diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b0dca47..61b10b3 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -118,3 +118,16 @@ jobs:
     - name: Test
       run: |
         stack test --system-ghc
+
+  hlint:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+
+    - name: Run HLint
+      env:
+         HLINT_VERSION: "3.2.7"
+      run: |
+        curl -L https://github.com/ndmitchell/hlint/releases/download/v${HLINT_VERSION}/hlint-${HLINT_VERSION}-x86_64-linux.tar.gz --output hlint.tar.gz
+        tar -xvf hlint.tar.gz
+        ./hlint-${HLINT_VERSION}/hlint src/ test/
diff --git a/.hlint.yaml b/.hlint.yaml
new file mode 100644
index 0000000..111320d
--- /dev/null
+++ b/.hlint.yaml
@@ -0,0 +1,76 @@
+# HLint configuration file
+# https://github.com/ndmitchell/hlint
+##########################
+
+# This file contains a template configuration file, which is typically
+# placed as .hlint.yaml in the root of your project
+
+
+# Warnings currently triggered by your code
+- ignore: {name: "Parse error: lexical error at character 'A'"}
+- ignore: {name: "Use newtype instead of data"}
+- ignore: {name: "Redundant lambda"}
+- ignore: {name: "Redundant <$>"}
+- ignore: {name: "Use section"}
+- ignore: {name: "Monoid law, right identity"}
+- ignore: {name: "Monoid law, left identity"}
+
+
+# Specify additional command line arguments
+#
+# - arguments: [--color, --cpp-simple, -XQuasiQuotes]
+
+
+# Control which extensions/flags/modules/functions can be used
+#
+# - extensions:
+#   - default: false # all extension are banned by default
+#   - name: [PatternGuards, ViewPatterns] # only these listed extensions can be used
+#   - {name: CPP, within: CrossPlatform} # CPP can only be used in a given module
+#
+# - flags:
+#   - {name: -w, within: []} # -w is allowed nowhere
+#
+# - modules:
+#   - {name: [Data.Set, Data.HashSet], as: Set} # if you import Data.Set qualified, it must be as 'Set'
+#   - {name: Control.Arrow, within: []} # Certain modules are banned entirely
+#
+# - functions:
+#   - {name: unsafePerformIO, within: []} # unsafePerformIO can only appear in no modules
+
+
+# Add custom hints for this project
+#
+# Will suggest replacing "wibbleMany [myvar]" with "wibbleOne myvar"
+# - error: {lhs: "wibbleMany [x]", rhs: wibbleOne x}
+
+# The hints are named by the string they display in warning messages.
+# For example, if you see a warning starting like
+#
+# Main.hs:116:51: Warning: Redundant ==
+#
+# You can refer to that hint with `{name: Redundant ==}` (see below).
+
+# Turn on hints that are off by default
+#
+# Ban "module X(module X) where", to require a real export list
+# - warn: {name: Use explicit module export list}
+#
+# Replace a $ b $ c with a . b $ c
+# - group: {name: dollar, enabled: true}
+#
+# Generalise map to fmap, ++ to <>
+# - group: {name: generalise, enabled: true}
+
+
+# Ignore some builtin hints
+# - ignore: {name: Use let}
+# - ignore: {name: Use const, within: SpecialModule} # Only within certain modules
+
+
+# Define some custom infix operators
+# - fixity: infixr 3 ~^#^~
+
+
+# To generate a suitable file for HLint do:
+# $ hlint --default > .hlint.yaml
diff --git a/README.md b/README.md
index 3839e0d..a47bf26 100644
--- a/README.md
+++ b/README.md
@@ -230,6 +230,14 @@ make it unique:
    Haskell types and `htoml*` libraries use typeclasses-based approach
    via `aeson` library.
 
+## Development
+
+`tomland` uses `hlint`, you can run it through:
+
+```
+hlint .
+```
+
 ## Acknowledgement
 
 Icons made by [Freepik](http://www.freepik.com) from [www.flaticon.com](https://www.flaticon.com/) is licensed by [CC 3.0 BY](http://creativecommons.org/licenses/by/3.0/).
diff --git a/src/Toml/Codec/BiMap/Conversion.hs b/src/Toml/Codec/BiMap/Conversion.hs
index d14221b..6425745 100644
--- a/src/Toml/Codec/BiMap/Conversion.hs
+++ b/src/Toml/Codec/BiMap/Conversion.hs
@@ -315,7 +315,7 @@ _ByteStringText :: TomlBiMap ByteString Text
 _ByteStringText = prism T.encodeUtf8 eitherText
   where
     eitherText :: ByteString -> Either TomlBiMapError Text
-    eitherText = either (\err -> Left $ ArbitraryError $ tShow err) Right . T.decodeUtf8'
+    eitherText = either (Left . ArbitraryError . tShow) Right . T.decodeUtf8'
 {-# INLINE _ByteStringText #-}
 
 {- | UTF-8 encoded 'ByteString' 'BiMap' for 'AnyValue'.
diff --git a/src/Toml/Codec/Generic.hs b/src/Toml/Codec/Generic.hs
index 1d6f40f..313f930 100644
--- a/src/Toml/Codec/Generic.hs
+++ b/src/Toml/Codec/Generic.hs
@@ -6,7 +6,6 @@
 {-# LANGUAGE PolyKinds            #-}
 {-# LANGUAGE Rank2Types           #-}
 {-# LANGUAGE TypeOperators        #-}
-{-# LANGUAGE TypeSynonymInstances #-}
 {-# LANGUAGE UndecidableInstances #-}
 
 {- |
diff --git a/src/Toml/Codec/Types.hs b/src/Toml/Codec/Types.hs
index ca112d0..c9c4970 100644
--- a/src/Toml/Codec/Types.hs
+++ b/src/Toml/Codec/Types.hs
@@ -119,8 +119,8 @@ instance Applicative (Codec i) where
 instance Alternative (Codec i) where
     empty :: Codec i o
     empty = Codec
-        { codecRead  = \_ -> empty
-        , codecWrite = \_ -> empty
+        { codecRead  = const empty
+        , codecWrite = const empty
         }
     {-# INLINE empty #-}
 
@@ -212,7 +212,7 @@ instance (s ~ TOML) => MonadState s TomlState where
     {-# INLINE get #-}
 
     put :: TOML -> TomlState ()
-    put t = TomlState (\_ -> (Just (), t))
+    put t = TomlState (const (Just (), t))
     {-# INLINE put #-}
 
 {- | Transform 'Either' into 'TomlState'.
diff --git a/src/Toml/Parser/Value.hs b/src/Toml/Parser/Value.hs
index fd64ad7..6d6d770 100644
--- a/src/Toml/Parser/Value.hs
+++ b/src/Toml/Parser/Value.hs
@@ -43,16 +43,16 @@ decimalP = do
         case value of
           Left _ -> do
             try more
-          Right _ -> 
+          Right _ ->
             fail "Leading zero."
 
   where
     leadingZeroP :: Parser String
     leadingZeroP = do
-               count 1 (char '0') >>= (\_ -> some digitChar)
+               count 1 (char '0') >> some digitChar
 
     more :: Parser Integer
-    more  = check =<< readMaybe . concat <$> sepBy1 (some digitChar) (char '_')
+    more  = check . readMaybe . concat =<< sepBy1 (some digitChar) (char '_')
 
     check :: Maybe Integer -> Parser Integer
     check = maybe (fail "Not an integer") pure
@@ -64,7 +64,7 @@ numberP :: Parser Integer -> Parser Char -> String -> Parser Integer
 numberP parseInteger parseDigit errorMessage = more
   where
     more :: Parser Integer
-    more = check =<< intValueMaybe . concat <$> sepBy1 (some parseDigit) (char '_')
+    more = check . intValueMaybe . concat =<< sepBy1 (some parseDigit) (char '_')
 
     intValueMaybe :: String -> Maybe Integer
     intValueMaybe = parseMaybe parseInteger . fromString
diff --git a/src/Toml/Type/PrefixTree.hs b/src/Toml/Type/PrefixTree.hs
index 31cb72c..12519fd 100644
--- a/src/Toml/Type/PrefixTree.hs
+++ b/src/Toml/Type/PrefixTree.hs
@@ -155,7 +155,7 @@ lookupT lk (Branch pref mv prefMap) =
     case keysDiff pref lk of
         Equal       -> mv
         NoPrefix    -> Nothing
-        Diff _ _ _  -> Nothing
+        Diff {}     -> Nothing
         SndIsPref _ -> Nothing
         FstIsPref k -> lookup k prefMap
 
@@ -185,7 +185,7 @@ toListT (Leaf k v) = [(k, v)]
 toListT (Branch pref ma prefMap) = case ma of
     Just a  -> (:) (pref, a)
     Nothing -> id
-    $ map (\(k, v) -> (pref <> k, v)) $ toList prefMap
+    $ map (first ((<>) pref)) $ toList prefMap
 
 {- | Converts 'PrefixMap' to the list of pairs.
 
@@ -224,14 +224,14 @@ differenceWithT f pt1 pt2 = case (pt1, pt2) of
             [(_, aNew)] -> Just $ addPrefixT k aNew
             -- shouldn't happen, but for some reasons
             _ : _ : _ -> Nothing
-        Diff _ _ _ -> Just l
+        Diff {} -> Just l
 
     (br@(Branch p ma pma), Leaf k b) -> case keysDiff p k of
         Equal -> compressTree $ Branch p (ma >>= \a -> f a b) pma
         NoPrefix -> Just br
         FstIsPref kSuf -> compressTree $ Branch p ma (differenceWith f pma $ single kSuf b)
         SndIsPref _ -> Just br
-        Diff _ _ _ -> Just br
+        Diff {} -> Just br
 
     (b1@(Branch p1 ma pma), Branch p2 mb pmb) -> case keysDiff p1 p2 of
         Equal -> compressTree $
@@ -242,4 +242,4 @@ differenceWithT f pt1 pt2 = case (pt1, pt2) of
         SndIsPref p1Suf@(p1Head :|| _) -> case HashMap.lookup p1Head pmb of
             Nothing -> Just b1
             Just ch -> addPrefixT p2 <$> differenceWithT f (Branch p1Suf ma pma) ch
-        Diff _ _ _ -> Just b1
+        Diff {} -> Just b1