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    <p>You should not ask your User (model) to talk to the UI (view). That's bad/not MVC-clean. That's what a controller is for. </p> <p>You can use either return values, exceptions, or <code>throw/catch</code> (which is not exception handling) to pass information from your model to your controller. For example, using return values:</p> <pre><code>post "/create_user" do flash[:notice] = case User.something when User then "User Created!" when :nono then "That's not allowed" when :later then "User queued to be created later." end end class User def self.something if authorized if can_create_now new(...) else queue_create(...) :later end else :nono end end end </code></pre> <hr> <p>Since I mentioned them above, following are examples using <strong>throw/catch</strong> and <strong>begin/rescue</strong> (exceptions). As the advisability of using either of these constructs is questionable, let us take a moment of silence to ponder if this is a good idea.</p> <hr> <p>Here is an example using <strong>throw/catch</strong>:</p> <pre><code>post "/create_user" do result = catch(:msg){ User.something } flash[:notice] = case when :nono then "That's not allowed" when :later then "User queued to be created later." else "User Created!" end end class User def self.something throw :msg, :nono unless authorized if can_create_now new(...) else queue_create(...) throw :msg, :later end end end </code></pre> <p>Finally, here's an example using <strong>exceptions</strong>, though I'm not convinced that this will be appropriate for all (non-disastrous) cases where you want to flash unique messages to the user:</p> <pre><code>post "/create_user" do flash[:notice] = "User Created!" # Assume all good begin User.something rescue User::Trouble=&gt;e flash[:notice] = case e when Unauthorized then "That's not allowed" when DelayedCreate then "User queued to be created later." else "Uh...Something bad happened." end end end class User class Trouble &lt; RuntimeError; end class Unauthorized &lt; Trouble; end class DelayedCreate &lt; Trouble; end def self.something raise Unauthorized unless authorized if can_create_now new(...) else queue_create(...) raise DelayedCreate end end end </code></pre> <p>Exceptions let you pass an additional data along (e.g. <code>raise Unauthorized.new "No such account"</code>, or any custom properties you want to add to your class), and so may be more useful (when appropriate). Just remember to pass <em>semantic</em> results from your model to your controller, and let it translate them into <em>user-facing</em> messages.</p>
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    1. POAccessing Sinatra scope from another class
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    1. COffs don't use throw or exceptions. You're replacing a bad design decision with an even worse design decision.
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    2. CO@ReinHenrichs If a method that is supposed to create a user cannot create a user, I think that an Exception is wholly appropriate. Further, catching any Exception and translating it into a human-readable error message is also a good idea. As I stated above, I believe this is only appropriate when you would normally raise an exception. As for `throw/catch`...I have only used it once in 12 years of programming Ruby, so I can't really justify it as strongly. :)
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    3. CO@ReinHenrichs Perhaps you would like to amend your answer with a suggestion for how to actually implement your advice.
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