Add cabal-helper for decoding Cabal-1.22 setup-configs

.. without having to worry about Cabal version conflicts

ghc-mod.cabal

@@ -186,6 +186,21 @@ Executable ghc-modi
                       , ghc
                       , ghc-mod
 
+Executable cabal-helper
+  Default-Language:     Haskell2010
+  Main-Is:              GHCModCabal.hs
+  GHC-Options:          -Wall
+  HS-Source-Dirs:       src
+  X-Install-Target:     $libexecdir
+  Build-Depends:        base >= 4.0 && < 5
+                      , bytestring
+                      , binary
+                      , directory
+  if flag(cabal-122)
+    Build-Depends:      Cabal >= 1.22
+  else
+    Buildable:          False
+
 Test-Suite doctest
   Type:                 exitcode-stdio-1.0
   Default-Language:     Haskell2010

src/GHCModCabal.hs

@@ -0,0 +1,28 @@
+{-# LANGUAGE BangPatterns #-}
+module Main where
+
+import Control.Applicative
+
+import Distribution.Simple.Utils (cabalVersion)
+import Distribution.Simple.Configure
+import Distribution.Text ( display )
+import System.Environment
+import System.Directory
+
+main :: IO ()
+main = do
+  args <- getArgs
+  case args of
+    "version":[] -> do
+        putStrLn $ "using version " ++ display cabalVersion ++ " of the Cabal library"
+    "print-setup-config":args' -> do
+        mfile <- findFile ["dist"] "setup-config"
+
+        let file = case mfile of
+                     Just f -> f
+                     Nothing -> let !(f:[]) = args' in f
+
+        putStrLn =<< show <$> getConfigStateFile file
+
+    cmd:_ -> error $ "Unknown command: " ++ cmd
+    [] -> error "No command given"