rails g channel chat

/Users/hoge/.rvm/gems/ruby-2.3.0@alpha/gems/opal-0.8.1/lib/opal/sprockets/server.rb:7:in `require': cannot load such file -- rack/showexceptions (LoadError)

おお、もう…。

で、なぜこうなるかというと、Rack 2.x系はrack/showexceptionsがrack/show_exceptionsにリネームされているから。

つまり、

/Users/hoge/.rvm/gems/ruby-2.3.0@alpha/gems/opal-0.8.1/lib/opal/sprockets/server.rb

require 'rack/file'
require 'rack/static'
require 'rack/urlmap'
require 'rack/builder'
require 'rack/deflater'
require 'rack/directory'
require 'rack/showexceptions'
require 'opal/source_map'
require 'sprockets'
require 'sourcemap'
require 'erb'
require 'opal/sprockets/source_map_server'
require 'opal/sprockets/source_map_header_patch'

（以下略）

を、

/Users/hoge/.rvm/gems/ruby-2.3.0@alpha/gems/opal-0.8.1/lib/opal/sprockets/server.rb

require 'rack/file'
require 'rack/static'
require 'rack/urlmap'
require 'rack/builder'
require 'rack/deflater'
require 'rack/directory'
require 'rack/show_exceptions'
require 'opal/source_map'
require 'sprockets'
require 'sourcemap'
require 'erb'
require 'opal/sprockets/source_map_server'
require 'opal/sprockets/source_map_header_patch'

（以下略）

こうしてやれば動く。

rails g channel chat
Running via Spring preloader in process 54449
      create  app/channels/chat_channel.rb
      create  app/assets/javascripts/channels/chat.coffee

create app/assets/javascripts/channels/chat.coffee…。
結局のところ、opal-railsにchannel_generatorがないので、書かないといけない模様。