D.Compat.Binary: backport binary generics to binary-0.5

GHC generics are used to derive binary instances for types appearing
in the persistent build config, which requires GHC >= 7.2 and
binary >= 0.7. Unfortunately, GHC < 7.8 ships with binary == 0.5.*.
The missing module is Data.Binary.Generics, which we have copied from
binary-0.7.2.3 to Distribution.Compat.Binary.Generics. To provide
generic implementations for the Binary class, we also have to provide
our own implementation, which is copied from binary-0.7.2.3 to
Distribution.Compat.Binary.Class. The interface required by Cabal is
exported from Distribution.Compat.Binary. This is only done if
bootstrapping Cabal with GHC < 7.8 or if binary >= 0.7 is not available,
otherwise Distribution.Compat.Binary simply re-exports Data.Binary.

Cabal/Cabal.cabal

@@ -126,10 +126,12 @@ source-repository head
   location: https://github.com/haskell/cabal/
   subdir:   Cabal
 
+flag bundled-binary-generic
+  default: False
+
 library
   build-depends:
     base       >= 4.4 && < 5,
-    binary     >= 0.7 && < 0.8,
     deepseq    >= 1.3 && < 1.5,
     filepath   >= 1   && < 1.4,
     directory  >= 1   && < 1.3,
@@ -140,6 +142,11 @@ library
     pretty     >= 1   && < 1.2,
     bytestring >= 0.9
 
+  if flag(bundled-binary-generic)
+    build-depends: binary >= 0.5 && < 0.7
+  else
+    build-depends: binary >= 0.7 && < 0.8
+
   -- Needed for GHC.Generics before GHC 7.6
   if impl(ghc < 7.6)
     build-depends: ghc-prim >= 0.2 && < 0.3
@@ -225,6 +232,7 @@ library
     Language.Haskell.Extension
 
   other-modules:
+    Distribution.Compat.Binary
     Distribution.Compat.CopyFile
     Distribution.Compat.TempFile
     Distribution.GetOpt
@@ -234,6 +242,11 @@ library
     Distribution.Simple.GHC.ImplInfo
     Paths_Cabal
 
+  if flag(bundled-binary-generic)
+    other-modules:
+      Distribution.Compat.Binary.Class
+      Distribution.Compat.Binary.Generic
+
   default-language: Haskell98
   default-extensions: CPP
 
@@ -294,7 +307,6 @@ test-suite package-tests
   hs-source-dirs: tests
   build-depends:
     base,
-    binary     >= 0.7 && < 0.8,
     containers,
     test-framework,
     test-framework-quickcheck2 >= 0.2.12,
@@ -312,4 +324,4 @@ test-suite package-tests
     build-depends: unix
   ghc-options: -Wall
   default-extensions: CPP
-  default-language: Haskell98
+  default-language: Haskell98

Cabal/Distribution/Compat/Binary.hs

@@ -0,0 +1,57 @@
+{-# LANGUAGE CPP #-}
+
+#ifndef MIN_VERSION_binary
+#define MIN_VERSION_binary(x, y, z) 0
+#endif
+
+module Distribution.Compat.Binary
+       ( decodeOrFailIO
+#if __GLASGOW_HASKELL__ >= 708 || MIN_VERSION_binary(0,7,0)
+       , module Data.Binary
+#else
+       , Binary(..)
+       , decode, encode
+#endif
+       ) where
+
+import Data.ByteString.Lazy (ByteString)
+
+#if __GLASGOW_HASKELL__ >= 708 || MIN_VERSION_binary(0,7,0)
+
+import Data.Binary
+
+decodeOrFailIO :: Binary a => ByteString -> IO (Either String a)
+decodeOrFailIO bs =
+  return $ case decodeOrFail bs of
+    Left (_, _, msg) -> Left msg
+    Right (_, _, a) -> Right a
+
+#else
+
+import Control.Exception (ErrorCall(..), catch, evaluate)
+import Data.Binary.Get
+import Data.Binary.Put
+#if __GLASGOW_HASKELL__ < 706
+import Prelude hiding (catch)
+#endif
+
+import Distribution.Compat.Binary.Class
+import Distribution.Compat.Binary.Generic ()
+
+-- | Decode a value from a lazy ByteString, reconstructing the original structure.
+--
+decode :: Binary a => ByteString -> a
+decode = runGet get
+
+-- | Encode a value using binary serialisation to a lazy ByteString.
+--
+encode :: Binary a => a -> ByteString
+encode = runPut . put
+{-# INLINE encode #-}
+
+decodeOrFailIO :: Binary a => ByteString -> IO (Either String a)
+decodeOrFailIO bs =
+  catch (evaluate (decode bs) >>= return . Right)
+  $ \(ErrorCall str) -> return $ Left str
+
+#endif