diff --git a/LICENSE b/LICENSE
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--- /dev/null
+++ b/LICENSE
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+              GNU GENERAL PUBLIC LICENSE
+                Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <http://fsf.org/>
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+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+              END OF TERMS AND CONDITIONS
+
+     How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<http://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<http://www.gnu.org/philosophy/why-not-lgpl.html>.
diff --git a/cabal-fmt.cabal b/cabal-fmt.cabal
new file mode 100644
--- /dev/null
+++ b/cabal-fmt.cabal
@@ -0,0 +1,103 @@
+cabal-version:      2.2
+name:               cabal-fmt
+version:            0.1
+synopsis:           Format .cabal files
+category:           Development
+description:
+  Format @.cabal@ files preserving the original ordering and comments.
+  .
+  Tuned for Oleg's preference, but has some knobs still.
+
+license:            GPL-3.0-or-later
+license-file:       LICENSE
+author:             Oleg Grenrus <oleg.grenrus@iki.fi>
+maintainer:         Oleg Grenrus <oleg.grenrus@iki.fi>
+tested-with:        GHC ==8.4.4 || ==8.6.5
+extra-source-files:
+  fixtures/*.cabal
+  fixtures/*.format
+
+source-repository head
+  type:     git
+  location: https://github.com/phadej/cabal-fmt.git
+
+library cabal-fmt-internal
+  default-language: Haskell2010
+  hs-source-dirs:   src
+
+  -- GHC boot libraries
+  build-depends:
+    , base        ^>=4.11.1.0 || ^>=4.12.0.0
+    , bytestring  ^>=0.10.8.2
+    , Cabal       ^>=3.0.0.0
+    , containers  ^>=0.5.11.0 || ^>=0.6.0.1
+    , filepath    ^>=1.4.2
+    , mtl         ^>=2.2.2
+    , parsec      ^>=3.1.13.0
+    , pretty      ^>=1.1.3.6
+
+  -- cabal-fmt: expand src
+  --
+  -- Note: the module list is expanded only when cabal-fmt is run from a
+  -- command line, with a single file. This is to get right relative
+  -- working directory.
+  --
+  exposed-modules:
+    CabalFmt
+    CabalFmt.Comments
+    CabalFmt.Error
+    CabalFmt.Fields
+    CabalFmt.Fields.BuildDepends
+    CabalFmt.Fields.Extensions
+    CabalFmt.Fields.Modules
+    CabalFmt.Fields.TestedWith
+    CabalFmt.Monad
+    CabalFmt.Options
+    CabalFmt.Parser
+    CabalFmt.Refactoring
+
+  other-extensions:
+    DeriveFunctor
+    DerivingStrategies
+    ExistentialQuantification
+    FlexibleContexts
+    GeneralizedNewtypeDeriving
+    OverloadedStrings
+    RankNTypes
+
+executable cabal-fmt
+  default-language: Haskell2010
+  hs-source-dirs:   cli
+  main-is:          Main.hs
+
+  -- dependencies in library
+  build-depends:
+    , base
+    , bytestring
+    , cabal-fmt-internal
+    , filepath
+
+  -- extra dependencies
+  build-depends:
+    , directory             ^>=1.3.1.5
+    , optparse-applicative  >=0.14.3.0 && <0.16
+
+test-suite golden
+  type:             exitcode-stdio-1.0
+  default-language: Haskell2010
+  hs-source-dirs:   tests
+  main-is:          Golden.hs
+
+  -- dependencies in library
+  build-depends:
+    , base
+    , bytestring
+    , Cabal
+    , cabal-fmt-internal
+
+  -- test dependencies
+  build-depends:
+    , filepath
+    , process
+    , tasty
+    , tasty-golden
diff --git a/cli/Main.hs b/cli/Main.hs
new file mode 100644
--- /dev/null
+++ b/cli/Main.hs
@@ -0,0 +1,108 @@
+module Main (main) where
+
+import Control.Applicative (many, (<**>))
+import Data.Foldable       (for_)
+import System.Directory    (doesDirectoryExist, getDirectoryContents)
+import System.Exit         (exitFailure)
+import System.FilePath     (takeDirectory, (</>))
+import System.IO.Unsafe    (unsafeInterleaveIO)
+
+import qualified Data.ByteString     as BS
+import qualified Options.Applicative as O
+
+import CabalFmt         (cabalFmt)
+import CabalFmt.Error   (renderError)
+import CabalFmt.Monad   (runCabalFmt)
+import CabalFmt.Options
+
+main :: IO ()
+main = do
+    (inplace, opts', filepaths) <- O.execParser optsP'
+
+    -- glob all files, only when a single filepath is given.
+    files <- case filepaths of
+        [fp] -> getFiles (takeDirectory fp)
+        _    -> return []
+    let opts = opts' { optFileList = files }
+
+    case filepaths of
+        []    -> BS.getContents       >>= main' False opts "<stdin>"
+        (_:_) -> for_ filepaths $ \filepath ->
+            BS.readFile filepath >>= main' inplace opts filepath
+  where
+    optsP' = O.info (optsP <**> O.helper) $ mconcat
+        [ O.fullDesc
+        , O.progDesc "Reformat .cabal files"
+        , O.header "cabal-fmt - .cabal file reformatter"
+        ]
+
+main' :: Bool -> Options -> FilePath -> BS.ByteString -> IO ()
+main' inplace opts filepath input =
+    case runCabalFmt opts (cabalFmt filepath input) of
+        Right output
+            | inplace   -> writeFile filepath output
+            | otherwise -> putStr output
+        Left err     -> do
+            renderError err
+            exitFailure
+
+-------------------------------------------------------------------------------
+-- Options parser
+-------------------------------------------------------------------------------
+
+optsP :: O.Parser (Bool, Options, [FilePath])
+optsP = (,,)
+    <$> O.flag False True (O.short 'i' <> O.long "inplace" <> O.help "process files in-place")
+    <*> optsP'
+    <*> many (O.strArgument (O.metavar "FILE" <> O.help "input files"))
+  where
+    optsP' = Options
+        <$> O.option O.auto (O.long "indent" <> O.value (optIndent defaultOptions) <> O.help "Indentation" <> O.showDefault)
+        <*> pure (optSpecVersion defaultOptions)
+        <*> pure []
+
+-------------------------------------------------------------------------------
+-- Files
+-------------------------------------------------------------------------------
+
+getFiles :: FilePath -> IO [FilePath]
+getFiles = getDirectoryContentsRecursive' check where
+    check "dist-newstyle" = False
+    check ('.' : _)       = False
+    check _               = True
+
+-- | List all the files in a directory and all subdirectories.
+--
+-- The order places files in sub-directories after all the files in their
+-- parent directories. The list is generated lazily so is not well defined if
+-- the source directory structure changes before the list is used.
+--
+-- /Note:/ From @Cabal@'s "Distribution.Simple.Utils"
+getDirectoryContentsRecursive'
+    :: (FilePath -> Bool) -- ^ Check, whether to recurse
+    -> FilePath           -- ^ top dir
+    -> IO [FilePath]
+getDirectoryContentsRecursive' ignore' topdir = recurseDirectories [""]
+  where
+    recurseDirectories :: [FilePath] -> IO [FilePath]
+    recurseDirectories []         = return []
+    recurseDirectories (dir:dirs) = unsafeInterleaveIO $ do
+      (files, dirs') <- collect [] [] =<< getDirectoryContents (topdir </> dir)
+      files' <- recurseDirectories (dirs' ++ dirs)
+      return (files ++ files')
+
+      where
+        collect files dirs' []              = return (reverse files
+                                                     ,reverse dirs')
+        collect files dirs' (entry:entries) | ignore entry
+                                            = collect files dirs' entries
+        collect files dirs' (entry:entries) = do
+          let dirEntry = dir </> entry
+          isDirectory <- doesDirectoryExist (topdir </> dirEntry)
+          if isDirectory
+            then collect files (dirEntry:dirs') entries
+            else collect (dirEntry:files) dirs' entries
+
+        ignore ['.']      = True
+        ignore ['.', '.'] = True
+        ignore x          = not (ignore' x)
diff --git a/fixtures/Cabal.cabal b/fixtures/Cabal.cabal
new file mode 100644
--- /dev/null
+++ b/fixtures/Cabal.cabal
@@ -0,0 +1,652 @@
+name:          Cabal
+version:       2.5.0.0
+copyright:     2003-2018, Cabal Development Team (see AUTHORS file)
+license:       BSD3
+license-file:  LICENSE
+author:        Cabal Development Team <cabal-devel@haskell.org>
+maintainer:    cabal-devel@haskell.org
+homepage:      http://www.haskell.org/cabal/
+bug-reports:   https://github.com/haskell/cabal/issues
+synopsis:      A framework for packaging Haskell software
+description:
+  The Haskell Common Architecture for Building Applications and
+  Libraries: a framework defining a common interface for authors to more
+  easily build their Haskell applications in a portable way.
+  .
+  The Haskell Cabal is part of a larger infrastructure for distributing,
+  organizing, and cataloging Haskell libraries and tools.
+category:       Distribution
+cabal-version:  >=1.10
+build-type:     Simple
+-- If we use a new Cabal feature, this needs to be changed to Custom so
+-- we can bootstrap.
+
+extra-source-files:
+  README.md tests/README.md ChangeLog.md
+  doc/bugs-and-stability.rst doc/concepts-and-development.rst
+  doc/conf.py doc/config-and-install.rst doc/developing-packages.rst
+  doc/images/Cabal-dark.png doc/index.rst doc/installing-packages.rst
+  doc/intro.rst doc/misc.rst doc/nix-local-build-overview.rst
+  doc/nix-local-build.rst doc/file-format-changelog.rst doc/README.md
+  doc/references.inc
+
+  -- Generated with 'make gen-extra-source-files'
+  -- Do NOT edit this section manually; instead, run the script.
+  -- BEGIN gen-extra-source-files
+  tests/ParserTests/errors/common1.cabal
+  tests/ParserTests/errors/common1.errors
+  tests/ParserTests/errors/common2.cabal
+  tests/ParserTests/errors/common2.errors
+  tests/ParserTests/errors/common3.cabal
+  tests/ParserTests/errors/common3.errors
+  tests/ParserTests/errors/forward-compat.cabal
+  tests/ParserTests/errors/forward-compat.errors
+  tests/ParserTests/errors/forward-compat2.cabal
+  tests/ParserTests/errors/forward-compat2.errors
+  tests/ParserTests/errors/forward-compat3.cabal
+  tests/ParserTests/errors/forward-compat3.errors
+  tests/ParserTests/errors/issue-5055-2.cabal
+  tests/ParserTests/errors/issue-5055-2.errors
+  tests/ParserTests/errors/issue-5055.cabal
+  tests/ParserTests/errors/issue-5055.errors
+  tests/ParserTests/errors/leading-comma-2.cabal
+  tests/ParserTests/errors/leading-comma-2.errors
+  tests/ParserTests/errors/leading-comma-2b.cabal
+  tests/ParserTests/errors/leading-comma-2b.errors
+  tests/ParserTests/errors/leading-comma-2c.cabal
+  tests/ParserTests/errors/leading-comma-2c.errors
+  tests/ParserTests/errors/leading-comma.cabal
+  tests/ParserTests/errors/leading-comma.errors
+  tests/ParserTests/errors/multiple-libs.cabal
+  tests/ParserTests/errors/multiple-libs.errors
+  tests/ParserTests/errors/noVersion.cabal
+  tests/ParserTests/errors/noVersion.errors
+  tests/ParserTests/errors/noVersion2.cabal
+  tests/ParserTests/errors/noVersion2.errors
+  tests/ParserTests/errors/range-ge-wild.cabal
+  tests/ParserTests/errors/range-ge-wild.errors
+  tests/ParserTests/errors/removed-fields.cabal
+  tests/ParserTests/errors/removed-fields.errors
+  tests/ParserTests/errors/spdx-1.cabal
+  tests/ParserTests/errors/spdx-1.errors
+  tests/ParserTests/errors/spdx-2.cabal
+  tests/ParserTests/errors/spdx-2.errors
+  tests/ParserTests/errors/spdx-3.cabal
+  tests/ParserTests/errors/spdx-3.errors
+  tests/ParserTests/ipi/Includes2.cabal
+  tests/ParserTests/ipi/Includes2.expr
+  tests/ParserTests/ipi/Includes2.format
+  tests/ParserTests/ipi/internal-preprocessor-test.cabal
+  tests/ParserTests/ipi/internal-preprocessor-test.expr
+  tests/ParserTests/ipi/internal-preprocessor-test.format
+  tests/ParserTests/ipi/issue-2276-ghc-9885.cabal
+  tests/ParserTests/ipi/issue-2276-ghc-9885.expr
+  tests/ParserTests/ipi/issue-2276-ghc-9885.format
+  tests/ParserTests/ipi/transformers.cabal
+  tests/ParserTests/ipi/transformers.expr
+  tests/ParserTests/ipi/transformers.format
+  tests/ParserTests/regressions/MiniAgda.cabal
+  tests/ParserTests/regressions/MiniAgda.check
+  tests/ParserTests/regressions/Octree-0.5.cabal
+  tests/ParserTests/regressions/Octree-0.5.expr
+  tests/ParserTests/regressions/Octree-0.5.format
+  tests/ParserTests/regressions/bad-glob-syntax.cabal
+  tests/ParserTests/regressions/bad-glob-syntax.check
+  tests/ParserTests/regressions/cc-options-with-optimization.cabal
+  tests/ParserTests/regressions/cc-options-with-optimization.check
+  tests/ParserTests/regressions/common-conditional.cabal
+  tests/ParserTests/regressions/common-conditional.expr
+  tests/ParserTests/regressions/common-conditional.format
+  tests/ParserTests/regressions/common.cabal
+  tests/ParserTests/regressions/common.expr
+  tests/ParserTests/regressions/common.format
+  tests/ParserTests/regressions/common2.cabal
+  tests/ParserTests/regressions/common2.expr
+  tests/ParserTests/regressions/common2.format
+  tests/ParserTests/regressions/cxx-options-with-optimization.cabal
+  tests/ParserTests/regressions/cxx-options-with-optimization.check
+  tests/ParserTests/regressions/elif.cabal
+  tests/ParserTests/regressions/elif.expr
+  tests/ParserTests/regressions/elif.format
+  tests/ParserTests/regressions/elif2.cabal
+  tests/ParserTests/regressions/elif2.expr
+  tests/ParserTests/regressions/elif2.format
+  tests/ParserTests/regressions/encoding-0.8.cabal
+  tests/ParserTests/regressions/encoding-0.8.expr
+  tests/ParserTests/regressions/encoding-0.8.format
+  tests/ParserTests/regressions/extensions-paths-5054.cabal
+  tests/ParserTests/regressions/extensions-paths-5054.check
+  tests/ParserTests/regressions/generics-sop.cabal
+  tests/ParserTests/regressions/generics-sop.expr
+  tests/ParserTests/regressions/generics-sop.format
+  tests/ParserTests/regressions/ghc-option-j.cabal
+  tests/ParserTests/regressions/ghc-option-j.check
+  tests/ParserTests/regressions/haddock-api-2.18.1-check.cabal
+  tests/ParserTests/regressions/haddock-api-2.18.1-check.check
+  tests/ParserTests/regressions/hidden-main-lib.cabal
+  tests/ParserTests/regressions/hidden-main-lib.expr
+  tests/ParserTests/regressions/hidden-main-lib.format
+  tests/ParserTests/regressions/issue-5055.cabal
+  tests/ParserTests/regressions/issue-5055.expr
+  tests/ParserTests/regressions/issue-5055.format
+  tests/ParserTests/regressions/issue-774.cabal
+  tests/ParserTests/regressions/issue-774.check
+  tests/ParserTests/regressions/issue-774.expr
+  tests/ParserTests/regressions/issue-774.format
+  tests/ParserTests/regressions/jaeger-flamegraph.cabal
+  tests/ParserTests/regressions/jaeger-flamegraph.expr
+  tests/ParserTests/regressions/jaeger-flamegraph.format
+  tests/ParserTests/regressions/leading-comma-2.cabal
+  tests/ParserTests/regressions/leading-comma-2.expr
+  tests/ParserTests/regressions/leading-comma-2.format
+  tests/ParserTests/regressions/leading-comma.cabal
+  tests/ParserTests/regressions/leading-comma.expr
+  tests/ParserTests/regressions/leading-comma.format
+  tests/ParserTests/regressions/multiple-libs-2.cabal
+  tests/ParserTests/regressions/multiple-libs-2.check
+  tests/ParserTests/regressions/multiple-libs-2.expr
+  tests/ParserTests/regressions/multiple-libs-2.format
+  tests/ParserTests/regressions/noVersion.cabal
+  tests/ParserTests/regressions/noVersion.expr
+  tests/ParserTests/regressions/noVersion.format
+  tests/ParserTests/regressions/nothing-unicode.cabal
+  tests/ParserTests/regressions/nothing-unicode.check
+  tests/ParserTests/regressions/nothing-unicode.expr
+  tests/ParserTests/regressions/nothing-unicode.format
+  tests/ParserTests/regressions/pre-1.6-glob.cabal
+  tests/ParserTests/regressions/pre-1.6-glob.check
+  tests/ParserTests/regressions/pre-2.4-globstar.cabal
+  tests/ParserTests/regressions/pre-2.4-globstar.check
+  tests/ParserTests/regressions/shake.cabal
+  tests/ParserTests/regressions/shake.expr
+  tests/ParserTests/regressions/shake.format
+  tests/ParserTests/regressions/spdx-1.cabal
+  tests/ParserTests/regressions/spdx-1.expr
+  tests/ParserTests/regressions/spdx-1.format
+  tests/ParserTests/regressions/spdx-2.cabal
+  tests/ParserTests/regressions/spdx-2.expr
+  tests/ParserTests/regressions/spdx-2.format
+  tests/ParserTests/regressions/spdx-3.cabal
+  tests/ParserTests/regressions/spdx-3.expr
+  tests/ParserTests/regressions/spdx-3.format
+  tests/ParserTests/regressions/th-lift-instances.cabal
+  tests/ParserTests/regressions/th-lift-instances.expr
+  tests/ParserTests/regressions/th-lift-instances.format
+  tests/ParserTests/regressions/wl-pprint-indef.cabal
+  tests/ParserTests/regressions/wl-pprint-indef.expr
+  tests/ParserTests/regressions/wl-pprint-indef.format
+  tests/ParserTests/warnings/bom.cabal
+  tests/ParserTests/warnings/bool.cabal
+  tests/ParserTests/warnings/deprecatedfield.cabal
+  tests/ParserTests/warnings/doubledash.cabal
+  tests/ParserTests/warnings/extratestmodule.cabal
+  tests/ParserTests/warnings/gluedop.cabal
+  tests/ParserTests/warnings/multiplesingular.cabal
+  tests/ParserTests/warnings/nbsp.cabal
+  tests/ParserTests/warnings/newsyntax.cabal
+  tests/ParserTests/warnings/oldsyntax.cabal
+  tests/ParserTests/warnings/subsection.cabal
+  tests/ParserTests/warnings/tab.cabal
+  tests/ParserTests/warnings/trailingfield.cabal
+  tests/ParserTests/warnings/unknownfield.cabal
+  tests/ParserTests/warnings/unknownsection.cabal
+  tests/ParserTests/warnings/utf8.cabal
+  tests/ParserTests/warnings/versiontag.cabal
+  tests/hackage/check.sh
+  tests/hackage/download.sh
+  tests/hackage/unpack.sh
+  tests/misc/ghc-supported-languages.hs
+  -- END gen-extra-source-files
+
+source-repository head
+  type:     git
+  location: https://github.com/haskell/cabal/
+  subdir:   Cabal
+
+flag bundled-binary-generic
+  default: False
+
+library
+  build-depends:
+    array      >= 0.4.0.1  && < 0.6,
+    base       >= 4.6      && < 5,
+    bytestring >= 0.10.0.0 && < 0.11,
+    containers >= 0.5.0.0  && < 0.7,
+    deepseq    >= 1.3.0.1  && < 1.5,
+    directory  >= 1.2      && < 1.4,
+    filepath   >= 1.3.0.1  && < 1.5,
+    pretty     >= 1.1.1    && < 1.2,
+    process    >= 1.1.0.2  && < 1.7,
+    time       >= 1.4.0.1  && < 1.10
+
+  if flag(bundled-binary-generic)
+    build-depends: binary >= 0.5.1.1 && < 0.7
+  else
+    build-depends: binary >= 0.7 && < 0.9
+
+  if os(windows)
+    build-depends: Win32 >= 2.3.0.0 && < 2.9
+  else
+    build-depends: unix  >= 2.6.0.0 && < 2.8
+
+  ghc-options: -Wall -fno-ignore-asserts -fwarn-tabs
+  if impl(ghc >= 8.0)
+    ghc-options: -Wcompat -Wnoncanonical-monad-instances
+                 -Wnoncanonical-monadfail-instances
+
+  exposed-modules:
+    Distribution.Backpack
+    Distribution.Backpack.Configure
+    Distribution.Backpack.ComponentsGraph
+    Distribution.Backpack.ConfiguredComponent
+    Distribution.Backpack.DescribeUnitId
+    Distribution.Backpack.FullUnitId
+    Distribution.Backpack.LinkedComponent
+    Distribution.Backpack.ModSubst
+    Distribution.Backpack.ModuleShape
+    Distribution.Backpack.PreModuleShape
+    Distribution.CabalSpecVersion
+    Distribution.Utils.IOData
+    Distribution.Utils.LogProgress
+    Distribution.Utils.MapAccum
+    Distribution.Compat.CreatePipe
+    Distribution.Compat.Directory
+    Distribution.Compat.Environment
+    Distribution.Compat.Exception
+    Distribution.Compat.Graph
+    Distribution.Compat.Internal.TempFile
+    Distribution.Compat.Newtype
+    Distribution.Compat.Prelude.Internal
+    Distribution.Compat.Semigroup
+    Distribution.Compat.Stack
+    Distribution.Compat.Time
+    Distribution.Compat.DList
+    Distribution.Compiler
+    Distribution.InstalledPackageInfo
+    Distribution.Types.AbiDependency
+    Distribution.Types.ExposedModule
+    Distribution.Types.InstalledPackageInfo
+    Distribution.Types.InstalledPackageInfo.FieldGrammar
+    Distribution.License
+    Distribution.Make
+    Distribution.ModuleName
+    Distribution.Package
+    Distribution.PackageDescription
+    Distribution.PackageDescription.Check
+    Distribution.PackageDescription.Configuration
+    Distribution.PackageDescription.PrettyPrint
+    Distribution.PackageDescription.Utils
+    Distribution.ReadE
+    Distribution.Simple
+    Distribution.Simple.Bench
+    Distribution.Simple.Build
+    Distribution.Simple.Build.Macros
+    Distribution.Simple.Build.PathsModule
+    Distribution.Simple.BuildPaths
+    Distribution.Simple.BuildTarget
+    Distribution.Simple.BuildToolDepends
+    Distribution.Simple.CCompiler
+    Distribution.Simple.Command
+    Distribution.Simple.Compiler
+    Distribution.Simple.Configure
+    Distribution.Simple.Flag
+    Distribution.Simple.GHC
+    Distribution.Simple.GHCJS
+    Distribution.Simple.Haddock
+    Distribution.Simple.Doctest
+    Distribution.Simple.Glob
+    Distribution.Simple.HaskellSuite
+    Distribution.Simple.Hpc
+    Distribution.Simple.Install
+    Distribution.Simple.InstallDirs
+    Distribution.Simple.LocalBuildInfo
+    Distribution.Simple.PackageIndex
+    Distribution.Simple.PreProcess
+    Distribution.Simple.PreProcess.Unlit
+    Distribution.Simple.Program
+    Distribution.Simple.Program.Ar
+    Distribution.Simple.Program.Builtin
+    Distribution.Simple.Program.Db
+    Distribution.Simple.Program.Find
+    Distribution.Simple.Program.GHC
+    Distribution.Simple.Program.HcPkg
+    Distribution.Simple.Program.Hpc
+    Distribution.Simple.Program.Internal
+    Distribution.Simple.Program.Ld
+    Distribution.Simple.Program.ResponseFile
+    Distribution.Simple.Program.Run
+    Distribution.Simple.Program.Script
+    Distribution.Simple.Program.Strip
+    Distribution.Simple.Program.Types
+    Distribution.Simple.Register
+    Distribution.Simple.Setup
+    Distribution.Simple.SrcDist
+    Distribution.Simple.Test
+    Distribution.Simple.Test.ExeV10
+    Distribution.Simple.Test.LibV09
+    Distribution.Simple.Test.Log
+    Distribution.Simple.UHC
+    Distribution.Simple.UserHooks
+    Distribution.Simple.Utils
+    Distribution.SPDX
+    Distribution.SPDX.License
+    Distribution.SPDX.LicenseId
+    Distribution.SPDX.LicenseExceptionId
+    Distribution.SPDX.LicenseExpression
+    Distribution.SPDX.LicenseListVersion
+    Distribution.SPDX.LicenseReference
+    Distribution.System
+    Distribution.TestSuite
+    Distribution.Text
+    Distribution.Pretty
+    Distribution.Types.AbiHash
+    Distribution.Types.AnnotatedId
+    Distribution.Types.Benchmark
+    Distribution.Types.BenchmarkInterface
+    Distribution.Types.BenchmarkType
+    Distribution.Types.BuildInfo
+    Distribution.Types.BuildType
+    Distribution.Types.ComponentInclude
+    Distribution.Types.Dependency
+    Distribution.Types.ExeDependency
+    Distribution.Types.LegacyExeDependency
+    Distribution.Types.PkgconfigDependency
+    Distribution.Types.DependencyMap
+    Distribution.Types.ComponentId
+    Distribution.Types.MungedPackageId
+    Distribution.Types.PackageId
+    Distribution.Types.UnitId
+    Distribution.Types.Executable
+    Distribution.Types.ExecutableScope
+    Distribution.Types.Library
+    Distribution.Types.LibraryVisibility
+    Distribution.Types.ForeignLib
+    Distribution.Types.ForeignLibType
+    Distribution.Types.ForeignLibOption
+    Distribution.Types.Module
+    Distribution.Types.ModuleReexport
+    Distribution.Types.ModuleRenaming
+    Distribution.Types.ComponentName
+    Distribution.Types.LibraryName
+    Distribution.Types.MungedPackageName
+    Distribution.Types.PackageName
+    Distribution.Types.PkgconfigName
+    Distribution.Types.UnqualComponentName
+    Distribution.Types.IncludeRenaming
+    Distribution.Types.Mixin
+    Distribution.Types.SetupBuildInfo
+    Distribution.Types.TestSuite
+    Distribution.Types.TestSuiteInterface
+    Distribution.Types.TestType
+    Distribution.Types.GenericPackageDescription
+    Distribution.Types.Condition
+    Distribution.Types.CondTree
+    Distribution.Types.HookedBuildInfo
+    Distribution.Types.PackageDescription
+    Distribution.Types.SourceRepo
+    Distribution.Types.Component
+    Distribution.Types.ComponentLocalBuildInfo
+    Distribution.Types.LocalBuildInfo
+    Distribution.Types.ComponentRequestedSpec
+    Distribution.Types.TargetInfo
+    Distribution.Types.Version
+    Distribution.Types.VersionRange
+    Distribution.Types.VersionInterval
+    Distribution.Types.GivenComponent
+    Distribution.Types.PackageVersionConstraint
+    Distribution.Utils.Generic
+    Distribution.Utils.NubList
+    Distribution.Utils.ShortText
+    Distribution.Utils.Progress
+    Distribution.Verbosity
+    Distribution.Version
+    Language.Haskell.Extension
+    Distribution.Compat.Binary
+
+  -- Parsec parser-related modules
+  build-depends:
+    -- transformers-0.4.0.0 doesn't have record syntax e.g. for Identity
+    -- See also https://github.com/ekmett/transformers-compat/issues/35
+    transformers (>= 0.3      && < 0.4) || (>=0.4.1.0 && <0.6),
+    mtl           >= 2.1      && < 2.3,
+    text          >= 1.2.3.0  && < 1.3,
+    parsec        >= 3.1.13.0 && < 3.2
+  exposed-modules:
+    Distribution.Compat.Parsing
+    Distribution.Compat.CharParsing
+    Distribution.FieldGrammar
+    Distribution.FieldGrammar.Class
+    Distribution.FieldGrammar.FieldDescrs
+    Distribution.FieldGrammar.Parsec
+    Distribution.FieldGrammar.Pretty
+    Distribution.PackageDescription.FieldGrammar
+    Distribution.PackageDescription.Parsec
+    Distribution.PackageDescription.Quirks
+    Distribution.Parsec
+    Distribution.Parsec.Error
+    Distribution.Parsec.Newtypes
+    Distribution.Parsec.Position
+    Distribution.Parsec.Warning
+    Distribution.Parsec.FieldLineStream
+    Distribution.Fields
+    Distribution.Fields.ConfVar
+    Distribution.Fields.Field
+    Distribution.Fields.Lexer
+    Distribution.Fields.LexerMonad
+    Distribution.Fields.ParseResult
+    Distribution.Fields.Parser
+    Distribution.Fields.Pretty
+
+  -- Lens functionality
+  exposed-modules:
+    Distribution.Compat.Lens
+    Distribution.Types.Lens
+    Distribution.Types.Benchmark.Lens
+    Distribution.Types.BuildInfo.Lens
+    Distribution.Types.Executable.Lens
+    Distribution.Types.ForeignLib.Lens
+    Distribution.Types.GenericPackageDescription.Lens
+    Distribution.Types.InstalledPackageInfo.Lens
+    Distribution.Types.Library.Lens
+    Distribution.Types.PackageDescription.Lens
+    Distribution.Types.PackageId.Lens
+    Distribution.Types.SetupBuildInfo.Lens
+    Distribution.Types.SourceRepo.Lens
+    Distribution.Types.TestSuite.Lens
+
+  other-modules:
+    Distribution.Backpack.PreExistingComponent
+    Distribution.Backpack.ReadyComponent
+    Distribution.Backpack.MixLink
+    Distribution.Backpack.ModuleScope
+    Distribution.Backpack.UnifyM
+    Distribution.Backpack.Id
+    Distribution.Utils.UnionFind
+    Distribution.Utils.Base62
+    Distribution.Compat.CopyFile
+    Distribution.Compat.GetShortPathName
+    Distribution.Compat.MonadFail
+    Distribution.Compat.Prelude
+    Distribution.Compat.SnocList
+    Distribution.GetOpt
+    Distribution.Lex
+    Distribution.Utils.String
+    Distribution.Simple.GHC.EnvironmentParser
+    Distribution.Simple.GHC.Internal
+    Distribution.Simple.GHC.ImplInfo
+    Paths_Cabal
+
+  if flag(bundled-binary-generic)
+    other-modules:
+      Distribution.Compat.Binary.Class
+      Distribution.Compat.Binary.Generic
+
+  default-language: Haskell2010
+  other-extensions:
+    BangPatterns
+    CPP
+    DefaultSignatures
+    DeriveDataTypeable
+    DeriveFoldable
+    DeriveFunctor
+    DeriveGeneric
+    DeriveTraversable
+    ExistentialQuantification
+    FlexibleContexts
+    FlexibleInstances
+    GeneralizedNewtypeDeriving
+    ImplicitParams
+    KindSignatures
+    NondecreasingIndentation
+    OverloadedStrings
+    RankNTypes
+    RecordWildCards
+    ScopedTypeVariables
+    StandaloneDeriving
+    Trustworthy
+    TypeFamilies
+    TypeOperators
+    TypeSynonymInstances
+    UndecidableInstances
+
+  if impl(ghc >= 7.11)
+    other-extensions: PatternSynonyms
+
+-- Small, fast running tests.
+test-suite unit-tests
+  type: exitcode-stdio-1.0
+  hs-source-dirs: tests
+  other-modules:
+    Test.Laws
+    Test.QuickCheck.Utils
+    UnitTests.Distribution.Compat.CreatePipe
+    UnitTests.Distribution.Compat.Time
+    UnitTests.Distribution.Compat.Graph
+    UnitTests.Distribution.Simple.Glob
+    UnitTests.Distribution.Simple.Program.Internal
+    UnitTests.Distribution.Simple.Utils
+    UnitTests.Distribution.SPDX
+    UnitTests.Distribution.System
+    UnitTests.Distribution.Types.GenericPackageDescription
+    UnitTests.Distribution.Utils.Generic
+    UnitTests.Distribution.Utils.NubList
+    UnitTests.Distribution.Utils.ShortText
+    UnitTests.Distribution.Version
+  main-is: UnitTests.hs
+  build-depends:
+    array,
+    base,
+    bytestring,
+    containers,
+    directory,
+    filepath,
+    integer-logarithms >= 1.0.2 && <1.1,
+    tasty >= 1.1.0.3 && < 1.2,
+    tasty-hunit,
+    tasty-quickcheck,
+    tagged,
+    temporary,
+    text,
+    pretty,
+    QuickCheck >= 2.11.3 && < 2.12,
+    Cabal
+  ghc-options: -Wall
+  default-language: Haskell2010
+
+test-suite parser-tests
+  type: exitcode-stdio-1.0
+  hs-source-dirs: tests
+  main-is: ParserTests.hs
+  build-depends:
+    base,
+    base-compat >=0.10.4 && <0.11,
+    bytestring,
+    directory,
+    filepath,
+    tasty >= 1.1.0.3 && < 1.2,
+    tasty-hunit,
+    tasty-quickcheck,
+    tasty-golden >=2.3.1.1 && <2.4,
+    Diff >=0.3.4 && <0.4,
+    Cabal
+  ghc-options: -Wall
+  default-language: Haskell2010
+
+  if impl(ghc >= 7.8)
+    build-depends:
+      tree-diff      >= 0.0.1 && <0.1
+    other-modules:
+      Instances.TreeDiff
+      Instances.TreeDiff.Language
+      Instances.TreeDiff.SPDX
+      Instances.TreeDiff.Version
+
+test-suite check-tests
+  type: exitcode-stdio-1.0
+  hs-source-dirs: tests
+  main-is: CheckTests.hs
+  build-depends:
+    base,
+    bytestring,
+    directory,
+    filepath,
+    tasty >= 1.1.0.3 && < 1.2,
+    tasty-golden >=2.3.1.1 && <2.4,
+    Diff >=0.3.4 && <0.4,
+    Cabal
+  ghc-options: -Wall
+  default-language: Haskell2010
+
+test-suite custom-setup-tests
+  type: exitcode-stdio-1.0
+  hs-source-dirs: tests/custom-setup
+  main-is: CustomSetupTests.hs
+  other-modules:
+    CabalDoctestSetup
+    IdrisSetup
+  build-depends:
+    Cabal,
+    base,
+    directory,
+    filepath,
+    process
+  default-language: Haskell2010
+
+test-suite hackage-tests
+  type: exitcode-stdio-1.0
+  main-is: HackageTests.hs
+
+  -- TODO: need to get 01-index.tar on appveyor
+  if os(windows)
+    buildable: False
+
+  hs-source-dirs: tests
+
+  build-depends:
+    base,
+    Cabal,
+    bytestring,
+    deepseq,
+    containers,
+    directory,
+    filepath
+
+  build-depends:
+    base-compat          >=0.10.4   && <0.11,
+    base-orphans         >=0.6      && <0.9,
+    optparse-applicative >=0.13.2.0 && <0.15,
+    stm                  >=2.4.5.0  && <2.6,
+    tar                  >=0.5.0.3  && <0.6
+
+  if impl(ghc >= 7.8)
+    build-depends:
+      tree-diff      >= 0.0.1 && <0.1
+    other-modules:
+      Instances.TreeDiff
+      Instances.TreeDiff.Language
+      Instances.TreeDiff.SPDX
+      Instances.TreeDiff.Version
+
+  ghc-options: -Wall -rtsopts -threaded
+  default-extensions: CPP
+  default-language: Haskell2010
diff --git a/fixtures/Cabal.format b/fixtures/Cabal.format
new file mode 100644
--- /dev/null
+++ b/fixtures/Cabal.format
@@ -0,0 +1,673 @@
+name:               Cabal
+version:            2.5.0.0
+copyright:          2003-2018, Cabal Development Team (see AUTHORS file)
+license:            BSD3
+license-file:       LICENSE
+author:             Cabal Development Team <cabal-devel@haskell.org>
+maintainer:         cabal-devel@haskell.org
+homepage:           http://www.haskell.org/cabal/
+bug-reports:        https://github.com/haskell/cabal/issues
+synopsis:           A framework for packaging Haskell software
+description:
+  The Haskell Common Architecture for Building Applications and
+  Libraries: a framework defining a common interface for authors to more
+  easily build their Haskell applications in a portable way.
+  .
+  The Haskell Cabal is part of a larger infrastructure for distributing,
+  organizing, and cataloging Haskell libraries and tools.
+
+category:           Distribution
+cabal-version:      >=1.10
+build-type:         Simple
+
+-- If we use a new Cabal feature, this needs to be changed to Custom so
+-- we can bootstrap.
+
+extra-source-files:
+  README.md
+  tests/README.md
+  ChangeLog.md
+  doc/bugs-and-stability.rst
+  doc/concepts-and-development.rst
+  doc/conf.py
+  doc/config-and-install.rst
+  doc/developing-packages.rst
+  doc/images/Cabal-dark.png
+  doc/index.rst
+  doc/installing-packages.rst
+  doc/intro.rst
+  doc/misc.rst
+  doc/nix-local-build-overview.rst
+  doc/nix-local-build.rst
+  doc/file-format-changelog.rst
+  doc/README.md
+  doc/references.inc
+  tests/ParserTests/errors/common1.cabal
+  tests/ParserTests/errors/common1.errors
+  tests/ParserTests/errors/common2.cabal
+  tests/ParserTests/errors/common2.errors
+  tests/ParserTests/errors/common3.cabal
+  tests/ParserTests/errors/common3.errors
+  tests/ParserTests/errors/forward-compat.cabal
+  tests/ParserTests/errors/forward-compat.errors
+  tests/ParserTests/errors/forward-compat2.cabal
+  tests/ParserTests/errors/forward-compat2.errors
+  tests/ParserTests/errors/forward-compat3.cabal
+  tests/ParserTests/errors/forward-compat3.errors
+  tests/ParserTests/errors/issue-5055-2.cabal
+  tests/ParserTests/errors/issue-5055-2.errors
+  tests/ParserTests/errors/issue-5055.cabal
+  tests/ParserTests/errors/issue-5055.errors
+  tests/ParserTests/errors/leading-comma-2.cabal
+  tests/ParserTests/errors/leading-comma-2.errors
+  tests/ParserTests/errors/leading-comma-2b.cabal
+  tests/ParserTests/errors/leading-comma-2b.errors
+  tests/ParserTests/errors/leading-comma-2c.cabal
+  tests/ParserTests/errors/leading-comma-2c.errors
+  tests/ParserTests/errors/leading-comma.cabal
+  tests/ParserTests/errors/leading-comma.errors
+  tests/ParserTests/errors/multiple-libs.cabal
+  tests/ParserTests/errors/multiple-libs.errors
+  tests/ParserTests/errors/noVersion.cabal
+  tests/ParserTests/errors/noVersion.errors
+  tests/ParserTests/errors/noVersion2.cabal
+  tests/ParserTests/errors/noVersion2.errors
+  tests/ParserTests/errors/range-ge-wild.cabal
+  tests/ParserTests/errors/range-ge-wild.errors
+  tests/ParserTests/errors/removed-fields.cabal
+  tests/ParserTests/errors/removed-fields.errors
+  tests/ParserTests/errors/spdx-1.cabal
+  tests/ParserTests/errors/spdx-1.errors
+  tests/ParserTests/errors/spdx-2.cabal
+  tests/ParserTests/errors/spdx-2.errors
+  tests/ParserTests/errors/spdx-3.cabal
+  tests/ParserTests/errors/spdx-3.errors
+  tests/ParserTests/ipi/Includes2.cabal
+  tests/ParserTests/ipi/Includes2.expr
+  tests/ParserTests/ipi/Includes2.format
+  tests/ParserTests/ipi/internal-preprocessor-test.cabal
+  tests/ParserTests/ipi/internal-preprocessor-test.expr
+  tests/ParserTests/ipi/internal-preprocessor-test.format
+  tests/ParserTests/ipi/issue-2276-ghc-9885.cabal
+  tests/ParserTests/ipi/issue-2276-ghc-9885.expr
+  tests/ParserTests/ipi/issue-2276-ghc-9885.format
+  tests/ParserTests/ipi/transformers.cabal
+  tests/ParserTests/ipi/transformers.expr
+  tests/ParserTests/ipi/transformers.format
+  tests/ParserTests/regressions/MiniAgda.cabal
+  tests/ParserTests/regressions/MiniAgda.check
+  tests/ParserTests/regressions/Octree-0.5.cabal
+  tests/ParserTests/regressions/Octree-0.5.expr
+  tests/ParserTests/regressions/Octree-0.5.format
+  tests/ParserTests/regressions/bad-glob-syntax.cabal
+  tests/ParserTests/regressions/bad-glob-syntax.check
+  tests/ParserTests/regressions/cc-options-with-optimization.cabal
+  tests/ParserTests/regressions/cc-options-with-optimization.check
+  tests/ParserTests/regressions/common-conditional.cabal
+  tests/ParserTests/regressions/common-conditional.expr
+  tests/ParserTests/regressions/common-conditional.format
+  tests/ParserTests/regressions/common.cabal
+  tests/ParserTests/regressions/common.expr
+  tests/ParserTests/regressions/common.format
+  tests/ParserTests/regressions/common2.cabal
+  tests/ParserTests/regressions/common2.expr
+  tests/ParserTests/regressions/common2.format
+  tests/ParserTests/regressions/cxx-options-with-optimization.cabal
+  tests/ParserTests/regressions/cxx-options-with-optimization.check
+  tests/ParserTests/regressions/elif.cabal
+  tests/ParserTests/regressions/elif.expr
+  tests/ParserTests/regressions/elif.format
+  tests/ParserTests/regressions/elif2.cabal
+  tests/ParserTests/regressions/elif2.expr
+  tests/ParserTests/regressions/elif2.format
+  tests/ParserTests/regressions/encoding-0.8.cabal
+  tests/ParserTests/regressions/encoding-0.8.expr
+  tests/ParserTests/regressions/encoding-0.8.format
+  tests/ParserTests/regressions/extensions-paths-5054.cabal
+  tests/ParserTests/regressions/extensions-paths-5054.check
+  tests/ParserTests/regressions/generics-sop.cabal
+  tests/ParserTests/regressions/generics-sop.expr
+  tests/ParserTests/regressions/generics-sop.format
+  tests/ParserTests/regressions/ghc-option-j.cabal
+  tests/ParserTests/regressions/ghc-option-j.check
+  tests/ParserTests/regressions/haddock-api-2.18.1-check.cabal
+  tests/ParserTests/regressions/haddock-api-2.18.1-check.check
+  tests/ParserTests/regressions/hidden-main-lib.cabal
+  tests/ParserTests/regressions/hidden-main-lib.expr
+  tests/ParserTests/regressions/hidden-main-lib.format
+  tests/ParserTests/regressions/issue-5055.cabal
+  tests/ParserTests/regressions/issue-5055.expr
+  tests/ParserTests/regressions/issue-5055.format
+  tests/ParserTests/regressions/issue-774.cabal
+  tests/ParserTests/regressions/issue-774.check
+  tests/ParserTests/regressions/issue-774.expr
+  tests/ParserTests/regressions/issue-774.format
+  tests/ParserTests/regressions/jaeger-flamegraph.cabal
+  tests/ParserTests/regressions/jaeger-flamegraph.expr
+  tests/ParserTests/regressions/jaeger-flamegraph.format
+  tests/ParserTests/regressions/leading-comma-2.cabal
+  tests/ParserTests/regressions/leading-comma-2.expr
+  tests/ParserTests/regressions/leading-comma-2.format
+  tests/ParserTests/regressions/leading-comma.cabal
+  tests/ParserTests/regressions/leading-comma.expr
+  tests/ParserTests/regressions/leading-comma.format
+  tests/ParserTests/regressions/multiple-libs-2.cabal
+  tests/ParserTests/regressions/multiple-libs-2.check
+  tests/ParserTests/regressions/multiple-libs-2.expr
+  tests/ParserTests/regressions/multiple-libs-2.format
+  tests/ParserTests/regressions/noVersion.cabal
+  tests/ParserTests/regressions/noVersion.expr
+  tests/ParserTests/regressions/noVersion.format
+  tests/ParserTests/regressions/nothing-unicode.cabal
+  tests/ParserTests/regressions/nothing-unicode.check
+  tests/ParserTests/regressions/nothing-unicode.expr
+  tests/ParserTests/regressions/nothing-unicode.format
+  tests/ParserTests/regressions/pre-1.6-glob.cabal
+  tests/ParserTests/regressions/pre-1.6-glob.check
+  tests/ParserTests/regressions/pre-2.4-globstar.cabal
+  tests/ParserTests/regressions/pre-2.4-globstar.check
+  tests/ParserTests/regressions/shake.cabal
+  tests/ParserTests/regressions/shake.expr
+  tests/ParserTests/regressions/shake.format
+  tests/ParserTests/regressions/spdx-1.cabal
+  tests/ParserTests/regressions/spdx-1.expr
+  tests/ParserTests/regressions/spdx-1.format
+  tests/ParserTests/regressions/spdx-2.cabal
+  tests/ParserTests/regressions/spdx-2.expr
+  tests/ParserTests/regressions/spdx-2.format
+  tests/ParserTests/regressions/spdx-3.cabal
+  tests/ParserTests/regressions/spdx-3.expr
+  tests/ParserTests/regressions/spdx-3.format
+  tests/ParserTests/regressions/th-lift-instances.cabal
+  tests/ParserTests/regressions/th-lift-instances.expr
+  tests/ParserTests/regressions/th-lift-instances.format
+  tests/ParserTests/regressions/wl-pprint-indef.cabal
+  tests/ParserTests/regressions/wl-pprint-indef.expr
+  tests/ParserTests/regressions/wl-pprint-indef.format
+  tests/ParserTests/warnings/bom.cabal
+  tests/ParserTests/warnings/bool.cabal
+  tests/ParserTests/warnings/deprecatedfield.cabal
+  tests/ParserTests/warnings/doubledash.cabal
+  tests/ParserTests/warnings/extratestmodule.cabal
+  tests/ParserTests/warnings/gluedop.cabal
+  tests/ParserTests/warnings/multiplesingular.cabal
+  tests/ParserTests/warnings/nbsp.cabal
+  tests/ParserTests/warnings/newsyntax.cabal
+  tests/ParserTests/warnings/oldsyntax.cabal
+  tests/ParserTests/warnings/subsection.cabal
+  tests/ParserTests/warnings/tab.cabal
+  tests/ParserTests/warnings/trailingfield.cabal
+  tests/ParserTests/warnings/unknownfield.cabal
+  tests/ParserTests/warnings/unknownsection.cabal
+  tests/ParserTests/warnings/utf8.cabal
+  tests/ParserTests/warnings/versiontag.cabal
+  tests/hackage/check.sh
+  tests/hackage/download.sh
+  tests/hackage/unpack.sh
+  tests/misc/ghc-supported-languages.hs
+
+-- Generated with 'make gen-extra-source-files'
+-- Do NOT edit this section manually; instead, run the script.
+-- BEGIN gen-extra-source-files
+-- END gen-extra-source-files
+
+source-repository head
+  type:     git
+  location: https://github.com/haskell/cabal/
+  subdir:   Cabal
+
+flag bundled-binary-generic
+  default: False
+
+library
+  build-depends:
+      array       >=0.4.0.1  && <0.6
+    , base        >=4.6      && <5
+    , bytestring  >=0.10.0.0 && <0.11
+    , containers  >=0.5.0.0  && <0.7
+    , deepseq     >=1.3.0.1  && <1.5
+    , directory   >=1.2      && <1.4
+    , filepath    >=1.3.0.1  && <1.5
+    , pretty      >=1.1.1    && <1.2
+    , process     >=1.1.0.2  && <1.7
+    , time        >=1.4.0.1  && <1.10
+
+  if flag(bundled-binary-generic)
+    build-depends: binary >=0.5.1.1 && <0.7
+
+  else
+    build-depends: binary >=0.7 && <0.9
+
+  if os(windows)
+    build-depends: Win32 >=2.3.0.0 && <2.9
+
+  else
+    build-depends: unix >=2.6.0.0 && <2.8
+
+  ghc-options:      -Wall -fno-ignore-asserts -fwarn-tabs
+
+  if impl(ghc >=8.0)
+    ghc-options:
+      -Wcompat -Wnoncanonical-monad-instances
+      -Wnoncanonical-monadfail-instances
+
+  exposed-modules:
+    Distribution.Backpack
+    Distribution.Backpack.ComponentsGraph
+    Distribution.Backpack.Configure
+    Distribution.Backpack.ConfiguredComponent
+    Distribution.Backpack.DescribeUnitId
+    Distribution.Backpack.FullUnitId
+    Distribution.Backpack.LinkedComponent
+    Distribution.Backpack.ModSubst
+    Distribution.Backpack.ModuleShape
+    Distribution.Backpack.PreModuleShape
+    Distribution.CabalSpecVersion
+    Distribution.Compat.Binary
+    Distribution.Compat.CreatePipe
+    Distribution.Compat.DList
+    Distribution.Compat.Directory
+    Distribution.Compat.Environment
+    Distribution.Compat.Exception
+    Distribution.Compat.Graph
+    Distribution.Compat.Internal.TempFile
+    Distribution.Compat.Newtype
+    Distribution.Compat.Prelude.Internal
+    Distribution.Compat.Semigroup
+    Distribution.Compat.Stack
+    Distribution.Compat.Time
+    Distribution.Compiler
+    Distribution.InstalledPackageInfo
+    Distribution.License
+    Distribution.Make
+    Distribution.ModuleName
+    Distribution.Package
+    Distribution.PackageDescription
+    Distribution.PackageDescription.Check
+    Distribution.PackageDescription.Configuration
+    Distribution.PackageDescription.PrettyPrint
+    Distribution.PackageDescription.Utils
+    Distribution.Pretty
+    Distribution.ReadE
+    Distribution.SPDX
+    Distribution.SPDX.License
+    Distribution.SPDX.LicenseExceptionId
+    Distribution.SPDX.LicenseExpression
+    Distribution.SPDX.LicenseId
+    Distribution.SPDX.LicenseListVersion
+    Distribution.SPDX.LicenseReference
+    Distribution.Simple
+    Distribution.Simple.Bench
+    Distribution.Simple.Build
+    Distribution.Simple.Build.Macros
+    Distribution.Simple.Build.PathsModule
+    Distribution.Simple.BuildPaths
+    Distribution.Simple.BuildTarget
+    Distribution.Simple.BuildToolDepends
+    Distribution.Simple.CCompiler
+    Distribution.Simple.Command
+    Distribution.Simple.Compiler
+    Distribution.Simple.Configure
+    Distribution.Simple.Doctest
+    Distribution.Simple.Flag
+    Distribution.Simple.GHC
+    Distribution.Simple.GHCJS
+    Distribution.Simple.Glob
+    Distribution.Simple.Haddock
+    Distribution.Simple.HaskellSuite
+    Distribution.Simple.Hpc
+    Distribution.Simple.Install
+    Distribution.Simple.InstallDirs
+    Distribution.Simple.LocalBuildInfo
+    Distribution.Simple.PackageIndex
+    Distribution.Simple.PreProcess
+    Distribution.Simple.PreProcess.Unlit
+    Distribution.Simple.Program
+    Distribution.Simple.Program.Ar
+    Distribution.Simple.Program.Builtin
+    Distribution.Simple.Program.Db
+    Distribution.Simple.Program.Find
+    Distribution.Simple.Program.GHC
+    Distribution.Simple.Program.HcPkg
+    Distribution.Simple.Program.Hpc
+    Distribution.Simple.Program.Internal
+    Distribution.Simple.Program.Ld
+    Distribution.Simple.Program.ResponseFile
+    Distribution.Simple.Program.Run
+    Distribution.Simple.Program.Script
+    Distribution.Simple.Program.Strip
+    Distribution.Simple.Program.Types
+    Distribution.Simple.Register
+    Distribution.Simple.Setup
+    Distribution.Simple.SrcDist
+    Distribution.Simple.Test
+    Distribution.Simple.Test.ExeV10
+    Distribution.Simple.Test.LibV09
+    Distribution.Simple.Test.Log
+    Distribution.Simple.UHC
+    Distribution.Simple.UserHooks
+    Distribution.Simple.Utils
+    Distribution.System
+    Distribution.TestSuite
+    Distribution.Text
+    Distribution.Types.AbiDependency
+    Distribution.Types.AbiHash
+    Distribution.Types.AnnotatedId
+    Distribution.Types.Benchmark
+    Distribution.Types.BenchmarkInterface
+    Distribution.Types.BenchmarkType
+    Distribution.Types.BuildInfo
+    Distribution.Types.BuildType
+    Distribution.Types.Component
+    Distribution.Types.ComponentId
+    Distribution.Types.ComponentInclude
+    Distribution.Types.ComponentLocalBuildInfo
+    Distribution.Types.ComponentName
+    Distribution.Types.ComponentRequestedSpec
+    Distribution.Types.CondTree
+    Distribution.Types.Condition
+    Distribution.Types.Dependency
+    Distribution.Types.DependencyMap
+    Distribution.Types.ExeDependency
+    Distribution.Types.Executable
+    Distribution.Types.ExecutableScope
+    Distribution.Types.ExposedModule
+    Distribution.Types.ForeignLib
+    Distribution.Types.ForeignLibOption
+    Distribution.Types.ForeignLibType
+    Distribution.Types.GenericPackageDescription
+    Distribution.Types.GivenComponent
+    Distribution.Types.HookedBuildInfo
+    Distribution.Types.IncludeRenaming
+    Distribution.Types.InstalledPackageInfo
+    Distribution.Types.InstalledPackageInfo.FieldGrammar
+    Distribution.Types.LegacyExeDependency
+    Distribution.Types.Library
+    Distribution.Types.LibraryName
+    Distribution.Types.LibraryVisibility
+    Distribution.Types.LocalBuildInfo
+    Distribution.Types.Mixin
+    Distribution.Types.Module
+    Distribution.Types.ModuleReexport
+    Distribution.Types.ModuleRenaming
+    Distribution.Types.MungedPackageId
+    Distribution.Types.MungedPackageName
+    Distribution.Types.PackageDescription
+    Distribution.Types.PackageId
+    Distribution.Types.PackageName
+    Distribution.Types.PackageVersionConstraint
+    Distribution.Types.PkgconfigDependency
+    Distribution.Types.PkgconfigName
+    Distribution.Types.SetupBuildInfo
+    Distribution.Types.SourceRepo
+    Distribution.Types.TargetInfo
+    Distribution.Types.TestSuite
+    Distribution.Types.TestSuiteInterface
+    Distribution.Types.TestType
+    Distribution.Types.UnitId
+    Distribution.Types.UnqualComponentName
+    Distribution.Types.Version
+    Distribution.Types.VersionInterval
+    Distribution.Types.VersionRange
+    Distribution.Utils.Generic
+    Distribution.Utils.IOData
+    Distribution.Utils.LogProgress
+    Distribution.Utils.MapAccum
+    Distribution.Utils.NubList
+    Distribution.Utils.Progress
+    Distribution.Utils.ShortText
+    Distribution.Verbosity
+    Distribution.Version
+    Language.Haskell.Extension
+
+  -- Parsec parser-related modules
+  build-depends:
+      mtl           >=2.1      && <2.3
+    , parsec        >=3.1.13.0 && <3.2
+    , text          >=1.2.3.0  && <1.3
+    , transformers  >=0.3      && <0.4 || >=0.4.1.0 && <0.6
+
+  -- transformers-0.4.0.0 doesn't have record syntax e.g. for Identity
+  -- See also https://github.com/ekmett/transformers-compat/issues/35
+  exposed-modules:
+    Distribution.Compat.CharParsing
+    Distribution.Compat.Parsing
+    Distribution.FieldGrammar
+    Distribution.FieldGrammar.Class
+    Distribution.FieldGrammar.FieldDescrs
+    Distribution.FieldGrammar.Parsec
+    Distribution.FieldGrammar.Pretty
+    Distribution.Fields
+    Distribution.Fields.ConfVar
+    Distribution.Fields.Field
+    Distribution.Fields.Lexer
+    Distribution.Fields.LexerMonad
+    Distribution.Fields.ParseResult
+    Distribution.Fields.Parser
+    Distribution.Fields.Pretty
+    Distribution.PackageDescription.FieldGrammar
+    Distribution.PackageDescription.Parsec
+    Distribution.PackageDescription.Quirks
+    Distribution.Parsec
+    Distribution.Parsec.Error
+    Distribution.Parsec.FieldLineStream
+    Distribution.Parsec.Newtypes
+    Distribution.Parsec.Position
+    Distribution.Parsec.Warning
+
+  -- Lens functionality
+  exposed-modules:
+    Distribution.Compat.Lens
+    Distribution.Types.Benchmark.Lens
+    Distribution.Types.BuildInfo.Lens
+    Distribution.Types.Executable.Lens
+    Distribution.Types.ForeignLib.Lens
+    Distribution.Types.GenericPackageDescription.Lens
+    Distribution.Types.InstalledPackageInfo.Lens
+    Distribution.Types.Lens
+    Distribution.Types.Library.Lens
+    Distribution.Types.PackageDescription.Lens
+    Distribution.Types.PackageId.Lens
+    Distribution.Types.SetupBuildInfo.Lens
+    Distribution.Types.SourceRepo.Lens
+    Distribution.Types.TestSuite.Lens
+
+  other-modules:
+    Distribution.Backpack.Id
+    Distribution.Backpack.MixLink
+    Distribution.Backpack.ModuleScope
+    Distribution.Backpack.PreExistingComponent
+    Distribution.Backpack.ReadyComponent
+    Distribution.Backpack.UnifyM
+    Distribution.Compat.CopyFile
+    Distribution.Compat.GetShortPathName
+    Distribution.Compat.MonadFail
+    Distribution.Compat.Prelude
+    Distribution.Compat.SnocList
+    Distribution.GetOpt
+    Distribution.Lex
+    Distribution.Simple.GHC.EnvironmentParser
+    Distribution.Simple.GHC.ImplInfo
+    Distribution.Simple.GHC.Internal
+    Distribution.Utils.Base62
+    Distribution.Utils.String
+    Distribution.Utils.UnionFind
+    Paths_Cabal
+
+  if flag(bundled-binary-generic)
+    other-modules:
+      Distribution.Compat.Binary.Class
+      Distribution.Compat.Binary.Generic
+
+  default-language: Haskell2010
+  other-extensions:
+    BangPatterns
+    CPP
+    DefaultSignatures
+    DeriveDataTypeable
+    DeriveFoldable
+    DeriveFunctor
+    DeriveGeneric
+    DeriveTraversable
+    ExistentialQuantification
+    FlexibleContexts
+    FlexibleInstances
+    GeneralizedNewtypeDeriving
+    ImplicitParams
+    KindSignatures
+    NondecreasingIndentation
+    OverloadedStrings
+    RankNTypes
+    RecordWildCards
+    ScopedTypeVariables
+    StandaloneDeriving
+    Trustworthy
+    TypeFamilies
+    TypeOperators
+    TypeSynonymInstances
+    UndecidableInstances
+
+  if impl(ghc >=7.11)
+    other-extensions: PatternSynonyms
+
+-- Small, fast running tests.
+test-suite unit-tests
+  type:             exitcode-stdio-1.0
+  hs-source-dirs:   tests
+  other-modules:
+    Test.Laws
+    Test.QuickCheck.Utils
+    UnitTests.Distribution.Compat.CreatePipe
+    UnitTests.Distribution.Compat.Graph
+    UnitTests.Distribution.Compat.Time
+    UnitTests.Distribution.SPDX
+    UnitTests.Distribution.Simple.Glob
+    UnitTests.Distribution.Simple.Program.Internal
+    UnitTests.Distribution.Simple.Utils
+    UnitTests.Distribution.System
+    UnitTests.Distribution.Types.GenericPackageDescription
+    UnitTests.Distribution.Utils.Generic
+    UnitTests.Distribution.Utils.NubList
+    UnitTests.Distribution.Utils.ShortText
+    UnitTests.Distribution.Version
+
+  main-is:          UnitTests.hs
+  build-depends:
+      array
+    , base
+    , bytestring
+    , Cabal
+    , containers
+    , directory
+    , filepath
+    , integer-logarithms  >=1.0.2   && <1.1
+    , pretty
+    , QuickCheck          >=2.11.3  && <2.12
+    , tagged
+    , tasty               >=1.1.0.3 && <1.2
+    , tasty-hunit
+    , tasty-quickcheck
+    , temporary
+    , text
+
+  ghc-options:      -Wall
+  default-language: Haskell2010
+
+test-suite parser-tests
+  type:             exitcode-stdio-1.0
+  hs-source-dirs:   tests
+  main-is:          ParserTests.hs
+  build-depends:
+      base
+    , base-compat       >=0.10.4  && <0.11
+    , bytestring
+    , Cabal
+    , Diff              >=0.3.4   && <0.4
+    , directory
+    , filepath
+    , tasty             >=1.1.0.3 && <1.2
+    , tasty-golden      >=2.3.1.1 && <2.4
+    , tasty-hunit
+    , tasty-quickcheck
+
+  ghc-options:      -Wall
+  default-language: Haskell2010
+
+  if impl(ghc >=7.8)
+    build-depends: tree-diff >=0.0.1 && <0.1
+    other-modules:
+      Instances.TreeDiff
+      Instances.TreeDiff.Language
+      Instances.TreeDiff.SPDX
+      Instances.TreeDiff.Version
+
+test-suite check-tests
+  type:             exitcode-stdio-1.0
+  hs-source-dirs:   tests
+  main-is:          CheckTests.hs
+  build-depends:
+      base
+    , bytestring
+    , Cabal
+    , Diff          >=0.3.4   && <0.4
+    , directory
+    , filepath
+    , tasty         >=1.1.0.3 && <1.2
+    , tasty-golden  >=2.3.1.1 && <2.4
+
+  ghc-options:      -Wall
+  default-language: Haskell2010
+
+test-suite custom-setup-tests
+  type:             exitcode-stdio-1.0
+  hs-source-dirs:   tests/custom-setup
+  main-is:          CustomSetupTests.hs
+  other-modules:
+    CabalDoctestSetup
+    IdrisSetup
+
+  build-depends:
+      base
+    , Cabal
+    , directory
+    , filepath
+    , process
+
+  default-language: Haskell2010
+
+test-suite hackage-tests
+  type:               exitcode-stdio-1.0
+  main-is:            HackageTests.hs
+
+  -- TODO: need to get 01-index.tar on appveyor
+  if os(windows)
+    buildable: False
+
+  hs-source-dirs:     tests
+  build-depends:
+      base
+    , bytestring
+    , Cabal
+    , containers
+    , deepseq
+    , directory
+    , filepath
+
+  build-depends:
+      base-compat           >=0.10.4   && <0.11
+    , base-orphans          >=0.6      && <0.9
+    , optparse-applicative  >=0.13.2.0 && <0.15
+    , stm                   >=2.4.5.0  && <2.6
+    , tar                   >=0.5.0.3  && <0.6
+
+  if impl(ghc >=7.8)
+    build-depends: tree-diff >=0.0.1 && <0.1
+    other-modules:
+      Instances.TreeDiff
+      Instances.TreeDiff.Language
+      Instances.TreeDiff.SPDX
+      Instances.TreeDiff.Version
+
+  ghc-options:        -Wall -rtsopts -threaded
+  default-extensions: CPP
+  default-language:   Haskell2010
diff --git a/fixtures/cabal-fmt.cabal b/fixtures/cabal-fmt.cabal
new file mode 100644
--- /dev/null
+++ b/fixtures/cabal-fmt.cabal
@@ -0,0 +1,16 @@
+cabal-version: 2.4
+name: cabal-fmt
+version: 0
+
+-- An example formatter
+executable cabal-fmt
+    default-language: Haskell2010
+    hs-source-dirs: src 
+    main-is: CabalFmt.hs
+    -- build depends will be in
+    -- a nice tabular format
+    build-depends: base >=4.11 && <4.13, pretty >=1.1.3.6 && <1.2, bytestring, Cabal ^>=2.5, containers ^>=0.5.11.0 || ^>=0.6.0.1
+    -- extensions will be sorted
+    other-extensions:
+      DeriveFunctor FlexibleContexts ExistentialQuantification OverloadedStrings
+      RankNTypes
diff --git a/fixtures/cabal-fmt.format b/fixtures/cabal-fmt.format
new file mode 100644
--- /dev/null
+++ b/fixtures/cabal-fmt.format
@@ -0,0 +1,26 @@
+cabal-version: 2.4
+name:          cabal-fmt
+version:       0
+
+-- An example formatter
+executable cabal-fmt
+  default-language: Haskell2010
+  hs-source-dirs:   src
+  main-is:          CabalFmt.hs
+
+  -- build depends will be in
+  -- a nice tabular format
+  build-depends:
+    , base        >=4.11     && <4.13
+    , bytestring
+    , Cabal       ^>=2.5
+    , containers  ^>=0.5.11.0 || ^>=0.6.0.1
+    , pretty      ^>=1.1.3.6
+
+  -- extensions will be sorted
+  other-extensions:
+    DeriveFunctor
+    ExistentialQuantification
+    FlexibleContexts
+    OverloadedStrings
+    RankNTypes
diff --git a/fixtures/simple-example.cabal b/fixtures/simple-example.cabal
new file mode 100644
--- /dev/null
+++ b/fixtures/simple-example.cabal
@@ -0,0 +1,51 @@
+name:                simple-example
+version:             1
+synopsis:            Simple example of cabal-doctest demonstrating a library
+                     with doctests
+description:         A package skeleton, to demostrate the use of cabal-doctest
+homepage:            https://github.com/phadej/cabal-doctest
+license:             BSD3
+license-file:        LICENSE
+author:              Oleg Grenrus <oleg.grenrus@iki.fi>
+maintainer:          Oleg Grenrus <oleg.grenrus@iki.fi>
+copyright:           (c) 2017 Oleg Grenrus
+category:            Example
+build-type:          Custom
+cabal-version:       1.12
+
+tested-with:
+  GHC==7.0.4,
+  GHC==7.2.2,
+  GHC==7.4.2,
+  GHC==7.6.3,
+  GHC==7.8.4,
+  GHC==7.10.3,
+  GHC==8.0.2,
+  GHC==8.2.2,
+  GHC==8.4.4,
+  GHC==8.6.3
+
+custom-setup
+  setup-depends:
+    base, Cabal, cabal-doctest >=1.0.6 && <1.1
+
+library
+  exposed-modules:     Example
+  default-extensions:  ScopedTypeVariables RankNTypes
+  build-depends:       base >=4.3 && <4.13
+  hs-source-dirs:      src
+  default-language:    Haskell2010
+
+test-suite doctests
+  type:                exitcode-stdio-1.0
+  main-is:             doctests.hs
+  build-depends:
+    base,
+    doctest >= 0.15 && <0.17,
+    QuickCheck ==2.12.*,
+    simple-example,
+    template-haskell
+
+  ghc-options:         -Wall -threaded
+  hs-source-dirs:      tests
+  default-language:    Haskell2010
diff --git a/fixtures/simple-example.format b/fixtures/simple-example.format
new file mode 100644
--- /dev/null
+++ b/fixtures/simple-example.format
@@ -0,0 +1,48 @@
+name:          simple-example
+version:       1
+synopsis:
+  Simple example of cabal-doctest demonstrating a library
+  with doctests
+
+description:   A package skeleton, to demostrate the use of cabal-doctest
+homepage:      https://github.com/phadej/cabal-doctest
+license:       BSD3
+license-file:  LICENSE
+author:        Oleg Grenrus <oleg.grenrus@iki.fi>
+maintainer:    Oleg Grenrus <oleg.grenrus@iki.fi>
+copyright:     (c) 2017 Oleg Grenrus
+category:      Example
+build-type:    Custom
+cabal-version: 1.12
+tested-with:
+  GHC ==8.6.3 || ==8.4.4 || ==8.2.2 || ==8.0.2 || ==7.10.3 || ==7.8.4 || ==7.6.3 || ==7.4.2 || ==7.2.2 || ==7.0.4
+
+custom-setup
+  setup-depends:
+      base
+    , Cabal
+    , cabal-doctest  >=1.0.6 && <1.1
+
+library
+  exposed-modules:    Example
+  default-extensions:
+    RankNTypes
+    ScopedTypeVariables
+
+  build-depends:      base >=4.3 && <4.13
+  hs-source-dirs:     src
+  default-language:   Haskell2010
+
+test-suite doctests
+  type:             exitcode-stdio-1.0
+  main-is:          doctests.hs
+  build-depends:
+      base
+    , doctest           >=0.15 && <0.17
+    , QuickCheck        >=2.12 && <2.13
+    , simple-example
+    , template-haskell
+
+  ghc-options:      -Wall -threaded
+  hs-source-dirs:   tests
+  default-language: Haskell2010
diff --git a/src/CabalFmt.hs b/src/CabalFmt.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt.hs
@@ -0,0 +1,137 @@
+{-# LANGUAGE FlexibleContexts  #-}
+{-# LANGUAGE OverloadedStrings #-}
+-- | This is a demo application of how you can make Cabal-like
+-- file formatter.
+--
+module CabalFmt (cabalFmt) where
+
+import Control.Monad        (join)
+import Control.Monad.Except (catchError)
+import Control.Monad.Reader (asks, local)
+import Data.Maybe           (fromMaybe)
+
+import qualified Data.ByteString                              as BS
+import qualified Distribution.CabalSpecVersion                as C
+import qualified Distribution.FieldGrammar.Parsec             as C
+import qualified Distribution.Fields                          as C
+import qualified Distribution.Fields.ConfVar                  as C
+import qualified Distribution.Fields.Pretty                   as C
+import qualified Distribution.PackageDescription.FieldGrammar as C
+import qualified Distribution.Parsec                          as C
+import qualified Distribution.Pretty                          as C
+import qualified Distribution.Simple.Utils                    as C
+import qualified Distribution.Types.Condition                 as C
+import qualified Distribution.Types.GenericPackageDescription as C
+import qualified Distribution.Types.PackageDescription        as C
+import qualified Distribution.Types.Version                   as C
+import qualified Text.PrettyPrint                             as PP
+
+import CabalFmt.Comments
+import CabalFmt.Fields
+import CabalFmt.Fields.BuildDepends
+import CabalFmt.Fields.Extensions
+import CabalFmt.Fields.Modules
+import CabalFmt.Fields.TestedWith
+import CabalFmt.Refactoring
+import CabalFmt.Monad
+import CabalFmt.Options
+import CabalFmt.Parser
+
+-------------------------------------------------------------------------------
+-- Main
+-------------------------------------------------------------------------------
+
+cabalFmt :: FilePath -> BS.ByteString -> CabalFmt String
+cabalFmt filepath contents = do
+    opts         <- asks id
+    indentWith   <- asks optIndent
+    gpd          <- parseGpd filepath contents
+    inputFields' <- parseFields contents
+    let inputFields = foldr (\r f -> r opts f) (attachComments contents inputFields') refactorings 
+
+    let v = C.cabalSpecFromVersionDigits
+          $ C.versionNumbers
+          $ C.specVersion
+          $ C.packageDescription gpd
+
+    local (\o -> o { optSpecVersion = v }) $ do
+
+        outputPrettyFields <- C.genericFromParsecFields
+            prettyFieldLines
+            prettySectionArgs
+            inputFields
+
+        return $ C.showFields' fromComments indentWith outputPrettyFields
+
+fromComments :: Comments -> [String]
+fromComments (Comments bss) = map C.fromUTF8BS bss
+
+-------------------------------------------------------------------------------
+-- Refactorings
+-------------------------------------------------------------------------------
+
+refactorings :: [Refactoring]
+refactorings =
+    [ refactoringExpandExposedModules
+    ]
+
+-------------------------------------------------------------------------------
+-- Field prettyfying
+-------------------------------------------------------------------------------
+
+prettyFieldLines :: C.FieldName -> [C.FieldLine ann] -> CabalFmt PP.Doc
+prettyFieldLines fn fls =
+    fromMaybe (C.prettyFieldLines fn fls) <$> knownField fn fls
+
+knownField :: C.FieldName -> [C.FieldLine ann] -> CabalFmt (Maybe PP.Doc)
+knownField fn fls = do
+    v <- asks optSpecVersion
+    return $ join $ fieldDescrLookup (fieldDescrs v) fn $ \p pp ->
+        case C.runParsecParser' v p "<input>" (C.fieldLinesToStream fls) of
+            Right x -> Just (pp x)
+            Left _  -> Nothing
+
+fieldDescrs :: C.CabalSpecVersion -> FieldDescrs () ()
+fieldDescrs v
+    =  buildDependsF v
+    <> setupDependsF v
+    <> defaultExtensionsF
+    <> otherExtensionsF
+    <> exposedModulesF
+    <> otherModulesF
+    <> testedWithF
+    <> coerceFieldDescrs C.packageDescriptionFieldGrammar
+    <> coerceFieldDescrs C.buildInfoFieldGrammar
+
+-------------------------------------------------------------------------------
+-- Sections
+-------------------------------------------------------------------------------
+
+prettySectionArgs :: C.FieldName -> [C.SectionArg ann] -> CabalFmt [PP.Doc]
+prettySectionArgs x args =
+    prettySectionArgs' x args `catchError` \_ ->
+        return (C.prettySectionArgs x args)
+
+prettySectionArgs' :: a -> [C.SectionArg ann] -> CabalFmt [PP.Doc]
+prettySectionArgs' _ args = do
+    c <- runParseResult "<args>" "" $ C.parseConditionConfVar (map (C.zeroPos <$) args)
+    return [ppCondition c]
+
+-------------------------------------------------------------------------------
+-- PrettyPrint condition
+-------------------------------------------------------------------------------
+
+-- This is originally from Cabal
+
+ppCondition :: C.Condition C.ConfVar -> PP.Doc
+ppCondition (C.Var x)      = ppConfVar x
+ppCondition (C.Lit b)      = PP.text (show b)
+ppCondition (C.CNot c)     = PP.char '!' PP.<> ppCondition c
+ppCondition (C.COr c1 c2)  = PP.parens (PP.hsep [ppCondition c1, PP.text "||", ppCondition c2])
+ppCondition (C.CAnd c1 c2) = PP.parens (PP.hsep [ppCondition c1, PP.text "&&", ppCondition c2])
+
+ppConfVar :: C.ConfVar -> PP.Doc
+ppConfVar (C.OS os)     = PP.text "os"   PP.<> PP.parens (C.pretty os)
+ppConfVar (C.Arch arch) = PP.text "arch" PP.<> PP.parens (C.pretty arch)
+ppConfVar (C.Flag name) = PP.text "flag" PP.<> PP.parens (C.pretty name)
+ppConfVar (C.Impl c v)  = PP.text "impl" PP.<> PP.parens (C.pretty c PP.<+> C.pretty v)
diff --git a/src/CabalFmt/Comments.hs b/src/CabalFmt/Comments.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Comments.hs
@@ -0,0 +1,127 @@
+{-# LANGUAGE BangPatterns               #-}
+{-# LANGUAGE DerivingStrategies         #-}
+{-# LANGUAGE GeneralizedNewtypeDeriving #-}
+{-# LANGUAGE OverloadedStrings          #-}
+{-# LANGUAGE ScopedTypeVariables        #-}
+module CabalFmt.Comments where
+
+import Data.Foldable (toList)
+import Data.Maybe    (fromMaybe)
+
+import qualified Data.ByteString           as BS
+import qualified Data.ByteString.Char8     as BS8
+import qualified Data.Map.Strict           as Map
+import qualified Distribution.Fields       as C
+import qualified Distribution.Fields.Field as C
+import qualified Distribution.Parsec       as C
+
+-------------------------------------------------------------------------------
+-- Comments wrapper
+-------------------------------------------------------------------------------
+
+newtype Comments = Comments [BS.ByteString]
+  deriving stock Show
+  deriving newtype (Semigroup, Monoid)
+
+-------------------------------------------------------------------------------
+-- Attach comments
+-------------------------------------------------------------------------------
+
+attachComments
+    :: BS.ByteString        -- ^ source with comments
+    -> [C.Field C.Position] -- ^ parsed source fields
+    -> [C.Field Comments]
+attachComments input inputFields = overAnn attach inputFields where
+    inputFieldsU :: [(FieldPath, C.Field C.Position)]
+    inputFieldsU = fieldUniverseN inputFields
+
+    comments :: [(Int, Comments)]
+    comments = extractComments input
+
+    -- todo: warning when comments are omitted
+    comments' :: Map.Map FieldPath Comments
+    comments' = Map.fromListWith (flip (<>))
+        [ (path, cs)
+        | (l, cs) <- comments
+        , path <- toList (findPath C.fieldAnn l inputFieldsU)
+        ]
+
+    attach :: FieldPath -> C.Position -> Comments
+    attach fp _pos = fromMaybe mempty (Map.lookup fp comments')
+
+overAnn :: forall a b. (FieldPath -> a -> b) -> [C.Field a] -> [C.Field b]
+overAnn f = go' id where
+    go :: (FieldPath -> FieldPath) -> Int -> C.Field a -> C.Field b
+    go g i (C.Field (C.Name a name) fls) =
+        C.Field (C.Name b name) (b <$$ fls)
+      where
+        b = f (g (Nth i End)) a
+
+    go g i (C.Section (C.Name a name) args fls) =
+        C.Section (C.Name b name) (b <$$ args) (go' (g . Nth i) fls)
+      where
+        b = f (g (Nth i End)) a
+
+    go' :: (FieldPath -> FieldPath) -> [C.Field a] -> [C.Field b]
+    go' g xs = zipWith (go g) [0..] xs
+
+    (<$$) :: (Functor f, Functor g) => x -> f (g y) -> f (g x)
+    x <$$ y = (x <$) <$> y
+
+-------------------------------------------------------------------------------
+-- Find comments in the input
+-------------------------------------------------------------------------------
+
+extractComments :: BS.ByteString -> [(Int, Comments)]
+extractComments = go . zip [1..] . map (BS.dropWhile isSpace8) . BS8.lines where
+    go :: [(Int, BS.ByteString)] -> [(Int, Comments)]
+    go [] = []
+    go ((n, bs) : rest)
+        | isComment bs = case span ((isComment .|| BS.null) . snd) rest of
+            (h,t) -> (n, Comments $ bs : map snd h) : go t
+        | otherwise = go rest
+
+    (f .|| g) x = f x || g x
+
+    isSpace8 w = w == 9 || w == 32
+
+    isComment :: BS.ByteString -> Bool
+    isComment = BS.isPrefixOf "--"
+
+-------------------------------------------------------------------------------
+-- FieldPath
+-------------------------------------------------------------------------------
+
+-- | Paths input paths. Essentially a list of offsets. Own type ofr safety.
+data FieldPath
+    = End
+    | Nth Int FieldPath -- nth field
+  deriving (Eq, Ord, Show)
+
+fieldPathSize :: FieldPath -> Int
+fieldPathSize = go 0 where
+    go !acc End = acc
+    go !acc (Nth _ fp) = go (succ acc) fp
+
+fieldUniverseN :: [C.Field ann] -> [(FieldPath, C.Field ann)]
+fieldUniverseN = concat . zipWith g [0..] where
+    g n f' = [ (Nth n p, f'') | (p, f'') <- fieldUniverse f' ]
+
+fieldUniverse :: C.Field ann -> [(FieldPath, C.Field ann)]
+fieldUniverse f@(C.Section _ _ fs) = (End,f) : concat (zipWith g [0..] fs) where
+    g n f' = [ (Nth n p, f'') | (p, f'') <- fieldUniverse f' ]
+fieldUniverse f@(C.Field _ _)      = [(End, f)]
+
+-- note: fieldUniverse* should produce 'FieldPath's in increasing order
+-- that helps
+findPath :: (a -> C.Position) -> Int -> [(FieldPath, a)] -> Maybe FieldPath
+findPath _ _ [] = Nothing
+findPath f l [(p, x)]
+    | C.Position k _ <- f x =
+        if l < k then Just p else Nothing
+findPath f l ((_, x) : rest@((p, x') : _))
+    | C.Position k  _ <- f x
+    , C.Position k' _ <- f x' =
+        if k < l && l < k'
+        then Just p
+        else findPath f l rest
diff --git a/src/CabalFmt/Error.hs b/src/CabalFmt/Error.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Error.hs
@@ -0,0 +1,115 @@
+module CabalFmt.Error (Error (..), renderError) where
+
+import System.FilePath   (normalise)
+import Text.Parsec.Error (ParseError)
+
+import qualified Data.ByteString            as BS
+import qualified Data.ByteString.Char8      as BS8
+import qualified Distribution.Parsec        as C
+import qualified Distribution.Simple.Utils  as C (fromUTF8BS)
+import qualified Distribution.Types.Version as C
+
+data Error
+    = SomeError String
+    | CabalParseError FilePath BS.ByteString [C.PError] (Maybe C.Version) [C.PWarning]
+    | PanicCannotParseInput  ParseError
+  deriving (Show)
+
+renderError :: Error -> IO ()
+renderError (SomeError err) = putStrLn $ "error: " ++ err
+renderError (PanicCannotParseInput err) = putStrLn $ "panic! " ++ show err
+renderError (CabalParseError filepath contents errors _ warnings) = 
+    putStr $ renderParseError filepath contents errors warnings
+
+-------------------------------------------------------------------------------
+-- Rendering of Cabal parser error
+-------------------------------------------------------------------------------
+
+-- | Render parse error highlighting the part of the input file.
+renderParseError
+    :: FilePath
+    -> BS.ByteString
+    -> [C.PError]
+    -> [C.PWarning]
+    -> String
+renderParseError filepath contents errors warnings = unlines $
+    [ "Errors encountered when parsing cabal file " <> filepath <> ":"
+    ]
+    ++ renderedErrors
+    ++ renderedWarnings
+  where
+    filepath' = normalise filepath
+
+    -- lines of the input file. 'lines' is taken, so they are called rows
+    -- contents, line number, whether it's empty line
+    rows :: [(String, Int, Bool)]
+    rows = zipWith f (BS8.lines contents) [1..] where
+        f bs i = let s = C.fromUTF8BS bs in (s, i, isEmptyOrComment s)
+
+    rowsZipper = listToZipper rows
+
+    isEmptyOrComment :: String -> Bool
+    isEmptyOrComment s = case dropWhile (== ' ') s of
+        ""          -> True   -- empty
+        ('-':'-':_) -> True   -- comment
+        _           -> False
+
+    renderedErrors   = concatMap renderError' errors
+    renderedWarnings = concatMap renderWarning warnings
+
+    renderError' :: C.PError -> [String]
+    renderError' (C.PError pos@(C.Position row col) msg)
+        -- if position is 0:0, then it doesn't make sense to show input
+        -- looks like, Parsec errors have line-feed in them
+        | pos == C.zeroPos = msgs
+        | otherwise        = msgs ++ formatInput row col
+      where
+        msgs = [ "", filepath' ++ ":" ++ C.showPos pos ++ ": error:", trimLF msg, "" ]
+
+    renderWarning :: C.PWarning -> [String]
+    renderWarning (C.PWarning _ pos@(C.Position row col) msg)
+        | pos == C.zeroPos = msgs
+        | otherwise        = msgs ++ formatInput row col
+      where
+        msgs = [ "", filepath' ++ ":" ++ C.showPos pos ++ ": warning:", trimLF msg, "" ]
+
+    -- sometimes there are (especially trailing) newlines.
+    trimLF :: String -> String
+    trimLF = dropWhile (== '\n') . reverse . dropWhile (== '\n') . reverse
+
+    -- format line: prepend the given line number
+    formatInput :: Int -> Int -> [String]
+    formatInput row col = case advance (row - 1) rowsZipper of
+        Zipper xs ys -> before ++ after where
+            before = case span (\(_, _, b) -> b) xs of
+                (_, [])     -> []
+                (zs, z : _) -> map formatInputLine $ z : reverse zs
+
+            after  = case ys of
+                []        -> []
+                (z : _zs) ->
+                    [ formatInputLine z                             -- error line
+                    , "      | " ++ replicate (col - 1) ' ' ++ "^"  -- pointer: ^
+                    ]
+                    -- do we need rows after?
+                    -- ++ map formatInputLine (take 1 zs)           -- one row after
+
+    formatInputLine :: (String, Int, Bool) -> String
+    formatInputLine (str, row, _) = leftPadShow row ++ " | " ++ str
+
+    -- hopefully we don't need to work with over 99999 lines .cabal files
+    -- at that point small glitches in error messages are hopefully fine.
+    leftPadShow :: Int -> String
+    leftPadShow n = let s = show n in replicate (5 - length s) ' ' ++ s
+
+data Zipper a = Zipper [a] [a]
+
+listToZipper :: [a] -> Zipper a
+listToZipper = Zipper []
+
+advance :: Int -> Zipper a -> Zipper a
+advance n z@(Zipper xs ys)
+    | n <= 0 = z
+    | otherwise = case ys of
+        []      -> z
+        (y:ys') -> advance (n - 1) $ Zipper (y:xs) ys'
diff --git a/src/CabalFmt/Fields.hs b/src/CabalFmt/Fields.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Fields.hs
@@ -0,0 +1,100 @@
+{-# LANGUAGE DeriveFunctor             #-}
+{-# LANGUAGE ExistentialQuantification #-}
+{-# LANGUAGE RankNTypes                #-}
+module CabalFmt.Fields (
+    FieldDescrs,
+    fieldDescrLookup,
+    coerceFieldDescrs,
+    singletonF,
+    ) where
+
+import Distribution.Compat.Newtype
+
+import qualified Data.Map.Strict           as Map
+import qualified Distribution.FieldGrammar as C
+import qualified Distribution.Fields.Field as C
+import qualified Distribution.Parsec       as C
+import qualified Distribution.Pretty       as C
+import qualified Distribution.Compat.CharParsing as C
+import qualified Text.PrettyPrint          as PP
+
+-------------------------------------------------------------------------------
+-- FieldDescr variant
+-------------------------------------------------------------------------------
+
+-- strict pair
+data SP = forall f. SP
+    { _pPretty :: !(f -> PP.Doc)
+    , _pParse  :: !(forall m. C.CabalParsing m => m f)
+    }
+
+-- | Lookup both pretty-printer and value parser.
+--
+-- As the value of the field is unknown, we have to work with it universally.
+--
+fieldDescrLookup
+    :: C.CabalParsing m
+    => FieldDescrs s a
+    -> C.FieldName
+    -> (forall f. m f -> (f -> PP.Doc) -> r)
+    -> Maybe r
+fieldDescrLookup (F m) fn kont = kont' <$> Map.lookup fn m where
+    kont' (SP a b) = kont b a
+
+-- | A collection field parsers and pretty-printers.
+newtype FieldDescrs s a = F { runF :: Map.Map C.FieldName SP }
+  deriving (Functor)
+
+coerceFieldDescrs :: FieldDescrs s a -> FieldDescrs () ()
+coerceFieldDescrs (F a) = F a
+
+instance Semigroup (FieldDescrs s a) where
+    F a <> F b = F (a <> b)
+
+instance Applicative (FieldDescrs s) where
+    pure _  = F mempty
+    f <*> x = F (mappend (runF f) (runF x))
+
+singletonF
+    :: C.FieldName
+    -> (f -> PP.Doc)
+    -> (forall m. C.CabalParsing m => m f)
+    -> FieldDescrs s a
+singletonF fn f g = F $ Map.singleton fn (SP f g)
+
+instance C.FieldGrammar FieldDescrs where
+    blurFieldGrammar _ (F m) = F m
+
+    booleanFieldDef fn _ _def = singletonF fn f C.parsec where
+        f :: Bool -> PP.Doc
+        f s = PP.text (show s)
+
+    uniqueFieldAla fn _pack _ =
+        singletonF fn (C.pretty . pack' _pack) (unpack' _pack <$> C.parsec)
+
+    optionalFieldAla fn _pack _ =
+        singletonF fn (C.pretty . pack' _pack) (unpack' _pack <$> C.parsec)
+
+    optionalFieldDefAla fn _pack _ def =
+        singletonF fn f (unpack' _pack <$> C.parsec)
+      where
+        f s | s == def  = PP.empty
+            | otherwise = C.pretty (pack' _pack s)
+
+    monoidalFieldAla fn _pack _ =
+        singletonF fn (C.pretty . pack' _pack) (unpack' _pack <$> C.parsec)
+
+    freeTextField fn _ = singletonF fn
+        PP.text
+        (C.munch $ const True)
+
+    freeTextFieldDef fn _ = singletonF fn
+        PP.text
+        (C.munch $ const True)
+
+    prefixedFields _fnPfx _l = F mempty
+    knownField _           = pure ()
+    deprecatedSince _  _ x = x
+    removedIn _ _ x        = x
+    availableSince _ _     = id
+    hiddenField _          = F mempty
diff --git a/src/CabalFmt/Fields/BuildDepends.hs b/src/CabalFmt/Fields/BuildDepends.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Fields/BuildDepends.hs
@@ -0,0 +1,124 @@
+{-# LANGUAGE OverloadedStrings #-}
+module CabalFmt.Fields.BuildDepends (
+    buildDependsF,
+    setupDependsF,
+    ) where
+
+import Control.Arrow               ((&&&))
+import Data.Char                   (toLower)
+import Data.List                   (sortOn)
+import Distribution.Compat.Newtype
+
+import qualified Distribution.CabalSpecVersion      as C
+import qualified Distribution.Parsec                as C
+import qualified Distribution.Parsec.Newtypes       as C
+import qualified Distribution.Pretty                as C
+import qualified Distribution.Types.Dependency      as C
+import qualified Distribution.Types.DependencyMap   as C
+import qualified Distribution.Types.PackageName     as C
+import qualified Distribution.Types.Version         as C
+import qualified Distribution.Types.VersionInterval as C
+import qualified Distribution.Types.VersionRange    as C
+import qualified Text.PrettyPrint                   as PP
+
+import CabalFmt.Fields
+
+setupDependsF :: C.CabalSpecVersion -> FieldDescrs () ()
+setupDependsF v = singletonF "setup-depends" (pretty v) parse
+
+buildDependsF :: C.CabalSpecVersion -> FieldDescrs () ()
+buildDependsF v = singletonF "build-depends" (pretty v) parse
+
+parse :: C.CabalParsing m => m [C.Dependency]
+parse = unpack' (C.alaList C.CommaVCat) <$> C.parsec
+
+pretty :: C.CabalSpecVersion -> [C.Dependency] -> PP.Doc
+pretty v deps = case deps of
+    [] -> PP.empty
+    [dep] -> C.pretty (C.depPkgName dep) PP.<+> prettyVR vr'
+      where
+        vr' = either (C.fromVersionIntervals . C.mkVersionIntervals) id
+            $ norm (C.asVersionIntervals $ C.depVerRange dep)
+        prettyVR vr | vr == C.anyVersion = PP.empty
+                    | otherwise          = C.pretty vr
+    _ -> PP.vcat (zipWith pretty' (True : repeat False) deps')
+      where
+        deps' = sortOn (map toLower . fst)
+              $ map (C.unPackageName . C.depPkgName &&& C.asVersionIntervals . C.depVerRange)
+              $ C.fromDepMap . C.toDepMap -- this combines duplicate packages
+              $ deps
+
+        width = maximum (0 : map (length . fst) deps') + 1
+        width' = maximum (0 : map (firstComponent . snd) deps')
+
+        -- we assume cabal-version: 2.2 or higher
+        pretty' :: Bool -> (String, [C.VersionInterval]) -> PP.Doc
+        pretty' isFirst (name, vis)
+            | empty vis = comma PP.<+> PP.text name
+            | otherwise = case norm vis of
+                Left [] -> comma PP.<+> PP.text name
+                Left (vi : vis') ->
+                    comma PP.<+>
+                    PP.text (leftpad width name) PP.<+>
+                    PP.hsep
+                        ( prettyVi vi
+                        : map (\vi' -> PP.text "||" PP.<+> prettyVi' vi') vis'
+                        )
+                Right vr ->
+                    comma PP.<+>
+                    PP.text (leftpad width name) PP.<+>
+                    C.pretty vr
+          where
+            comma | isFirst, v < C.CabalSpecV2_2 = PP.text " "
+                  | otherwise = PP.comma
+
+        -- indent first
+        prettyVi (C.LowerBound l lb, C.NoUpperBound) =
+            prettyLowerBound lb PP.<> C.pretty l
+        prettyVi (C.LowerBound l C.InclusiveBound, C.UpperBound u C.InclusiveBound)
+            | l == u = PP.text "==" PP.<> C.pretty l
+        prettyVi (C.LowerBound l lb, C.UpperBound u ub) =
+            prettyLowerBound lb PP.<> PP.text (leftpad width' l')
+            PP.<+> PP.text "&&" PP.<+>
+            prettyUpperBound ub PP.<> C.pretty u
+          where
+            l' = C.prettyShow l
+
+        prettyVi' (C.LowerBound l lb, C.NoUpperBound) =
+            prettyLowerBound lb PP.<> C.pretty l
+        prettyVi' (C.LowerBound l C.InclusiveBound, C.UpperBound u C.InclusiveBound)
+            | l == u = PP.text "==" PP.<> C.pretty l
+        prettyVi' (C.LowerBound l lb, C.UpperBound u ub) =
+            prettyLowerBound lb PP.<> C.pretty l PP.<+> PP.text "&&" PP.<+>
+            prettyUpperBound ub PP.<> C.pretty u
+
+        prettyLowerBound :: C.Bound -> PP.Doc
+        prettyLowerBound C.InclusiveBound = PP.text ">="
+        prettyLowerBound C.ExclusiveBound = PP.text ">"
+
+        prettyUpperBound :: C.Bound -> PP.Doc
+        prettyUpperBound C.InclusiveBound = PP.text "<="
+        prettyUpperBound C.ExclusiveBound = PP.text "<"
+  where
+    empty :: [C.VersionInterval] -> Bool
+    empty []                                                  = True
+    empty [(C.LowerBound l C.InclusiveBound, C.NoUpperBound)] = l == C.mkVersion [0]
+    empty _                                                   = False
+
+    norm :: [C.VersionInterval] -> Either [C.VersionInterval] C.VersionRange
+    norm []     = Right C.anyVersion
+    norm (i:is) = maybe (Left $ i:is) Right $
+        foldr1 C.unionVersionRanges <$> traverse f (i : is)
+      where
+        f :: C.VersionInterval -> Maybe C.VersionRange
+        f (C.LowerBound l C.InclusiveBound, C.UpperBound u C.ExclusiveBound)
+            | v >= C.CabalSpecV2_0, u == C.majorUpperBound l =
+                Just (C.majorBoundVersion l)
+        f _ = Nothing
+
+    firstComponent :: [C.VersionInterval] -> Int
+    firstComponent [] = 0
+    firstComponent ((C.LowerBound l _, _) : _) = length (C.prettyShow l)
+
+leftpad :: Int -> String -> String
+leftpad w s = s ++ replicate (w - length s) ' '
diff --git a/src/CabalFmt/Fields/Extensions.hs b/src/CabalFmt/Fields/Extensions.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Fields/Extensions.hs
@@ -0,0 +1,28 @@
+{-# LANGUAGE OverloadedStrings #-}
+module CabalFmt.Fields.Extensions (
+    otherExtensionsF,
+    defaultExtensionsF,
+    ) where
+
+import Data.List                   (sortOn)
+import Distribution.Compat.Newtype
+
+import qualified Distribution.Parsec          as C
+import qualified Distribution.Parsec.Newtypes as C
+import qualified Distribution.Pretty          as C
+import qualified Language.Haskell.Extension   as C
+import qualified Text.PrettyPrint             as PP
+
+import CabalFmt.Fields
+
+otherExtensionsF :: FieldDescrs () ()
+otherExtensionsF = singletonF "other-extensions" pretty parse
+
+defaultExtensionsF :: FieldDescrs () ()
+defaultExtensionsF = singletonF "default-extensions" pretty parse
+
+parse :: C.CabalParsing m => m [C.Extension]
+parse = unpack' (C.alaList' C.FSep C.MQuoted) <$> C.parsec
+
+pretty :: [C.Extension] -> PP.Doc
+pretty = PP.vcat . map C.pretty . sortOn show
diff --git a/src/CabalFmt/Fields/Modules.hs b/src/CabalFmt/Fields/Modules.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Fields/Modules.hs
@@ -0,0 +1,43 @@
+{-# LANGUAGE OverloadedStrings #-}
+module CabalFmt.Fields.Modules (
+    otherModulesF,
+    exposedModulesF,
+    ) where
+
+import Data.Function               (on)
+import Data.List                   (nub, sortBy)
+import Distribution.Compat.Newtype
+
+import qualified Distribution.ModuleName      as C
+import qualified Distribution.Parsec          as C
+import qualified Distribution.Parsec.Newtypes as C
+import qualified Distribution.Pretty          as C
+import qualified Text.PrettyPrint             as PP
+
+import CabalFmt.Fields
+
+exposedModulesF :: FieldDescrs () ()
+exposedModulesF = singletonF "exposed-modules" pretty parse
+
+otherModulesF :: FieldDescrs () ()
+otherModulesF = singletonF "other-modules" pretty parse
+
+parse :: C.CabalParsing m => m [C.ModuleName]
+parse = unpack' (C.alaList' C.VCat C.MQuoted) <$> C.parsec
+
+pretty :: [C.ModuleName] -> PP.Doc
+pretty = PP.vcat . map C.pretty . nub . sortBy (cmp `on` C.prettyShow)
+  where
+    cmp a b = case dropCommonPrefix a b of
+        ([], [])  -> EQ
+        ([], _:_) -> LT
+        (_:_, []) -> GT
+        (a', b')  -> compare a' b'
+
+dropCommonPrefix :: Eq a => [a] -> [a] -> ([a], [a])
+dropCommonPrefix [] [] = ([], [])
+dropCommonPrefix [] ys = ([], ys)
+dropCommonPrefix xs [] = (xs, [])
+dropCommonPrefix xs@(x:xs') ys@(y:ys')
+    | x == y    = dropCommonPrefix xs' ys'
+    | otherwise = (xs, ys)
diff --git a/src/CabalFmt/Fields/TestedWith.hs b/src/CabalFmt/Fields/TestedWith.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Fields/TestedWith.hs
@@ -0,0 +1,34 @@
+{-# LANGUAGE OverloadedStrings #-}
+module CabalFmt.Fields.TestedWith (
+    testedWithF,
+    ) where
+
+import Distribution.Compat.Newtype
+
+import qualified Distribution.Parsec                as C
+import qualified Distribution.Compiler                as C
+import qualified Distribution.Parsec.Newtypes       as C
+import qualified Distribution.Pretty                as C
+import qualified Distribution.Types.VersionRange    as C
+import qualified Text.PrettyPrint                   as PP
+import qualified Data.Map.Strict as Map
+
+import CabalFmt.Fields
+
+testedWithF :: FieldDescrs () ()
+testedWithF = singletonF "tested-with" pretty parse where
+    parse :: C.CabalParsing m => m [(C.CompilerFlavor, C.VersionRange)]
+    parse = unpack' (C.alaList' C.FSep C.TestedWith) <$> C.parsec
+
+    pretty :: [(C.CompilerFlavor, C.VersionRange)] -> PP.Doc
+    pretty tw0 = PP.fsep $ PP.punctuate PP.comma
+        [ prettyC c PP.<+> C.pretty vr
+        | (c, vr) <- Map.toList tw1
+        ]
+      where
+        tw1 :: Map.Map C.CompilerFlavor C.VersionRange
+        tw1 = Map.fromListWith C.unionVersionRanges tw0
+
+        prettyC C.GHC   = PP.text "GHC"
+        prettyC C.GHCJS = PP.text "GHCJS"
+        prettyC c       = C.pretty c
diff --git a/src/CabalFmt/Monad.hs b/src/CabalFmt/Monad.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Monad.hs
@@ -0,0 +1,17 @@
+{-# LANGUAGE DerivingStrategies, GeneralizedNewtypeDeriving #-}
+module CabalFmt.Monad (
+    CabalFmt,
+    runCabalFmt,
+    ) where
+
+import Control.Monad.Except (MonadError)
+import Control.Monad.Reader (MonadReader, ReaderT, runReaderT)
+
+import CabalFmt.Error
+import CabalFmt.Options
+
+newtype CabalFmt a = CabalFmt { unCabalFmt :: ReaderT Options (Either Error) a }
+  deriving newtype (Functor, Applicative, Monad, MonadReader Options, MonadError Error)
+
+runCabalFmt :: Options -> CabalFmt a -> Either Error a
+runCabalFmt opts m = runReaderT (unCabalFmt m) opts
diff --git a/src/CabalFmt/Options.hs b/src/CabalFmt/Options.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Options.hs
@@ -0,0 +1,20 @@
+module CabalFmt.Options (
+    Options (..),
+    defaultOptions,
+    ) where
+
+import qualified Distribution.CabalSpecVersion as C
+
+data Options = Options
+    { optIndent      :: !Int
+    , optSpecVersion :: !C.CabalSpecVersion
+    , optFileList    :: ![FilePath]
+    }
+  deriving Show
+
+defaultOptions :: Options
+defaultOptions = Options
+    { optIndent      = 2
+    , optSpecVersion = C.cabalSpecLatest
+    , optFileList    = []
+    }
diff --git a/src/CabalFmt/Parser.hs b/src/CabalFmt/Parser.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Parser.hs
@@ -0,0 +1,27 @@
+module CabalFmt.Parser where
+
+import Control.Monad.Except (throwError)
+
+import qualified Data.ByteString                              as BS
+import qualified Distribution.Fields                          as C
+import qualified Distribution.PackageDescription.Parsec       as C
+import qualified Distribution.Parsec                          as C
+import qualified Distribution.Types.GenericPackageDescription as C
+
+import CabalFmt.Error
+import CabalFmt.Monad
+
+runParseResult :: FilePath -> BS.ByteString -> C.ParseResult a -> CabalFmt a
+runParseResult filepath contents pr = case result of
+    Right gpd -> return gpd
+    Left (mspecVersion, errors) -> throwError $ CabalParseError filepath contents errors mspecVersion warnings
+  where
+    (warnings, result) = C.runParseResult pr
+
+parseGpd :: FilePath -> BS.ByteString -> CabalFmt C.GenericPackageDescription
+parseGpd filepath contents = runParseResult filepath contents $ C.parseGenericPackageDescription contents
+
+parseFields :: BS.ByteString -> CabalFmt [C.Field C.Position]
+parseFields contents = case C.readFields contents of
+    Left err -> throwError $ PanicCannotParseInput err
+    Right x  -> return x
diff --git a/src/CabalFmt/Refactoring.hs b/src/CabalFmt/Refactoring.hs
new file mode 100644
--- /dev/null
+++ b/src/CabalFmt/Refactoring.hs
@@ -0,0 +1,76 @@
+{-# LANGUAGE OverloadedStrings #-}
+module CabalFmt.Refactoring (
+    Refactoring,
+    refactoringExpandExposedModules,
+    ) where
+
+import Data.List       (intercalate, stripPrefix)
+import Data.Maybe      (catMaybes)
+import System.FilePath (dropExtension, splitDirectories)
+
+import qualified Distribution.Compat.CharParsing     as C
+import qualified Distribution.Fields                 as C
+import qualified Distribution.Parsec                 as C
+import qualified Distribution.Parsec.FieldLineStream as C
+import qualified Distribution.Simple.Utils           as C
+
+import CabalFmt.Comments
+import CabalFmt.Options
+
+-------------------------------------------------------------------------------
+-- Refactoring type
+-------------------------------------------------------------------------------
+
+type Refactoring = Options -> [C.Field Comments] -> [C.Field Comments]
+
+-------------------------------------------------------------------------------
+-- Expand exposed-modules
+-------------------------------------------------------------------------------
+
+refactoringExpandExposedModules :: Refactoring
+refactoringExpandExposedModules opts = overField refact where
+    refact name@(C.Name c n) fls
+        | n == "exposed-modules" || n == "other-modules"
+        , definitions <- parse c =
+            let newModules :: [C.FieldLine Comments]
+                newModules = catMaybes
+                    [ do rest <- stripPrefix prefix fp
+                         return $ C.FieldLine mempty $ C.toUTF8BS $ intercalate "." rest
+                    | prefix <- definitions
+                    , fp <- fileList
+                    ]
+            in (name, newModules ++ fls)
+        | otherwise = (name, fls)
+
+    fileList :: [[FilePath]]
+    fileList = map (splitDirectories . dropExtension) (optFileList opts)
+
+    parse :: Comments -> [[FilePath]]
+    parse (Comments bss) = catMaybes
+        [ either (const Nothing) Just
+        $ C.runParsecParser parser "<input>" $ C.fieldLineStreamFromBS bs
+        | bs <- bss
+        ]
+
+    parser :: C.ParsecParser [FilePath]
+    parser = do
+        _ <- C.string "--"
+        C.spaces
+        _ <- C.string "cabal-fmt:"
+        C.spaces
+        _ <- C.string "expand"
+        C.spaces
+        dir <- C.parsecToken
+        return (splitDirectories dir)
+
+-------------------------------------------------------------------------------
+-- Tools
+-------------------------------------------------------------------------------
+
+overField :: (C.Name Comments -> [C.FieldLine Comments] -> (C.Name Comments, [C.FieldLine Comments]))
+          -> [C.Field Comments] -> [C.Field Comments]
+overField f = goMany where
+    goMany = map go
+
+    go (C.Field name fls)       = let ~(name', fls') = f name fls in C.Field name' fls'
+    go (C.Section name args fs) = C.Section name args (goMany fs)
diff --git a/tests/Golden.hs b/tests/Golden.hs
new file mode 100644
--- /dev/null
+++ b/tests/Golden.hs
@@ -0,0 +1,43 @@
+module Main (main) where
+
+import Distribution.Simple.Utils  (fromUTF8BS, toUTF8BS)
+import System.FilePath            ((-<.>), (</>))
+import System.Process             (readProcessWithExitCode)
+import Test.Tasty                 (TestTree, defaultMain, testGroup)
+import Test.Tasty.Golden.Advanced (goldenTest)
+
+import qualified Data.ByteString as BS
+
+import CabalFmt         (cabalFmt)
+import CabalFmt.Monad   (runCabalFmt)
+import CabalFmt.Options (defaultOptions)
+
+main :: IO ()
+main = defaultMain $ testGroup "tests"
+    [ goldenTest' "cabal-fmt"
+    , goldenTest' "Cabal"
+    , goldenTest' "simple-example"
+    ]
+
+goldenTest' :: String -> TestTree
+goldenTest' n = goldenTest n readGolden makeTest cmp writeGolden
+  where
+    goldenPath = "fixtures" </> n -<.> "format"
+    inputPath  = "fixtures" </> n -<.> "cabal"
+
+    readGolden  = BS.readFile goldenPath
+    writeGolden = BS.writeFile goldenPath
+
+    makeTest = do
+        contents <- BS.readFile inputPath
+        case runCabalFmt defaultOptions $ cabalFmt inputPath contents of
+            Right output' -> return (toUTF8BS output')
+            Left err      -> fail (show err)
+
+    cmp a b | a == b    = return Nothing
+            | otherwise = Just <$> readProcess' "diff" ["-u", goldenPath, "-"] (fromUTF8BS b)
+
+
+    readProcess' proc args input = do
+        (_, out, _) <- readProcessWithExitCode proc args input
+        return out