As server-side web developers, we tend to think of our templating system as providing the View layer in a Model-View-Controller architecture. I have recently come to the conclusion that this is an incorrect approach. Instead, we need to think of the HTTP response as the View.
When an HTTP client makes an HTTP request, what does it get in return? It does not receive only the body of the HTTP response. The client receives the body and the headers of the HTTP response. The response as a whole is what the web application delivers as its presentation to the client.
Templating systems, as far as I can tell, deal only with the body of the HTTP response. In practice, the rest of the response is generally handled by the Controller; for example, setting the status, setting cookies, and modifying headers.
Let's say we accept the idea that the HTTP response is the View (i.e., the presentation that is delivered to the client). If we accept that idea, then for a clean separation of concerns a la SeparatedPresentation, we need to combine both the template work and the header work into their own layer. That layer needs to be completely separated from the Controller and Model.
Thus, anything that deals with the response, including the setting of headers, should be considered the View layer. If you are setting headers in a Controller, you are losing that clean separation of concerns. To remedy this, your Controller needs to hand off to a layer that builds the response on its own; this is the proper place for the tempalting system to be invoked, not the Controller.
In summary: the template is not the View. The response is the View. We should separate our concerns accordingly.
If you like the line of thinking presented here, please check out the Action-Domain-Responder refinement of the MVC pattern. It's still a work-in-progress so be sure to leave your comments and criticism either here on this blog or as a new issue at Github.