ghc: use resource

Formula/ghc.rb

@@ -1,16 +1,5 @@
 require 'formula'
 
-class Ghcbinary < Formula
-  if Hardware.is_64_bit? and not build.build_32_bit?
-    url 'http://www.haskell.org/ghc/dist/7.4.2/ghc-7.4.2-x86_64-apple-darwin.tar.bz2'
-    sha1 '7c655701672f4b223980c3a1068a59b9fbd08825'
-  else
-    url 'http://www.haskell.org/ghc/dist/7.4.2/ghc-7.4.2-i386-apple-darwin.tar.bz2'
-    sha1 '60f749893332d7c22bb4905004a67510992d8ef6'
-  end
-  version '7.4.2'
-end
-
 class Ghc < Formula
   homepage 'http://haskell.org/ghc/'
   url 'http://www.haskell.org/ghc/dist/7.6.3/ghc-7.6.3-src.tar.bz2'
@@ -30,6 +19,18 @@ class Ghc < Formula
   option '32-bit'
   option 'tests', 'Verify the build using the testsuite in Fast Mode, 5 min'
 
+  # build is not available in the resource's context, so exploit the closure.
+  build_32_bit = build.build_32_bit?
+  resource 'binary' do
+    if Hardware.is_64_bit? and not build_32_bit
+      url 'http://www.haskell.org/ghc/dist/7.4.2/ghc-7.4.2-x86_64-apple-darwin.tar.bz2'
+      sha1 '7c655701672f4b223980c3a1068a59b9fbd08825'
+    else
+      url 'http://www.haskell.org/ghc/dist/7.4.2/ghc-7.4.2-i386-apple-darwin.tar.bz2'
+      sha1 '60f749893332d7c22bb4905004a67510992d8ef6'
+    end
+  end
+
   resource 'testsuite' do
     url 'https://github.com/ghc/testsuite/archive/ghc-7.6.3-release.tar.gz'
     sha1 '6a1973ae3cccdb2f720606032ae84ffee8680ca1'
@@ -51,7 +52,7 @@ class Ghc < Formula
     # Move the main tarball contents into a subdirectory
     (buildpath+'Ghcsource').install Dir['*']
 
-    Ghcbinary.new.brew do
+    resource('binary').stage do
       # Define where the subformula will temporarily install itself
       subprefix = buildpath+'subfo'