Certain forms are too complicated to have them fit on one page. If, for example, a form involves large amounts of structured data, such as picking locations on a map, scheduling events in a calendar widget, or having certain parts of a form change depending on earlier input, it is of value to be able to break up a certain form over multiple pages.
This is easy to do with dynamic web pages and Javascript, as one would simply create a tab widget with different pages, and the actual submitted form would contain the whole tab widget and all of its input fields, yielding a single POST request for the entire operation.
Sometimes, however, it takes a long time to generate certain input fields; they might even be computationally intensive even after the page has been generated, taxing the low-end computer user's browser. Additionally, it becomes difficult or impossible to create forms that adapt themselves based on earlier input.
It therefore becomes necessary to split up a certain form over multiple full page requests.
This can prove to be difficult, especially since the first page of a form will POST to /location/a, which will issue a redirect to /location/b and requested as GET by the client. Passing the stored form data from POST /location/a to GET /location/b is where the difficulty lies.
Erwin Vervaet, the creator of Spring Web Flow (A subproject of the Spring framework, mostly known for its dependency injection capabilities) once wrote a blog article demonstrating this functionality in said framework, and also comparing it to the Lift Web Framework which implemented similar functionality. He then presents a challenge to other web frameworks, which is further described in a later article.
How would Yesod face this problem, especially considering its stateless REST-based nature?