This function starts by using the function to get a table containing the values it needs to present. When the user is browsing by song—when the what parameter is :song--you want to select complete rows from the database. But when they’re browsing by genre, artist, or album, you want to select only the distinct values for the given category. The database function select does most of the heavy lifting, with values-for-page mostly responsible for passing the right arguments depending on the value of what. This is also where you select a random subset of the matching rows if necessary.

Once you have the values you want to present, you need to do two things with them. The main task, of course, is to present them, which happens in the do-rows loop, leaving the rendering of each row to the function . That function renders :song rows one way and all other kinds another way.

The solution you’ll use is to send back a form that has enough information stashed away in a hidden INPUT element to allow the server to reconstitute the list of songs matching the browse page criteria. That information is the list of values returned by values-for-page and the value of the what parameter. This is where you use the parameter type; the function values->base64 extracts the values of a specified column from the table returned by values-for-page into a list and then makes a base 64-encoded string out of that list to embed in the form.