Better error when helper does not exist (see issue #4)

lib/ruby-handlebars/tree.rb

@@ -1,4 +1,7 @@
 module Handlebars
+  class UnknownHelper < StandardError
+  end
+
   module Tree
     class TreeItem < Struct
       def eval(context)
@@ -50,7 +53,9 @@ def _eval(context)
 
     class Helper < TreeItem.new(:name, :parameters, :block)
       def _eval(context)
-        context.get_helper(name.to_s).apply(context, parameters, block)
+        helper = context.get_helper(name.to_s)
+        raise(UnknownHelper, "Helper \"#{name}\" does not exist" )if helper.nil?
+        helper.apply(context, parameters, block)
       end
     end
 

spec/handlebars_spec.rb

@@ -435,4 +435,16 @@ def self.escape(value)
       end
     end
   end
+
+  context 'regression tests' do
+    context 'when an unknown helper is called in a template' do
+      it 'should provide a useful error message with inline helpers' do
+        expect{ evaluate('{{unknown "This will hardly work" }}') }.to raise_exception(Handlebars::UnknownHelper, 'Helper "unknown" does not exist')
+      end
+
+      it 'should provide a useful error message with block helpers' do
+        expect{ evaluate('{{#unknown}}This will hardly work{{/unknown}}') }.to raise_exception(Handlebars::UnknownHelper, 'Helper "unknown" does not exist')
+      end
+    end
+  end
 end