diff --git a/hs/src/Aoc/Parse.hs b/hs/src/Aoc/Parse.hs
index 7af5eb7..512bd7b 100644
--- a/hs/src/Aoc/Parse.hs
+++ b/hs/src/Aoc/Parse.hs
@@ -4,10 +4,13 @@ module Aoc.Parse
   , module Text.Megaparsec.Char.Lexer
   , Parser
   , manyLines
+  , oneSpace
+  , untilSpace
   , untilEol
   , lineChar
   ) where
 
+import           Data.Char
 import           Data.Void
 
 import qualified Data.Text                  as T
@@ -22,8 +25,14 @@ type Parser = Parsec Void T.Text
 manyLines :: Parser a -> Parser [a]
 manyLines p = endBy (try p) newline
 
+oneSpace :: Parser Char
+oneSpace = label "whitespace character" $ satisfy isSpace
+
+untilSpace :: Parser T.Text
+untilSpace = takeWhileP (Just "non-whitespace character") (not . isSpace)
+
 untilEol :: Parser T.Text
-untilEol = takeWhileP Nothing (/= '\n')
+untilEol = takeWhileP (Just "non-newline character") (/= '\n')
 
 lineChar :: Parser Char
-lineChar = satisfy (/= '\n')
+lineChar = label "non-newline character" $ satisfy (/= '\n')
diff --git a/hs/src/Aoc/Y2020.hs b/hs/src/Aoc/Y2020.hs
index 42c0ef3..bd854dc 100644
--- a/hs/src/Aoc/Y2020.hs
+++ b/hs/src/Aoc/Y2020.hs
@@ -6,10 +6,12 @@ import           Aoc.Day
 import qualified Aoc.Y2020.D01 as D01
 import qualified Aoc.Y2020.D02 as D02
 import qualified Aoc.Y2020.D03 as D03
+import qualified Aoc.Y2020.D04 as D04
 
 days :: [Day]
 days =
   [ D01.day
   , D02.day
   , D03.day
+  , D04.day
   ]
diff --git a/hs/src/Aoc/Y2020/D04.hs b/hs/src/Aoc/Y2020/D04.hs
new file mode 100644
index 0000000..d3f4a47
--- /dev/null
+++ b/hs/src/Aoc/Y2020/D04.hs
@@ -0,0 +1,170 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Aoc.Y2020.D04
+  ( day
+  ) where
+
+import           Control.Monad
+import           Data.Char
+import qualified Data.Text     as T
+import           Data.Void
+
+import           Aoc.Day
+import           Aoc.Parse
+
+data Field a = None | Invalid T.Text | Valid a
+  deriving (Show)
+
+isPresent :: Field a -> Bool
+isPresent None        = False
+isPresent (Invalid _) = True
+isPresent (Valid _)   = True
+
+isValid :: Field a -> Bool
+isValid None        = False
+isValid (Invalid _) = False
+isValid (Valid _)   = True
+
+instance Semigroup (Field a) where
+  _         <> Valid a   = Valid a
+  Valid a   <> _         = Valid a
+  _         <> Invalid t = Invalid t
+  Invalid t <> _         = Invalid t
+  None      <> None      = None
+
+instance Monoid (Field a) where
+  mempty = None
+
+data Height = Cm Int | In Int
+  deriving (Show)
+
+data Passport = Passport
+  { byr :: Field Int
+  , iyr :: Field Int
+  , eyr :: Field Int
+  , hgt :: Field Height
+  , hcl :: Field T.Text
+  , ecl :: Field T.Text
+  , pid :: Field Int
+  , cid :: Field Void
+  } deriving (Show)
+
+instance Semigroup Passport where
+  p1 <> p2 = Passport
+    { byr = byr p1 <> byr p2
+    , iyr = iyr p1 <> iyr p2
+    , eyr = eyr p1 <> eyr p2
+    , hgt = hgt p1 <> hgt p2
+    , hcl = hcl p1 <> hcl p2
+    , ecl = ecl p1 <> ecl p2
+    , pid = pid p1 <> pid p2
+    , cid = cid p1 <> cid p2
+    }
+
+instance Monoid Passport where
+  mempty = Passport mempty mempty mempty mempty mempty mempty mempty mempty
+
+pField :: T.Text -> Parser a -> Parser (Field a)
+pField name p = do
+  notFollowedBy oneSpace
+  void $ string name
+  void $ char ':'
+  (Valid <$> try p) <|> (Invalid <$> untilSpace)
+
+nDigits :: Int -> Parser Int
+nDigits n = do
+  digits <- takeWhileP (Just "digit") isDigit
+  guard $ T.length digits == n
+  pure $ read $ T.unpack digits
+
+nDigitsBetween :: Int -> Int -> Int -> Parser Int
+nDigitsBetween n low high = do
+  i <- nDigits n
+  guard $ low <= i && i <= high
+  pure i
+
+pByr :: Parser Passport
+pByr = do
+  f <- pField "byr" $ nDigitsBetween 4 1920 2002
+  pure mempty{byr = f}
+
+pIyr :: Parser Passport
+pIyr = do
+  f <- pField "iyr" $ nDigitsBetween 4 2010 2020
+  pure mempty{iyr = f}
+
+pEyr :: Parser Passport
+pEyr = do
+  f <- pField "eyr" $ nDigitsBetween 4 2020 2030
+  pure mempty{eyr = f}
+
+pHgt :: Parser Passport
+pHgt = do
+  f <- pField "hgt" $ do
+    i <- decimal
+    (string "cm" >> guard (150 <= i && i <= 193) >> pure (Cm i)) <|> (string "in" >> guard (59 <= i && i <= 76) >> pure (In i))
+  pure mempty{hgt = f}
+
+pHcl :: Parser Passport
+pHcl = do
+  f <- pField "hcl" $ do
+    void $ char '#'
+    t <- untilSpace
+    guard $ T.length t == 6 && T.all isHexDigit t
+    pure t
+  pure mempty{hcl = f}
+
+pEcl :: Parser Passport
+pEcl = do
+  f <- pField "ecl" $ foldr1 (<|>) ["amb", "blu", "brn", "gry", "grn", "hzl", "oth"]
+  pure mempty{ecl = f}
+
+pPid :: Parser Passport
+pPid = do
+  f <- pField "pid" $ nDigits 9
+  pure mempty{pid = f}
+
+pCid :: Parser Passport
+pCid = do
+  f <- pField "cid" $ fail "void"
+  pure mempty{cid = f}
+
+parser :: Parser [Passport]
+parser = passport `sepBy` newline
+  where
+    passport = mconcat <$> field `endBy1` oneSpace
+    field = pByr <|> pIyr <|> pEyr <|> pHgt <|> pHcl <|> pEcl <|> pPid <|> pCid
+
+hasRequiredKeys :: Passport -> Bool
+hasRequiredKeys p = and
+  [ isPresent $ byr p
+  , isPresent $ iyr p
+  , isPresent $ eyr p
+  , isPresent $ hgt p
+  , isPresent $ hcl p
+  , isPresent $ ecl p
+  , isPresent $ pid p
+  ]
+
+hasValidKeys :: Passport -> Bool
+hasValidKeys p = and
+  [ isValid $ byr p
+  , isValid $ iyr p
+  , isValid $ eyr p
+  , isValid $ hgt p
+  , isValid $ hcl p
+  , isValid $ ecl p
+  , isValid $ pid p
+  ]
+
+solver :: [Passport] -> IO ()
+solver passports = do
+  putStrLn ">> Part 1"
+  print $ length $ filter hasRequiredKeys passports
+
+  putStrLn ""
+  putStrLn ">> Part 2"
+  print $ length $ filter hasValidKeys passports
+
+day :: Day
+day = dayParse "2020_04" parser solver