:- http_handler(root(cart/add/ProductName), cart_add(ProductName), []).

% GET /cart/add/ProductName
cart_add(ProductName, Request) :-
    \+ product(ProductName, _), !,
    http_redirect(see_other, root(.), Request).

cart_add(ProductName, Request) :-
    http_session_assert(cart_entry(ProductName)),
    http_redirect(see_other, root(.), Request).