From a94ff15c5993ceaee3d28a8d2c117d53ada0a299 Mon Sep 17 00:00:00 2001
From: Chris Done <chrisdone@gmail.com>
Date: Tue, 13 Jan 2015 19:26:34 +0100
Subject: [PATCH] Only unpack when necessary

---
 Stackage/ShakeBuild.hs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/Stackage/ShakeBuild.hs b/Stackage/ShakeBuild.hs
index d821b08f..e4f4aaf1 100644
--- a/Stackage/ShakeBuild.hs
+++ b/Stackage/ShakeBuild.hs
@@ -88,7 +88,9 @@ packageTarget pb shakeDir name plan = do
               (M.keys (sdPackages (ppDesc plan))))
     pwd <- liftIO getCurrentDirectory
     env <- liftIO (fmap (Env . (++ defaultEnv pwd)) getEnvironment)
-    () <- cmd (Cwd shakeDir) "cabal" "unpack" nameVer
+    unpacked <- liftIO (doesDirectoryExist (shakeDir <//> nameVer))
+    unless unpacked
+           (cmd (Cwd shakeDir) "cabal" "unpack" nameVer)
     () <- cmd cwd env "cabal" "configure" (opts pwd)
     () <- cmd cwd env "cabal" "build"
     () <- cmd cwd env "cabal" "copy"