git-annex: 5.20140421

installing git-annex with cabal-install is quite long and requires to install
some heavy packages. It also has several external lib dependencies and needs a
few configuration flags to build so it is quite difficult to install too.
This formula should make it easy and quick to install with a bottle.

The huge number of haskell dependencies is built within the formula and statically
linked to git-annex. The haskell libraries built are discarded - cabal-install
should be used instead of this package in order to keep them.

Library/Homebrew/language/haskell.rb

@@ -0,0 +1,68 @@
+module Language
+  module Haskell
+    # module for formulas using cabal-install as build tool
+    module Cabal
+      def cabal_sandbox
+        pwd = Pathname.pwd
+        # force cabal to put its stuff here instead of the home directory by
+        # pretending the home is here. This also avoid to deal with many options
+        # to configure cabal. Note this is also useful with cabal sandbox to
+        # avoid touching ~/.cabal
+        home = ENV["HOME"]
+        ENV["HOME"] = pwd
+        # use cabal's sandbox feature if available
+        cabal_version = `cabal --version`[/[0-9.]+/].split('.').collect(&:to_i)
+        if (cabal_version <=> [1, 20]) > -1
+          system "cabal", "sandbox", "init"
+          cabal_sandbox_bin = pwd/".cabal-sandbox/bin"
+        else
+          # no or broken sandbox feature - just use the HOME trick
+          cabal_sandbox_bin = pwd/".cabal/bin"
+        end
+        # cabal may build useful tools that should be found in the PATH
+        mkdir_p cabal_sandbox_bin
+        path = ENV["PATH"]
+        ENV.prepend_path 'PATH', cabal_sandbox_bin
+        # update cabal package database
+        system "cabal", "update"
+        yield
+        # restore the environment
+        if (cabal_version <=> [1, 20]) > -1
+          system "cabal", "sandbox", "delete"
+        end
+        ENV["HOME"] = home
+        ENV["PATH"] = path
+      end
+
+      def cabal_install(*opts)
+        system "cabal", "install", "--jobs=#{ENV.make_jobs}", *opts
+      end
+
+      # install the tools passed in parameter and remove the packages that where
+      # used so they won't be in the way of the dependency solver for the main
+      # package. The tools are installed sequentially in order to make possible
+      # to install several tools that depends on each other
+      def cabal_install_tools(*opts)
+        opts.each {|t| cabal_install t}
+        rm_rf Dir[".cabal*/*packages.conf.d/"]
+      end
+
+      # remove the development files from the lib directory. cabal-install should
+      # be used instead to install haskell packages
+      def cabal_clean_lib
+        # a better approach may be needed here
+        rm_rf lib
+      end
+
+      def install_cabal_package
+        cabal_sandbox do
+          # the dependencies are built first and installed locally, and only the
+          # current package is actually installed in the destination dir
+          cabal_install "--only-dependencies"
+          cabal_install "--prefix=#{prefix}"
+        end
+        cabal_clean_lib
+      end
+    end
+  end
+end