安装 rails3 后，我在尝试使用 haml 时遇到问题。
我安装了更新的 gem，之后rails PROJECT_NAME，我haml --rails在它的根目录下安装了。
它显然工作得很好，因为我在插件 init.rb 中有 haml 文件夹，正如预期的那样。
但是当我尝试 rake 或 rails server 时，我得到：
rake aborted!
no such file to load -- haml
使用 --trace 我得到这个：
** Invoke default (first_time)
** Invoke test (first_time)
** Execute test
** Invoke test:units (first_time)
** Invoke db:test:prepare (first_time)
** Invoke db:abort_if_pending_migrations (first_time)
** Invoke environment (first_time)
** Execute environment
rake aborted!
no such file to load -- haml
/usr/local/lib/ruby/gems/1.8/gems/activesupport-3.0.0.beta/lib/active_support/dependencies.rb:167:in `require'
/usr/local/lib/ruby/gems/1.8/gems/activesupport-3.0.0.beta/lib/active_support/dependencies.rb:167:in `require'
/usr/local/lib/ruby/gems/1.8/gems/activesupport-3.0.0.beta/lib/active_support/dependencies.rb:537:in `new_constants_in'
/usr/local/lib/ruby/gems/1.8/gems/activesupport-3.0.0.beta/lib/active_support/dependencies.rb:167:in `require'
RAILS_PROJECT_ROOT/vendor/plugins/haml/init.rb:5
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/plugin.rb:49
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/initializable.rb:25:in `instance_exec'
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/initializable.rb:25:in `run'
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/initializable.rb:55:in `run_initializers'
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/initializable.rb:54:in `each'
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/initializable.rb:54:in `run_initializers'
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/application.rb:71:in `initialize!'
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/application.rb:112:in `initialize_tasks'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:636:in `call'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:636:in `execute'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:631:in `each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:631:in `execute'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:597:in `invoke_with_call_chain'
/usr/local/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:590:in `invoke_with_call_chain'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:607:in `invoke_prerequisites'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:604:in `each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:604:in `invoke_prerequisites'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:596:in `invoke_with_call_chain'
/usr/local/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:590:in `invoke_with_call_chain'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:607:in `invoke_prerequisites'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:604:in `each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:604:in `invoke_prerequisites'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:596:in `invoke_with_call_chain'
/usr/local/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:590:in `invoke_with_call_chain'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:607:in `invoke_prerequisites'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:604:in `each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:604:in `invoke_prerequisites'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:596:in `invoke_with_call_chain'
/usr/local/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:590:in `invoke_with_call_chain'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:583:in `invoke'
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/test_unit/testing.rake:45
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/test_unit/testing.rake:43:in `collect'
/usr/local/lib/ruby/gems/1.8/gems/railties-3.0.0.beta/lib/rails/test_unit/testing.rake:43
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:636:in `call'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:636:in `execute'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:631:in `each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:631:in `execute'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:597:in `invoke_with_call_chain'
/usr/local/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:590:in `invoke_with_call_chain'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:607:in `invoke_prerequisites'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:604:in `each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:604:in `invoke_prerequisites'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:596:in `invoke_with_call_chain'
/usr/local/lib/ruby/1.8/monitor.rb:242:in `synchronize'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:590:in `invoke_with_call_chain'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:583:in `invoke'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2051:in `invoke_task'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `top_level'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `each'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2029:in `top_level'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2068:in `standard_exception_handling'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2023:in `top_level'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2001:in `run'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:2068:in `standard_exception_handling'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/lib/rake.rb:1998:in `run'
/usr/local/lib/ruby/gems/1.8/gems/rake-0.8.7/bin/rake:31
/usr/local/bin/rake:19:in `load'
/usr/local/bin/rake:19