NRuby

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require 'rack'
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class MyMiddleware
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  def initialize(app)
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    @app = app
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  end
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  def call(env)
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    unless env['PATH_INFO'] == '/'
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      return [404, { 'Content-Type' => 'text/plain' }, ['Not Found']]
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    end
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    @app.call(env)
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  end
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end
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use MyMiddleware
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run ->(env) do
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  [200, { 'Content-Type' => 'text/plain' }, ['Hello, world!']]
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end
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