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Predicate use_module/1
Availability:built-in
Sourceuse_module(+Files)
Load the file(s) specified with Files just like ensure_loaded/1. The files must all be module files. All exported predicates from the loaded files are imported into the module from which this predicate is called. This predicate is equivalent to ensure_loaded/1, except that it raises an error if Files are not module files.

The imported predicates act as weak symbols in the module into which they are imported. This implies that a local definition of a predicate overrides (clobbers) the imported definition. If the flag warn_override_implicit_import is true (default), a warning is printed. Below is an example of a module that uses library(lists), but redefines flatten/2, giving it a totally different meaning: 

:- module(shapes, []).
:- use_module(library(lists)).

flatten(cube, square).
flatten(ball, circle).

Loading the above file prints the following message: 

Warning: /home/janw/Bugs/Import/t.pl:5:
        Local definition of shapes:flatten/2
        overrides weak import from lists

This warning can be avoided by (1) using use_module/2 to only import the predicates from the lists library that are actually used in the‘shapes’module, (2) using the except([flatten/2]) option of use_module/2, (3) use :- abolish(flatten/2). before the local definition or (4) setting warn_override_implicit_import to false. Globally disabling this warning is only recommended if overriding imported predicates is common as a result of design choices or the program is ported from a system that silently overrides imported predicates.

Note that it is always an error to import two modules with use_module/1 that export the same predicate. Such conflicts must be resolved with use_module/2 as described above.

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.
Convert a CSV with column names to JSON

Convert a CSV with column names to JSON

With the help of library(csv) and library(http/json), we read a CSV file and use the column names in the first row to emit JSON.

For this example, we use the file weather.csv:

city,temp_lo,temp_hi,prcp,date
San Francisco,46,50,0.25,1994-11-27
San Francisco,43,57,0,1994-11-29
Hayward,37,54,,1994-11-29

The script csv_to_json.pl reads the CSV from standard input or from the file provided as the first argument of the script. It converts the contents to a list of dicts, then writes this as JSON to standard output:

:- initialization(main, main).

:- use_module(library(csv)).
:- use_module(library(http/json)).

main :-
    (   current_prolog_flag(argv, [CSV_file|_])
    ->  csv_read_file(CSV_file, CSV, [])
    ;   csv_read_stream(current_input, CSV, [])
    ),
    CSV = [Colnames|Rows],
    Colnames =.. [row|Names],
    maplist(row_dict(Names), Rows, Dicts),
    json_write_dict(current_output, Dicts, [null('')]).

row_dict(Names, Row, Dict) :-
    Row =.. [row|Fields],
    pairs_keys_values(Data, Names, Fields),
    dict_create(Dict, _, Data).

The null('') option to json_write_dict/3 is necessary to convert the empty "prcp" field in the last row to a missing value represented as null in the JSON output.

This is how we can use it to convert weather.csv to JSON:

$ swipl csv_to_json.pl weather.csv
[
  {
    "city":"San Francisco",
    "date":"1994-11-27",
    "prcp":0.25,
    "temp_hi":50,
    "temp_lo":46
  },
  {
    "city":"San Francisco",
    "date":"1994-11-29",
    "prcp":0,
    "temp_hi":57,
    "temp_lo":43
  },
  {
    "city":"Hayward",
    "date":"1994-11-29",
    "prcp":null,
    "temp_hi":54,
    "temp_lo":37
  }
]
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LogicalCaptain said (2021-06-20T12:45:25):0 upvotes 0 0 downvotes
Picture of user LogicalCaptain.

Interestingly, a :- use_module(ModuleB) is not needed in ModuleA if ModuleB is loaded via the main init.pl file by issuing the :- use_module(ModuleB) directive there.

Run

?- list_undefined.

to find out which predicates are undefined.