Module Fmlib_browser.Navigation

When the user clicks a link in an application, the URL is not changed directly. Instead a url_request is passed to the application's on_url_request handler. The resulting message must be handled in the application's update function.

This mechanism allows the application to prepare for the URL change, e.g. persist important data on the server or in local storage.

A url_request can be either

• an Internal URL with the same origin as the application. The user is about to change pages within the application. push_url should be used to perform the URL change.

• an External URL with a different origin than the application. The user is about to be redirected to an external site. load should be used to continue.

NOTE: The origin consists of the protocol, the host, and the port of the URL. If any of those changes, the new URL is seen as an External URL.

This navigation key is handed to the application through its init function. It is needed for the navigation commands push_url, replace_url, back and forward.

This means that those commands are only available in a full application which controls the whole browser window and not in other kinds of programs that only control a part of it such as element. This restriction exists to prevent subtle synchronization bugs related to URL changes.

For example, when part of the website is controlled by an element and another part is controlled by a React component and one of them changes the URL, the other needs to be informed about it. The navigation key restriction forces the programmer to implement URL changes and a synchronization mechanism outside of both components.

The same restriction exists in elm/browser. See this document for a more detailed explanation.