:- use_module(library(http/http_dispatch)). [det]http_location_by_id(+ID, 
-Location)
[det]http_location_by_id(+ID, 
-Location)id(ID) appears in the option list of the handler, ID 
it is used and takes preference over using the predicate.Module:Pred
If the handler is declared with a pattern, e.g., root(user/User), 
the location to access a particular user may be accessed using 
e.g., user('Bob'). The number of arguments to the compound 
term must match the number of variables in the path pattern.
A plain atom ID can be used to find a handler with a 
pattern. The returned location is the path up to the first variable, 
e.g.,
/user/ in the example above.
User code is advised to use http_link_to_id/3 which can also add query parameters to the URL. This predicate is a helper for http_link_to_id/3.
existence_error(http_handler_id, Id).library(http/html_write) 
construct
location_by_id(ID) or its abbreviation #(ID)