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diff --git a/tests/Language/Haskell/Stylish/Config/Tests.hs b/tests/Language/Haskell/Stylish/Config/Tests.hs
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+++ b/tests/Language/Haskell/Stylish/Config/Tests.hs
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+module Language.Haskell.Stylish.Config.Tests
+    ( tests
+    ) where
+
+
+--------------------------------------------------------------------------------
+import           Control.Exception               hiding (assert)
+import qualified Data.Set                        as Set
+import           System.Directory
+import           System.FilePath                 ((</>))
+import           System.IO.Error
+import           System.Random
+import           Test.Framework                  (Test, testGroup)
+import           Test.Framework.Providers.HUnit  (testCase)
+import           Test.HUnit                      (Assertion, assert)
+--------------------------------------------------------------------------------
+import           Language.Haskell.Stylish.Config
+
+--------------------------------------------------------------------------------
+tests :: Test
+tests = testGroup "Language.Haskell.Stylish.Config"
+    [ testCase "Extensions extracted correctly from .cabal file"
+               testExtensionsFromDotCabal
+    , testCase "Extensions extracted correctly from .stylish-haskell.yaml file"
+               testExtensionsFromDotStylish
+    , testCase "Extensions extracted correctly from .stylish-haskell.yaml and .cabal files"
+               testExtensionsFromBoth
+    ]
+--------------------------------------------------------------------------------
+
+-- | Create a temporary directory with a randomised name built from the template provided
+createTempDirectory :: String -> IO FilePath
+createTempDirectory template  = do
+  tmpRootDir <- getTemporaryDirectory
+  dirId <- randomIO :: IO Word
+  findTempName tmpRootDir dirId
+  where
+    findTempName :: FilePath -> Word -> IO FilePath
+    findTempName tmpRootDir x = do
+      let dirpath = tmpRootDir </> template ++ show x
+      r <- try $ createDirectory dirpath
+      case r of
+        Right _ -> return dirpath
+        Left  e | isAlreadyExistsError e -> findTempName tmpRootDir (x+1)
+                | otherwise              -> ioError e
+
+-- | Perform an action inside a temporary directory tree and purge the tree afterwords
+withTestDirTree :: IO a -> IO a
+withTestDirTree action = bracket
+    ((,) <$> getCurrentDirectory <*> createTempDirectory "stylish_haskell")
+    (\(current, temp) ->
+        setCurrentDirectory current *>
+        removeDirectoryRecursive temp)
+    (\(_, temp) -> setCurrentDirectory temp *> action)
+
+-- | Put an example config files (.cabal/.stylish-haskell.yaml/both)
+-- into the current directory and extract extensions from it.
+createFilesAndGetExtensions :: [(FilePath, String)] -> IO Extensions
+createFilesAndGetExtensions files = withTestDirTree $ do
+  mapM_ (\(k, v) -> writeFile k v) files
+  -- create an empty directory and change into it
+  createDirectory "src"
+  setCurrentDirectory "src"
+  -- from that directory read the config file and extract extensions
+  -- to make sure the search for .cabal file works
+  config <- loadConfig (const (pure ())) Nothing
+  pure $ configLanguageExtensions config
+
+--------------------------------------------------------------------------------
+testExtensionsFromDotCabal :: Assertion
+testExtensionsFromDotCabal =
+  assert $ (expected ==) . Set.fromList <$>
+    createFilesAndGetExtensions [("test.cabal", dotCabal True)]
+    where
+      expected = Set.fromList ["ScopedTypeVariables", "DataKinds"]
+
+--------------------------------------------------------------------------------
+testExtensionsFromDotStylish :: Assertion
+testExtensionsFromDotStylish =
+  assert $ (expected ==) . Set.fromList <$>
+    createFilesAndGetExtensions [(".stylish-haskell.yaml", dotStylish)]
+    where
+      expected = Set.fromList ["TemplateHaskell", "QuasiQuotes"]
+
+--------------------------------------------------------------------------------
+testExtensionsFromBoth :: Assertion
+testExtensionsFromBoth =
+  assert $ (expected ==) . Set.fromList <$>
+    createFilesAndGetExtensions [ ("test.cabal", dotCabal True)
+                                , (".stylish-haskell.yaml", dotStylish)]
+    where
+      expected = Set.fromList
+        ["ScopedTypeVariables", "DataKinds", "TemplateHaskell", "QuasiQuotes"]
+
+-- | Example cabal file borrowed from
+--   https://www.haskell.org/cabal/users-guide/developing-packages.html
+--   with some default-extensions added
+dotCabal :: Bool -> String
+dotCabal includeExtensions = unlines $
+  [ "name:            TestPackage"
+  , "version:         0.0"
+  , "synopsis:        Package with library and two programs"
+  , "license:         BSD3"
+  , "author:          Angela Author"
+  , "build-type:      Simple"
+  , "cabal-version:   >= 1.2"
+  , ""
+  , "library"
+  , "   build-depends:   HUnit"
+  , "   exposed-modules: A, B, C"
+  ] ++
+  [if includeExtensions then "   default-extensions: ScopedTypeVariables"
+                        else ""]
+  ++
+  [ ""
+  , "executable program1"
+  , "   main-is:         Main.hs"
+  , "   hs-source-dirs:  prog1"
+  , "   other-modules:   A, B"
+  ] ++
+  [if includeExtensions then "   default-extensions: DataKinds"
+                        else ""]
+
+-- | Example .stylish-haskell.yaml
+dotStylish :: String
+dotStylish = unlines $
+  [ "steps:"
+  , "  - imports:"
+  , "      align: none"
+  , "      list_align: after_alias"
+  , "      long_list_align: inline"
+  , "      separate_lists: true"
+  , "  - language_pragmas:"
+  , "      style: vertical"
+  , "      align: false"
+  , "      remove_redundant: true"
+  , "  - trailing_whitespace: {}"
+  , "columns: 110"
+  , "language_extensions:"
+  , "  - TemplateHaskell"
+  , "  - QuasiQuotes"
+  ]