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This page is referenced from the comment section of Defining a Module

Prolog module loading example: Loading the "lynx" library

Suppose you want to load the lynx library.

Support $SWIPROLOG is the filesystem tree location where SWI Prolog has been installed (in my case, /usr/local/logic/swipl)

Then the relevant slice of the filesystem tree is as follows:

$SWIPROLOG
├── lib
│   └── swipl
│       ├── library
│       │   ├── lynx
│       │   │   ├── format.pl
│       │   │   ├── html_style.pl
│       │   │   ├── html_text.pl
│       │   │   └── pldoc_style.pl

In directory $SWIPROLOG/lib/swipl/library/lynx, we find:

  • File format.pl
    • part of module text_format
    • exporting predicates
      • format_paragraph/2
      • trim_line/2
  • File html_style.pl
    • part of module format_style
    • exporting predicates
      • element_css/3
      • css_block_options/5
      • css_inline_options/3
      • attrs_classes/2
      • style_css_attrs/2
  • File html_text.pl
    • part of module html_text
    • exporting predicates
      • html_text/1
      • html_text/2
  • File pldoc_style.pl
    • part of module pldoc_style
    • exporting nothing but extending html_text:style/3 via the multifile directive.

Problem

The lynx subdirectory of the SWI Prolog library directory is probably not in your library search path.

The files won't be found automatically, even though the library directory itself is on the search path:

?- bagof(X,file_search_path(library,X),L).
L = ['/usr/local/logic/swipl/lib/swipl/library',
     '/usr/local/logic/swipl/lib/swipl/library/clp',
     '/home/paquette/.local/share/swi-prolog/pack/sldnfdraw/prolog',
     pce('prolog/lib')].

Try to use format_paragraph/2, fully qualified:

?- text_format:format_paragraph("hello, world",[width(50),text_align(center)]).
ERROR: Unknown procedure: text_format:format_paragraph/2 (DWIM could not correct goal)

Solution

Let's make the predicates found in file format.pl. (not predicates found in module format) accessible.

(More precisely, "those predicates exported from some module foo, a part of which is defined in file format.pl. This is confusing.)

You can:

Use the relative path of a file underneath the library search path

In interactive mode, use the predicate:

use_module(library(lynx/format)).

Inside a source file or when reading [user], use the directive (actually, you can use the predicate, too - it will just be run when you close [user] instead of immediately):

:- use_module(library(lynx/format)).

Extend the library file search path by adding a mapping for atom library to a directory

In interactive mode, use assertz/2:

assertz(file_search_path(library,'/usr/local/logic/swipl/lib/swipl/library/lynx')).
use_module(library(format)).

Inside a source file or when reading [user], assert a new fact for file_search_path(library,DIR), then call the use_module/1 directive:

file_search_path(library,'/usr/local/logic/swipl/lib/swipl/library/lynx').
:- use_module(library(format)).

Create a new file search path and search that

In interactive mode, use assertz/2:

assertz(file_search_path(lynx,'/usr/local/logic/swipl/lib/swipl/library/lynx')).
use_module(lynx(format)).

Inside a source file or when reading [user], assert a new fact for file_search_apth(ALIAS,DIR), then call the use_module/1 directive:

file_search_path(lynx,'/usr/local/logic/swipl/lib/swipl/library/lynx').
:- use_module(lynx(format)).

Indicate full path to file

In interactive mode:

use_module('/usr/local/logic/swipl/lib/swipl/library/lynx/format.pl').

Inside a source file or when reading [user]:

:- use_module('/usr/local/logic/swipl/lib/swipl/library/lynx/format.pl').

Now the predicate is accessible!

?- format_paragraph("hello, world",[width(50),text_align(center)]).
                   hello, world

or with qualification of the predicate:

?- text_format:format_paragraph("hello, world",[width(50),text_align(center)]).
                   hello, world

Perfect.

Addendum

There is also a very nice predicate to examine predicate metadata, predicate_property/2:

Perhaps surprisingly, the term denoting the predicate is not given in functor/arity style,

% WRONG
?- predicate_property(format_paragraph/2,imported_from(M)).
M = yall.

but in the form of a "callable term":

% CORRECT
?- predicate_property(format_paragraph(_,_),imported_from(M)).
M = text_format.

?- predicate_property(format_paragraph(_,_),file(F)).
F = '/usr/local/logic/swipl/lib/swipl/library/lynx/format.pl'.