added assert() macro.

if failed, spits out this on compile:

 = FAILED assertion while evaluating node 'worm'
   - assertion: services.include? 'couchdb'
   - offending key: require_couchdb
 = bailing out

2 files changed, 28 insertions, 0 deletions

diff --git a/lib/leap_cli/config/macros.rb b/lib/leap_cli/config/macros.rb
index f46799c..b5f6a34 100644
--- a/lib/leap_cli/config/macros.rb
+++ b/lib/leap_cli/config/macros.rb
@@ -335,5 +335,27 @@ module LeapCli; module Config
       entries.join("\n")
     end
 
+    ##
+    ## UTILITY
+    ##
+
+    class AssertionFailed < Exception
+      attr_accessor :assertion
+      def initialize(assertion)
+        @assertion = assertion
+      end
+      def to_s
+        @assertion
+      end
+    end
+
+    def assert(assertion)
+      if instance_eval(assertion)
+        true
+      else
+        raise AssertionFailed.new(assertion)
+      end
+    end
+
   end
 end; end
diff --git a/lib/leap_cli/config/object.rb b/lib/leap_cli/config/object.rb
index 00997b1..1d296b3 100644
--- a/lib/leap_cli/config/object.rb
+++ b/lib/leap_cli/config/object.rb
@@ -282,6 +282,12 @@ module LeapCli
                 Util::log "error message: no file '#{exc}'", :indent => 1
               end
             end
+          rescue AssertionFailed => exc
+            Util.bail! do
+              Util::log :failed, "assertion while evaluating node '#{@node.name}'"
+              Util::log 'assertion: %s' % exc.assertion, :indent => 1
+              Util::log "offending key: #{key}", :indent => 1
+            end
           rescue SyntaxError, StandardError => exc
             Util::bail! do
               Util::log :error, "while evaluating node '#{@node.name}'"