add haml _form file and copy it into place if app template engine is haml

lib/generators/simple_form/install_generator.rb

@@ -13,7 +13,11 @@ module SimpleForm
       end
 
       def copy_scaffold_template
-        copy_file '_form.html.erb', 'lib/templates/erb/scaffold/_form.html.erb'
+        if Rails::Generators.options[:rails][:template_engine] == :haml
+          copy_file '_form.html.haml', 'lib/templates/haml/scaffold/_form.html.haml'
+        else
+          copy_file '_form.html.erb', 'lib/templates/erb/scaffold/_form.html.erb'
+        end
       end
     end
   end

lib/generators/simple_form/templates/_form.html.haml

@@ -0,0 +1,18 @@
+= simple_form_for(@<%= singular_name %>) do |f|
+  - if @<%= singular_name %>.errors.any?
+    #error_explanation
+      %h2
+        = pluralize(@<%= singular_name %>.errors.count, "error")
+        prohibited this <%= singular_name %> from being saved:
+
+      %ul
+      - @<%= singular_name %>.errors.full_messages.each do |msg|
+        %li= msg
+
+  .inputs
+  <%- attributes.each do |attribute| -%>
+    = f.<%= attribute.reference? ? :association : :input %> :<%= attribute.name %>
+  <%- end -%>
+
+  .actions
+    = f.button :submit