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diff --git a/lib/Language/Haskell/Stylish/Config/Cabal.hs b/lib/Language/Haskell/Stylish/Config/Cabal.hs
new file mode 100644
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+++ b/lib/Language/Haskell/Stylish/Config/Cabal.hs
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+--------------------------------------------------------------------------------
+module Language.Haskell.Stylish.Config.Cabal
+    ( findLanguageExtensions
+    ) where
+
+
+--------------------------------------------------------------------------------
+import           Data.Either                              (isRight)
+import           Data.List                                (nub)
+import           Data.Maybe                               (maybeToList)
+import qualified Distribution.PackageDescription          as Cabal
+import qualified Distribution.PackageDescription.Parsec   as Cabal
+import qualified Distribution.Simple.Utils                as Cabal
+import qualified Distribution.Types.CondTree              as Cabal
+import qualified Distribution.Verbosity                   as Cabal
+import qualified Language.Haskell.Extension               as Language
+import           Language.Haskell.Stylish.Verbose
+import           System.Directory                         (getCurrentDirectory)
+
+
+--------------------------------------------------------------------------------
+import           Language.Haskell.Stylish.Config.Internal
+
+
+--------------------------------------------------------------------------------
+findLanguageExtensions :: Verbose -> IO [Language.KnownExtension]
+findLanguageExtensions verbose =
+    findCabalFile verbose >>=
+    maybe (pure []) (readDefaultLanguageExtensions verbose)
+
+
+--------------------------------------------------------------------------------
+-- | Find the closest .cabal file, possibly going up the directory structure.
+findCabalFile :: Verbose -> IO (Maybe FilePath)
+findCabalFile verbose = do
+  potentialProjectRoots <- ancestors <$> getCurrentDirectory
+  potentialCabalFile <- filter isRight <$>
+    traverse Cabal.findPackageDesc potentialProjectRoots
+  case potentialCabalFile of
+    [Right cabalFile] -> return (Just cabalFile)
+    _ -> do
+      verbose $ ".cabal file not found, directories searched: " <>
+        show potentialProjectRoots
+      verbose $ "Stylish Haskell will work basing on LANGUAGE pragmas in source files."
+      return Nothing
+
+
+--------------------------------------------------------------------------------
+-- | Extract @default-extensions@ fields from a @.cabal@ file
+readDefaultLanguageExtensions :: Verbose -> FilePath -> IO [Language.KnownExtension]
+readDefaultLanguageExtensions verbose cabalFile = do
+  verbose $ "Parsing " <> cabalFile <> "..."
+  packageDescription <- Cabal.readGenericPackageDescription Cabal.silent cabalFile
+  let library :: [Cabal.Library]
+      library = maybeToList $ fst . Cabal.ignoreConditions <$>
+        Cabal.condLibrary packageDescription
+
+      subLibraries :: [Cabal.Library]
+      subLibraries = fst . Cabal.ignoreConditions . snd <$>
+        Cabal.condSubLibraries packageDescription
+
+      executables :: [Cabal.Executable]
+      executables = fst . Cabal.ignoreConditions . snd <$>
+        Cabal.condExecutables packageDescription
+
+      testSuites :: [Cabal.TestSuite]
+      testSuites = fst . Cabal.ignoreConditions . snd <$>
+        Cabal.condTestSuites packageDescription
+
+      benchmarks :: [Cabal.Benchmark]
+      benchmarks = fst . Cabal.ignoreConditions . snd <$>
+        Cabal.condBenchmarks packageDescription
+
+      gatherBuildInfos :: [Cabal.BuildInfo]
+      gatherBuildInfos = map Cabal.libBuildInfo library <>
+                         map Cabal.libBuildInfo subLibraries <>
+                         map Cabal.buildInfo executables <>
+                         map Cabal.testBuildInfo testSuites <>
+                         map Cabal.benchmarkBuildInfo benchmarks
+
+      defaultExtensions :: [Language.KnownExtension]
+      defaultExtensions = map fromEnabled . filter isEnabled $
+        concatMap Cabal.defaultExtensions gatherBuildInfos
+        where isEnabled (Language.EnableExtension _) = True
+              isEnabled _                            = False
+
+              fromEnabled (Language.EnableExtension x) = x
+              fromEnabled x                             =
+                error $ "Language.Haskell.Stylish.Config.readLanguageExtensions: " <>
+                        "invalid LANGUAGE pragma:  " <> show x
+  verbose $ "Gathered default-extensions: " <> show defaultExtensions
+  pure $ nub defaultExtensions