diff --git a/app/Main.hs b/app/Main.hs
index 8e366f2..c4ce311 100644
--- a/app/Main.hs
+++ b/app/Main.hs
@@ -1,4 +1,108 @@
+{-# LANGUAGE BinaryLiterals #-}
+{-# LANGUAGE DataKinds #-}
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE MultiParamTypeClasses #-}
+{-# LANGUAGE OverloadedRecordDot #-}
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE RankNTypes #-}
+{-# LANGUAGE ScopedTypeVariables #-}
+{-# LANGUAGE TypeOperators #-}
+
 module Main (main) where
 
+import Data.Foldable (for_)
+import Data.List (intercalate)
+import Data.Maybe (fromMaybe, maybeToList)
+import Data.Text (Text)
+import Data.Word (Word8)
+import Network.HTTP.Media ((//), (/:))
+import Network.Wai (Application)
+import Servant
+  ( Accept (..)
+  , Handler
+  , MimeRender (..)
+  , Server
+  , ServerError (..)
+  , Proxy (..)
+  , Capture
+  , Get
+  , err400
+  , serve
+  , throwError
+  , (:>)
+  )
+import Text.Blaze.Html5 ((!))
+import Text.Blaze.Html.Renderer.Utf8 (renderHtml)
+import Text.Printf (PrintfArg, printf)
+
+import qualified Network.Wai.Handler.Warp as Warp
+import qualified Text.Blaze.Html5 as H
+import qualified Text.Blaze.Html5.Attributes as A
+import qualified Unicode.Char.General.Blocks as UnicodeBlocks
+import qualified Unicode.Char.General.Names as UnicodeNames
+
+import qualified UToy.Decode as Decode
+import qualified UToy.Parsers as Parsers
+
 main :: IO ()
-main = putStrLn "Hello, World!"
+main = Warp.run 3000 app
+
+app :: Application
+app = serve (Proxy :: Proxy API) server
+
+type API =
+  "hex" :> Capture "bytes" Text :> Get '[HTML] HexModel
+
+server :: Server API
+server =
+  hex
+  where
+    hex bytesP = do
+      bytes <- Parsers.parseHexBytes bytesP `orThrow` const err400
+      pure $ HexModel $ Decode.decodeUtf8 bytes
+
+newtype HexModel = HexModel
+  { codepoints :: [([Word8], Either String Char)]
+  }
+
+instance MimeRender HTML HexModel where
+  mimeRender _ model = renderHtml $ H.docTypeHtml $ do
+    H.head $ do
+      H.meta ! A.charset "utf-8"
+      H.title "utoy"
+      H.style "html { font-size: 32px; font-family: 'Noto Sans', sans-serif; } td { padding: 0.5em 1em; } pre { margin: 0; font-size: 0.5em; } body { display: flex; justify-content: center; }"
+    H.body $ do
+      H.table $ for_ model.codepoints $ \(bytes, eiC) -> do
+        H.tr $ do
+          H.td $ H.pre $ H.toHtml $ unlines $ map unwords [map showByteHex bytes, map showByteBin bytes]
+          case eiC of
+            Left err ->
+              H.td ! A.colspan "4" $ H.toHtml $ "Decoding error: " ++ err
+            Right c -> do
+              H.td $ do
+                H.input ! A.value (H.toValue [c]) ! A.style "text-align: center; width: 2em; font-size: 1em;"
+              H.td $ H.code $ printfHtml "U+%04X" c
+              H.td $ H.code $ H.toHtml $ intercalate ", " $ maybeToList (UnicodeNames.name c) ++ map (++ "*") (UnicodeNames.nameAliases c)
+              H.td $ H.toHtml $ fromMaybe "" $ blockName c
+    where
+      showByteHex = printf "      %02X"
+      showByteBin = printf "%8b"
+
+      blockName :: Char -> Maybe String
+      blockName c = UnicodeBlocks.blockName . UnicodeBlocks.blockDefinition <$> UnicodeBlocks.block c
+
+-- Utilities
+
+orThrow :: Either a b -> (a -> ServerError) -> Handler b
+orThrow (Left err) f = throwError $ f err
+orThrow (Right val) _ = pure val
+
+printfHtml :: PrintfArg a => String -> a -> H.Html
+printfHtml fmt = (H.toHtml :: String -> H.Html) . printf fmt
+
+-- HTML routes
+
+data HTML
+
+instance Accept HTML where
+  contentType _ = "text" // "html" /: ("charset", "utf-8")
diff --git a/hie.yaml b/hie.yaml
index af9c93d..f68809f 100644
--- a/hie.yaml
+++ b/hie.yaml
@@ -1,7 +1,10 @@
 cradle:
   stack:
-    - path: "./test/Spec.hs"
-      component: "utoy:test:utoy-test"
+    - path: "./src"
+      component: "utoy:lib"
 
     - path: "./app/Main.hs"
       component: "utoy:exe:utoy-exe"
+
+    - path: "./test"
+      component: "utoy:test:utoy-test"
diff --git a/nix/haskell-deps.nix b/nix/haskell-deps.nix
index 7edd5e4..f9524a1 100644
--- a/nix/haskell-deps.nix
+++ b/nix/haskell-deps.nix
@@ -1,4 +1,12 @@
 haskellPackages: with haskellPackages; [
+  attoparsec
+  blaze-html
+  bytestring
+  http-media
+  servant-server
+  text
   unicode-data
   unicode-data-names
+  wai
+  warp
 ]
diff --git a/nix/pkgs.nix b/nix/pkgs.nix
index de9cae8..48dbd09 100644
--- a/nix/pkgs.nix
+++ b/nix/pkgs.nix
@@ -14,7 +14,11 @@ let
   };
 
   haskellOverlay = pkgs: final: prev: {
-    unicode-data = prev.unicode-data_0_4_0;
+    attoparsec-iso8601 = prev.attoparsec-iso8601_1_1_0_0;
+    http-api-data = prev.http-api-data_0_5;
+    servant = pkgs.haskell.lib.doJailbreak prev.servant;
+    servant-server = pkgs.haskell.lib.doJailbreak prev.servant-server;
+    unicode-data = prev.unicode-data_0_4_0_1;
     unicode-data-names = pkgs.haskell.lib.markUnbroken prev.unicode-data-names;
   };
 in
diff --git a/nix/sources.json b/nix/sources.json
index d486d47..f7baa89 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -12,15 +12,15 @@
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "nixpkgs": {
-        "branch": "nixos-22.11",
+        "branch": "nixpkgs-unstable",
         "description": "Nix Packages collection",
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "7076110064c09f0b3942f609f2134c1358ef2e50",
-        "sha256": "0j7idx8vyb0spwxqb7rr8pk15wi7yfyf5hp608wkhaz7wjw8k9nf",
+        "rev": "9952d6bc395f5841262b006fbace8dd7e143b634",
+        "sha256": "0kvpf63dda6nzbqd2kyr99qh1av89mva26xykp3zb4diyicp7yji",
         "type": "tarball",
-        "url": "https://github.com/NixOS/nixpkgs/archive/7076110064c09f0b3942f609f2134c1358ef2e50.tar.gz",
+        "url": "https://github.com/NixOS/nixpkgs/archive/9952d6bc395f5841262b006fbace8dd7e143b634.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }
diff --git a/package.yaml b/package.yaml
index a798d03..5c081c5 100644
--- a/package.yaml
+++ b/package.yaml
@@ -13,6 +13,8 @@ extra-source-files:
 
 dependencies:
 - base >= 4.7 && < 5
+- attoparsec
+- text
 
 ghc-options:
 - -Wall
@@ -38,6 +40,15 @@ executables:
     - -with-rtsopts=-N
     dependencies:
     - utoy
+    - blaze-html
+    - bytestring
+    - http-media
+    - servant-server
+    - text
+    - unicode-data
+    - unicode-data-names
+    - wai
+    - warp
     # Fix "Multiple files use the same module name", see
     # https://stackoverflow.com/questions/67519851/multiple-files-use-the-same-module-name
     when:
diff --git a/shell.nix b/shell.nix
index 33ce29d..e95d7f9 100644
--- a/shell.nix
+++ b/shell.nix
@@ -11,8 +11,9 @@ let
 
     # haskell tools
     stack
-    (pkgs.haskell.packages."${settings.ghc}".ghcWithPackages haskellDeps)
-    pkgs.haskell.packages."${settings.ghc}".haskell-language-server
+    (haskellPackages.ghcWithPackages haskellDeps)
+    haskellPackages.haskell-language-server
+    haskellPackages.implicit-hie
   ];
 in
   pkgs.mkShellNoCC {
diff --git a/src/UToy/.Decode.hs.swp b/src/UToy/.Decode.hs.swp
new file mode 100644
index 0000000..fb7123f
Binary files /dev/null and b/src/UToy/.Decode.hs.swp differ
diff --git a/src/UToy/Decode.hs b/src/UToy/Decode.hs
new file mode 100644
index 0000000..7fbd7d5
--- /dev/null
+++ b/src/UToy/Decode.hs
@@ -0,0 +1,64 @@
+{-# LANGUAGE BinaryLiterals #-}
+{-# LANGUAGE LambdaCase #-}
+{-# LANGUAGE NumericUnderscores #-}
+
+module UToy.Decode
+  ( decodeUtf8
+  ) where
+
+import Data.Bits (shiftL, (.&.), (.|.))
+import Data.Char (chr)
+import Data.Word (Word8)
+import Text.Printf (printf)
+
+decodeUtf8 :: [Word8] -> [([Word8], Either String Char)]
+decodeUtf8 = \case
+  (cu0:rest)
+    | isAscii cu0 ->
+      ([cu0], Right $ chr $ fromIntegral cu0) : decodeUtf8 rest
+  (cu0:cu1:rest)
+    | isTwoByteStarter cu0 && isContinuation cu1 ->
+      ( [cu0, cu1]
+      , Right $ chr $
+        fromIntegral (cu0 .&. 0b0001_1111) `shiftL` 6
+        .|. fromIntegral (cu1 .&. 0b0011_1111)
+      ) : decodeUtf8 rest
+  (cu0:cu1:cu2:rest)
+    | isThreeByteStarter cu0 && isContinuation cu1 && isContinuation cu2 ->
+      ( [cu0, cu1, cu2]
+      , Right $ chr $
+        fromIntegral (cu0 .&. 0b0000_1111) `shiftL` 12
+        .|. fromIntegral (cu1 .&. 0b0011_1111) `shiftL` 6
+        .|. fromIntegral (cu2 .&. 0b0011_1111)
+      ) : decodeUtf8 rest
+  (cu0:cu1:cu2:cu3:rest)
+    | isFourByteStarter cu0 && isContinuation cu1 && isContinuation cu2 && isContinuation cu3 ->
+      ( [cu0, cu1, cu2, cu3]
+      , let
+          codepoint = 
+            fromIntegral (cu0 .&. 0b0000_0111) `shiftL` 18
+            .|. fromIntegral (cu1 .&. 0b0011_1111) `shiftL` 12
+            .|. fromIntegral (cu2 .&. 0b0011_1111) `shiftL` 6
+            .|. fromIntegral (cu3 .&. 0b0011_1111)
+        in
+          if codepoint > 0x10_ffff then
+            Left $ printf "Code point U+%X would be too big (maximum: U+10FFFF)" codepoint
+          else
+            Right $ chr codepoint
+            
+      ) : decodeUtf8 rest
+  (cu0:rest) ->
+    ( [cu0]
+    , Left "Invalid start of code point"
+    ) : decodeUtf8 rest
+  [] ->
+    []
+
+  where
+    isAscii cu = cu .&. 0b1000_0000 == 0b0000_0000
+
+    isTwoByteStarter cu = cu .&. 0b1110_0000 == 0b1100_0000
+    isThreeByteStarter cu = cu .&. 0b1111_0000 == 0b1110_0000
+    isFourByteStarter cu = cu .&. 0b1111_1000 == 0b1111_0000
+
+    isContinuation cu = cu .&. 0b1100_0000 == 0b1000_0000
diff --git a/src/UToy/Parsers.hs b/src/UToy/Parsers.hs
new file mode 100644
index 0000000..9b65b4a
--- /dev/null
+++ b/src/UToy/Parsers.hs
@@ -0,0 +1,31 @@
+module UToy.Parsers
+  ( parseHexBytes
+  ) where
+
+import Data.Char (isHexDigit, ord)
+import Data.Text (Text)
+import Data.Word (Word8)
+import Text.Printf (printf)
+
+import qualified Data.Attoparsec.Text as Atto
+
+parseHexBytes :: Text -> Either String [Word8]
+parseHexBytes = Atto.parseOnly $ hexBytes <* Atto.endOfInput
+
+hexBytes :: Atto.Parser [Word8]
+hexBytes = hexByte `Atto.sepBy` separators
+  where
+    hexByte = do
+      high <- hexDigit
+      low <- hexDigit
+      pure $ fromIntegral $ high * 16 + low
+
+    hexDigit = hexDigitToInt <$> Atto.satisfy isHexDigit
+
+    hexDigitToInt c
+      | '0' <= c && c <= '9' = ord c - ord '0'
+      | 'A' <= c && c <= 'F' = ord c - ord 'A' + 10
+      | 'a' <= c && c <= 'f' = ord c - ord 'a' + 10
+      | otherwise = error $ printf "not a hex digit: %c" c
+    
+    separators = Atto.skipMany $ Atto.satisfy $ Atto.inClass " +."
diff --git a/utoy.cabal b/utoy.cabal
index 9712630..4af2fa6 100644
--- a/utoy.cabal
+++ b/utoy.cabal
@@ -1,6 +1,6 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.35.0.
+-- This file has been generated from package.yaml by hpack version 0.35.2.
 --
 -- see: https://github.com/sol/hpack
 
@@ -20,13 +20,18 @@ source-repository head
   location: https://git.pbrinkmeier.de/paul/utoy
 
 library
+  exposed-modules:
+      UToy.Decode
+      UToy.Parsers
   other-modules:
       Paths_utoy
   hs-source-dirs:
       src
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints
   build-depends:
-      base >=4.7 && <5
+      attoparsec
+    , base >=4.7 && <5
+    , text
   default-language: Haskell2010
 
 executable utoy-exe
@@ -35,8 +40,18 @@ executable utoy-exe
       app
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
   build-depends:
-      base >=4.7 && <5
+      attoparsec
+    , base >=4.7 && <5
+    , blaze-html
+    , bytestring
+    , http-media
+    , servant-server
+    , text
+    , unicode-data
+    , unicode-data-names
     , utoy
+    , wai
+    , warp
   default-language: Haskell2010
 
 test-suite utoy-test
@@ -48,6 +63,8 @@ test-suite utoy-test
       test
   ghc-options: -Wall -Wcompat -Widentities -Wincomplete-record-updates -Wincomplete-uni-patterns -Wmissing-export-lists -Wmissing-home-modules -Wpartial-fields -Wredundant-constraints -threaded -rtsopts -with-rtsopts=-N
   build-depends:
-      base >=4.7 && <5
+      attoparsec
+    , base >=4.7 && <5
+    , text
     , utoy
   default-language: Haskell2010