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Predicate http_handler/3
Availability::- use_module(library(http/http_dispatch)).
Source[det]http_handler(+Path, :Closure, +Options)
Register Closure as a handler for HTTP requests. Path is either an absolute path such as '/home.html' or a term Alias(Relative). Where Alias is associated with a concrete path using http:location/3 and resolved using http_absolute_location/3. Relative can be a single atom or a term‘Segment1/Segment2/...`, where each element is either an atom or a variable. If a segment is a variable it matches any segment and the binding may be passed to the closure. If the last segment is a variable it may match multiple segments. This allows registering REST paths, for example: 
:- http_handler(root(user/User), user(Method, User),
                [ method(Method),
                  methods([get,post,put])
                ]).

user(get, User, Request) :-
    ...
user(post, User, Request) :-
    ...

If an HTTP request arrives at the server that matches Path, Closure is called as below, where Request is the parsed HTTP request. 

call(Closure, Request)

Options is a list containing the following options:

authentication(+Type)
Demand authentication. Authentication methods are plugable. The library http_authenticate.pl provides a plugin for user/password based Basic HTTP authentication.
chunked
Use Transfer-encoding: chunked if the client allows for it.
condition(:Goal)
If present, the handler is ignored if Goal does not succeed.
content_type(+Term)
Specifies the content-type of the reply. This value is currently not used by this library. It enhances the reflexive capabilities of this library through http_current_handler/3.
id(+Atom)
Identifier of the handler. The default identifier is the predicate name. Used by http_location_by_id/2 and http_link_to_id/3.
hide_children(+Bool)
If true on a prefix-handler (see prefix), possible children are masked. This can be used to (temporary) overrule part of the tree.
method(+Method)
Declare that the handler processes Method. This is equivalent to methods([Method]). Using method(*) allows for all methods.
methods(+ListOfMethods)
Declare that the handler processes all of the given methods. If this option appears multiple times, the methods are combined.
prefix
Call Pred on any location that is a specialisation of Path. If multiple handlers match, the one with the longest path is used. Options defined with a prefix handler are the default options for paths that start with this prefix. Note that the handler acts as a fallback handler for the tree below it: 
:- http_handler(/, http_404([index('index.html')]),
                [spawn(my_pool),prefix]).
priority(+Integer)
If two handlers handle the same path, the one with the highest priority is used. If equal, the last registered is used. Please be aware that the order of clauses in multifile predicates can change due to reloading files. The default priority is 0 (zero).
spawn(+SpawnOptions)
Run the handler in a separate thread. If SpawnOptions is an atom, it is interpreted as a thread pool name (see create_thread_pool/3). Otherwise the options are passed to http_spawn/2 and from there to thread_create/3. These options are typically used to set the stack limits.
time_limit(+Spec)
One of infinite, default or a positive number (seconds). If default, the value from the setting http:time_limit is taken. The default of this setting is 300 (5 minutes). See setting/2.

Note that http_handler/3 is normally invoked as a directive and processed using term-expansion. Using term-expansion ensures proper update through make/0 when the specification is modified.

Errors
- existence_error(http_location, Location)
- permission_error(http_method, Method, Location)
See also
http_reply_file/3 and http_redirect/3 are generic handlers to serve files and achieve redirects.

Examples

Serve static files from an HTTP server

Serve static files from an HTTP server

The SWI-Prolog HTTP server doesn't not server static files by default. You explicitly have to add a handler telling it to do so. The declarations below tell the server to serve files from the current directory under / on the server.

:- use_module(library(http/http_server)).
:- use_module(library(http/http_files)).

:- http_handler(root(.), http_reply_from_files('.', []), [prefix]).

Now, start the server and visit http://localhost:8080/.

?- http_server([port(localhost:8080)]).
% Started server at http://localhost:8080/
true.

Remarks

  • Note that the localhost: only binds the loopback network, avoiding to expose your website to the outside world.
  • The first argument of http_reply_from_files/3 is subject to search using absolute_file_name/3. This allows to abstract the physical location of the directory as well as virtually merge the contents of multiple physical directories.
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localhost localhost said (2023-04-04T10:32:23):0 upvotes 0 0 downvotes
Picture of user localhost localhost.

With Prolog 7.6.4 for amd64 code similar to first example in documentation doesn't work! I get error "Arguments are not sufficiently instantiated" pointing to line given below:

:- http_handler(root(note), notes_action(Method), [ method(Method), methods([get, post]) ]).

LogicalCaptain said (2021-05-05T13:31:41):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.
I suppose the handlers are visible to all web-server threads that are running in the same SWIPL instance.