instr) (lines sf))
+parse s = case paragraphs s of
+    [sp, sf] ->
+        (map (runParser pos) (lines sp), map (runParser fold_instr) (lines sf))
+    _ -> error "bad input"
   where
     pos = Position <$> nat <* char ',' <*> nat
     fold_instr =
         FoldX <$ string "fold along x="  <*> nat <|>
         FoldY <$ string "fold along y="  <*> nat
-    [sp, sf] = paragraphs s
 
 -- Part One
 
diff --git a/2021/Day20.hs b/2021/Day20.hs
index 5ed6c4f..23fa8d1 100644
--- a/2021/Day20.hs
+++ b/2021/Day20.hs
@@ -13,12 +13,13 @@ type Pixels = Set Position
 type Input = (Algorithm, Pixels)
 
 parse :: String -> Input
-parse s = (algo, pixels)
-  where
-    [p1, p2] = paragraphs s
-    algo = Set.fromList
-        [n | (n, c) <- zip [0..] (filter (/= '\n') p1), c == '#']
-    pixels = Set.fromList [p | (p, c) <- readGrid p2, c == '#']
+parse s = case paragraphs s of
+    [p1, p2] -> (algo, pixels)
+      where
+        algo = Set.fromList
+            [n | (n, c) <- zip [0..] (filter (/= '\n') p1), c == '#']
+        pixels = Set.fromList [p | (p, c) <- readGrid p2, c == '#']
+    _ -> error "bad input"
 
 -- Part One
 
diff --git a/2021/Day21.hs b/2021/Day21.hs
index aa2a67b..3f7f689 100644
--- a/2021/Day21.hs
+++ b/2021/Day21.hs
@@ -11,9 +11,10 @@ import qualified Data.Map as Map
 type Input = (Int, Int)
 
 parse :: String -> Input
-parse s = (runParser startPos l1, runParser startPos l2)
+parse s = case lines s of
+    [l1, l2] -> (runParser startPos l1, runParser startPos l2)
+    _ -> error "bad input"
   where
-    [l1, l2] = lines s
     startPos = string "Player " *> digit *> string " starting position: " *> nat
 
 -- Part One
diff --git a/2022/Day22.hs b/2022/Day22.hs
index 838e68c..a714278 100644
--- a/2022/Day22.hs
+++ b/2022/Day22.hs
@@ -26,15 +26,17 @@ data FieldMap = FieldMap {
     deriving (Show)
 
 parse :: String -> Input
-parse s = (field_map, runParser path path_line)
+parse s = case paragraphs s of
+    [map_text, path_text] -> (field_map, runParser path path_line)
+      where
+        field_map = FieldMap {
+            open = Set.fromList [p | (p, c) <- pcs, c == '.'],
+            walls = Set.fromList [p | (p, c) <- pcs, c == '#']
+            }
+        pcs = readGrid map_text
+        path_line = head (lines path_text)
+    _ -> error "bad input"
   where
-    field_map = FieldMap {
-        open = Set.fromList [p | (p, c) <- pcs, c == '.'],
-        walls = Set.fromList [p | (p, c) <- pcs, c == '#']
-        }
-    pcs = readGrid map_text
-    [map_text, path_text] = paragraphs s
-    path_line = head (lines path_text)
     path = (,) <$> nat <*> many ((,) <$> enumValue <*> nat)
 
 -- Part One