yap-6.3/pl/modules.md

The YAP Module system

The YAP Module system (#YAPModules)

The YAP module system is based on the Quintus/SISCtus module system ˜\cite quintus . In this design, modules are named collections of predicates, and all predicates belong to a single module. By default, predicates are only visible within a module, or private to that module. The module may also define a list of predicates that are exported, that is, visible to other modules.

The main predicates in the module system are:

* module/2 associates a source file to a module. It has two arguments: the name of the new module, and a list of predicates exported by the module.

* use_module/1 and use_module/2 can be used to load a module. They take as first argument the source file for the module. Whereas use_module/1 loads all exported predicates, use_module/2 only takes the ones given by the second argument.

YAP pre-defines a number of modules. Most system predicates belong to the module prolog. Predicates from the module prolog are automatically visible to every module. The system module was introduced for SWI-Prolog compatibility, and in YAP mostly acts as an alias to prolog. The user module is also visible to all other modules.

The YAP engine is always associated to a module, the current source module or type-in module. By default, all predicates read-in and all calls to a goal will be made to predicates visible to the current source module, Initially, the source module for YAP is the module user. Thus Prolog programs that do not define modules will operate within the user module. In this case, all predicates will be visible to all source files.

YAP includes a number of libraries and packages, most of them defining their own modules. Note that there is no system mechanism to avoid clashes between module names, so it is up to the programmer to carefully choose the names for her own program modules.

The main mechanism to change the current type-in module is by using the module/2 declaration.This declaration sets the source module when it starts consulting a file, and resets it at the end. One can set the type-in module permanently by using the built-in module/1.

\subsection Explicit Naming

The module system allows one to explicitly specify the source mode for a clause by prefixing a clause with its module, say:

user:(a :- b).

it is also possible to type

user:a :- user:b.

both formulations describe the same clause, independently of the current type-in module.

In fact, it is sufficient to specify the source mode for the clause's head:

user:a :- b.

if the current type-in module is m, the clause could also be written as:

user:a :- m:b.

The compiler rewrites the source clauses to ensure that explicit calls are respected, and that implicit calls are made to the current source module.

A goal should refer to a predicate visible within the current type-in module. Thus, if a goal appears in a text file with a module declaration, the goal refers to that module's context (but see the initialization/1 directive for more details).

Again, one can override this rule by prefixing a goal with a module to be consulted. The following query:

?- nasa:launch(apollo,13).

invokes the goal launch(apollo,13) as if the current source module was nasa.

YAP and other Prolog systems allow the module prefix to see all predicates visible in the module, including predicates private to the module. This rule allows maximum flexibility, but it also breaks encapsulation and should be used with care. The ciao language proposes a different approach to this problem, see \cite DBLP:conf/cl/GrasH00 .

Modules are not always associated with a source-file. They may range over several files, by using the includedirective. Moreover, they may not be associated to any source file. As an example,

?- assert( nasa:launch(apollo,13) ).

will create a module nasa, if does not already exist. In fact it is sufficient to call a predicate from a module to implicitly create the module. Hence after this call:

?- nasa:launch(apollo,13).

there will be a nasamodule in the system, even if nasa:launch/2 is not at all defined.

\pred use_module( +Files ) is directive loads a module file

This predicate loads the file specified by Files, importing all their public predicates into the current type-in module. It is implemented as if by:

use_module(F) :-
	load_files(F, [if(not_loaded),must_be_module(true)]).

Notice that Files may be a single file, or a list with a number files. The Files are loaded in YAP only once, even if they have been updated meanwhile. YAP should also verify whether the files actually define modules. Please consult load_files/3 for other options when loading a file.

Predicate name clashes between two different modules may arise, either when trying to import predicates that are also defined in the current type-in module, or by trying to import the same predicate from two different modules.

In the first case, the local predicate is considered to have priority and use_module/1 simply gives a warning. As an example, if the file a.pl contains:

:- module( a, [a/1] ).

:- use_module(b).

a(1).
a(X) :- b(X).

and the file b.pl contains:

:- module( b, [a/1,b/1] ).

a(2).

b(1).

YAP will execute as follows:

?- [a].
 % consulting .../a.pl...
  % consulting .../b.pl...
  % consulted .../b.pl in module b, 0 msec 0 bytes
 % consulted .../a.pl in module a, 1 msec 0 bytes
true.
 ?- a(X).
X = 1 ? ;
X = 1.

The example shows that the query a(X)has a single answer, the one defined in a.pl. Calls to a(X)succeed in the top-level, because the module a was loaded into user. On the other hand, b(X)is not exported by a.pl, and is not available to calls, although it can be accessed as a predicate in the module 'a' by using the : operator.

Next, consider the three files c.pl, d1.pl, and d2.pl:

% c.pl
:- module( c, [a/1] ).

:- use_module([d1, d2]).

a(X) :-
	b(X).
a(X) :-
	c(X).
a(X) :-
   d(X).

% d1.pl
:- module( d1, [b/1,c/1] ).

b(2).
c(3).


% d2.pl
:- module( d2, [b/1,d/1] ).

b(1).
d(4).

The result is as follows:

./yap -l c
YAP 6.3.4 (x86_64-darwin13.3.0): Tue Jul 15 10:42:11 CDT 2014

     ERROR!!
     at line 3 in o/d2.pl,
     PERMISSION ERROR- loading .../c.pl: modules d1 and d2 both define b/1
 ?- a(X).
X = 2 ? ;
     ERROR!!
     EXISTENCE ERROR- procedure c/1 is undefined, called from context  prolog:$user_call/2
                 Goal was c:c(_131290)

The state of the module system after this error is undefined.

@\pred module(+ M:atom,+ L:list ) is directive the current file defines module M with exports L. The list may include

• predicate indicators

• operator definitions that look like calls to op/3.

The list L may include predicates imported from other modules. If you want to fully reexport a module, or a sub-set, also consider reexport/1.

Similar to module/2, this directive defines the file where it appears in as a module file; it must be the first declaration in the file. M must be an atom specifying the module name; L must be a list containing the module's public predicates specification, in the form [predicate_name/arity,...].

The last argument Options must be a list of options, which can be: + filename the filename for a module to import into the current module.

 + <b>library( +File )</b>
   a library file to import into the current module.

 + <b>hide( +Opt)</b>
    if  _Opt_ is `false`, keep source code for current module, if `true`, disable.

 + <b>export(+PredicateIndicator )</b>
	Add predicates to the public list of the context module. This implies
the predicate will be imported into another module if this module
is imported with use_module/1 and use_module/2.

 + <b>export_list(? _Mod_,? _ListOfPredicateIndicator_)</b>
   The list  _ListOfPredicateIndicator_ contains all predicates
 	exported by module  _Mod_

Note that predicates are normally exported using the directive module/2. The export/1 argumwnt is meant to allow export from dynamically created modules. The directive argument may also be a list of predicates.

@pred use_module(+Files, +Imports) loads a module file but only imports the named predicates

This predicate loads the file specified by Files, importing their public predicates specified by Imports into the current type-in module. It is implemented as if by:

use_module(Files, Imports) :-
	load_files(Files, [if(not_loaded),must_be_module(true),imports(Imports)]).

The Imports argument may be use to specify which predicates one wants to load. It can also be used to give the predicates a different name. As an example, the graphs library is implemented on top of the red-black trees library, and some predicates are just aliases:

:- use_module(library(rbtrees), [
	rb_min/3 as min_assoc,
	rb_max/3 as max_assoc,

        ...]).

Unfortunately it is still not possible to change argument order.

\pred module(+ M:atom,+ L:list ) is directive the current file defines module M with exports L. The list may include

• predicate indicators

• operator definitions that look like calls to op/3.

The list L may include predicates imported from other modules. If you want to fully reexport a module, or a sub-set, also consider reexport/1.

Similar to module/2, this directive defines the file where it appears in as a module file; it must be the first declaration in the file. M must be an atom specifying the module name; L must be a list containing the module's public predicates specification, in the form [predicate_name/arity,...].

The last argument Options must be a list of options, which can be: +filename the filename for a module to import into the current module.

+ <b>library( +File )</b>
   a library file to import into the current module.

+ <b>hide( +Opt)</b>
    if  _Opt_ is `false`, keep source code for current module, if `true`, disable.

+ <b>export(+PredicateIndicator )</b>
	Add predicates to the public list of the context module. This implies
the predicate will be imported into another module if this module
is imported with use_module/1 and use_module/2.

+ <b>export_list(? _Mod_,? _ListOfPredicateIndicator_)</b>
   The list  _ListOfPredicateIndicator_ contains all predicates
 	exported by module  _Mod_

Note that predicates are normally exported using the directive module/2. The export/1 argument is meant to allow export from dynamically created modules. The directive argument may also be a list of predicates.