mlton: use a requirement to check install location

Formula/mlton.rb

@@ -4,6 +4,20 @@ require 'formula'
 # Since MLton is written in ML, building from source
 # would require an existing ML compiler/interpreter for bootstrapping.
 
+class StandardHomebrewLocation < Requirement
+  def message; <<-EOS.undent
+    mlton won't work outside of /usr/local
+
+    Because this uses pre-compiled binaries, it will not work if
+    Homebrew is installed somewhere other than /usr/local; mlton
+    will be unable to find GMP.
+    EOS
+  end
+  def satisfied?
+    HOMEBREW_PREFIX.to_s == "/usr/local"
+  end
+end
+
 class Mlton < Formula
   homepage 'http://mlton.org'
   url 'http://mlton.org/pages/Download/attachments/mlton-20100608-1.amd64-darwin.gmp-static.tgz'
@@ -11,18 +25,12 @@ class Mlton < Formula
 
   # We download and install the version of MLton which is statically linked to libgmp, but all
   # generated executables will require gmp anyway, hence the dependency
+  depends_on StandardHomebrewLocation.new
   depends_on 'gmp'
 
   skip_clean :all
 
   def install
-    unless HOMEBREW_PREFIX.to_s == "/usr/local"
-      opoo "mlton won't work outside of /usr/local."
-      puts "Because this uses pre-compiled binaries, it will not work if"
-      puts "Homebrew is installed somewhere other than /usr/local; mlton"
-      puts "will be unable to find GMP."
-    end
-
     cd "local" do
       # Remove OS X droppings
       rm Dir["man/man1/._*"]