Prolog directives

### 5.1 Prolog directives

#### 5.1.1 Introduction

Prolog directives are annotations inserted in Prolog source files for the compiler. A Prolog directive is used to specify:

- the properties of some procedures defined in the source file.
- the format and the syntax for read-terms in the source file (using changeable Prolog flags).
- included source files.
- a goal to be executed at run-time.

#### 5.1.2 `dynamic/1`

Templates

  `dynamic(+predicate_indicator)
  dynamic(+predicate_indicator_list)
  dynamic(+predicate_indicator_sequence)`

Description
`dynamic(Pred)` specifies that the procedure whose predicate indicator is `Pred` is a dynamic procedure. This directive makes it possible to alter the definition of `Pred` by adding or removing clauses. For more information refer to the section about dynamic clause management (section 6.7.1).
This directive shall precede the definition of `Pred` in the source file.
If there is no clause for `Pred` in the source file, `Pred` exists however as an empty predicate (this means that `current_predicate(Pred)` succeeds).
In order to allow multiple definitions, `Pred` can also be a list of predicate indicators or a sequence of predicate indicators using `','/2` as separator.
Portability
ISO directive.

#### 5.1.3 `public/1`

Templates

  `public(+predicate_indicator)
  public(+predicate_indicator_list)
  public(+predicate_indicator_sequence)`

Description
`public(Pred)` specifies that the procedure whose predicate indicator is `Pred` is a public procedure. This directive makes it possible to inspect the clauses of `Pred`. For more information refer to the section about dynamic clause management (section 6.7.1).
This directive shall precede the definition of `Pred` in the source file. Since a dynamic procedure is also public. It is useless (but correct) to define a public directive for a predicate already declared as dynamic.
In order to allow multiple definitions, `Pred` can also be a list of predicate indicators or a sequence of predicate indicators using `','/2` as separator.
Portability
GNU Prolog directive. The ISO reference does not define any directive to declare a predicate public but it does distinguish public predicates. It is worth noting that in most Prolog systems the `public/1` directive is as a visibility declaration. Indeed, declaring a predicate as public makes it visible from any predicate defined in any other file (otherwise the predicate is only visible from predicates defined in the same source file as itself). When a module system is incorporated in GNU Prolog a more general visibility declaration shall be provided conforming to the ISO reference.

#### 5.1.4 `multifile/1`

Templates

  `multifile(+predicate_indicator)
  multifile(+predicate_indicator_list)
  multifile(+predicate_indicator_sequence)`

Description
`multifile(Pred)` is not supported by GNU Prolog. When such a directive is encountered it is simply ignored. All clauses for a given predicate must reside in a single file.
Portability
ISO directive. Not supported.

#### 5.1.5 `discontiguous/1`

Templates

  `discontiguous(+predicate_indicator)
  discontiguous(+predicate_indicator_list)
  discontiguous(+predicate_indicator_sequence)`

Description
`discontiguous(Pred)` specifies that the procedure whose predicate indicator is `Pred` is a discontiguous procedure. Namely, the clauses defining `Pred` are not restricted to be consecutive but can appear anywhere in the source file.
This directive shall precede the definition of `Pred` in the source file.
In order to allow multiple definitions, `Pred` can also be a list of predicate indicators or a sequence of predicate indicators using `','/2` as separator.
Portability
ISO directive. The ISO reference document states that if there is no clause for `Pred` in the source file, `Pred` exists however as an empty predicate (i.e. `current_predicate(Pred)` will succeed). This is not the case for GNU Prolog.

#### 5.1.6 `ensure_linked/1`

Templates

  `ensure_linked(+predicate_indicator)
  ensure_linked(+predicate_indicator_list)
  ensure_linked(+predicate_indicator_sequence)`

Description
`ensure_linked(Pred)` specifies that the procedure whose predicate indicator is `Pred` must be included by the linker. This directive is useful when compiling to native code to force the linker to include the code of a given predicate. Indeed, if the `gplc` is invoked with an option to reduce the size of the executable (section 2.4.3), the linker only includes the code of predicates that are statically referenced. However, the linker cannot detect dynamically referenced predicates (used as data passed to a meta-call predicate). The use of this directive prevents it to exclude the code of such predicates.
In order to allow multiple definitions, `Pred` can also be a list of predicate indicators or a sequence of predicate indicators using `','/2` as separator.
Portability
GNU Prolog directive.

#### 5.1.7 `built_in/0`, `built_in/1`, `built_in_fd/0`, `built_in_fd/1`

Templates

  `built_in
  built_in(+predicate_indicator)
  built_in(+predicate_indicator_list)
  built_in(+predicate_indicator_sequence)
  built_in_fd
  built_in_fd(+predicate_indicator)
  built_in_fd(+predicate_indicator_list)
  built_in_fd(+predicate_indicator_sequence)`

Description
`built_in` specifies that the procedures defined from now have the `built_in` property (section 6.8.2).
`built_in(Pred)` is similar to `built_in/0` but only affects the procedure whose predicate indicator is `Pred`.
This directive shall precede the definition of `Pred` in the source file.
In order to allow multiple definitions, `Pred` can also be a list of predicate indicators or a sequence of predicate indicators using `','/2` as separator.
`built_in_fd` (resp. `built_in_fd(Pred)`) is similar to `built_in` (resp. `built_in(Pred)`) but sets the `built_in_fd` predicate property (section 6.8.2).
Portability
GNU Prolog directives.

#### 5.1.8 `include/1`

Templates

  `include(+atom)`

Description
`include(File)` specifies that the content of the Prolog source `File` shall be inserted. The resulting Prolog text is identical to the Prolog text obtained by replacing the directive by the content of the Prolog source `File`.
See `absolute_file_name/2` for information about the syntax of `File` (section 6.26.1).
Portability
ISO directive.

#### 5.1.9 `ensure_loaded/1`

Templates

  `ensure_loaded(+atom)`

Description
`ensure_loaded(File)` is not supported by GNU Prolog. When such a directive is encountered it is simply ignored.
Portability
ISO directive. Not supported.

#### 5.1.10 `op/3`

Templates

  `op(+integer, +operator_specifier, +atom_or_atom_list)`

Description
`op(Priority, OpSpecifier, Operator)` alters the operator table. This directive is executed as soon as it is encountered by calling the built-in predicate `op/3` (section 6.14.10). A system directive is also generated to reflect the effect of this directive at run-time (section 2.4.4).
Portability
ISO directive.

#### 5.1.11 `char_conversion/2`

Templates

  `char_conversion(+character, +character)`

Description
`char_conversion(InChar, OutChar)` alters the character-conversion mapping. This directive is executed as soon as it is encountered by a call to the built-in predicate `char_conversion/2` (section 6.14.12). A system directive is also generated to reflect the effect of this directive at run-time (section 2.4.4).
Portability
ISO directive.

#### 5.1.12 `set_prolog_flag/2`

Templates

  `set_prolog_flag(+flag, +term)`

Description
`set_prolog_flag(Flag, Value)` sets the value of the Prolog flag `Flag` to `Value`. This directive is executed as soon as it is encountered by a call to the built-in predicate `set_prolog_flag/2` (section 6.22.1). A system directive is also generated to reflect the effect of this directive at run-time (section 2.4.4).
Portability
ISO directive.

#### 5.1.13 `initialization/1`

Templates

  `initialization(+callable_term)`

Description
`initialization(Goal)` adds `Goal` to the set of goal which shall be executed at run-time. A user directive is generated to execute `Goal` at run-time. If several initialization directives appear in the same file they are executed in the order of apparition (section 2.4.4).
Portability
ISO directive.

#### 5.1.14 `foreign/2`, `foreign/1`

Templates

  `foreign(+callable_term, +foreign_option_list)
  foreign(+callable_term)`

Description
`foreign(Template, Options)` defines an interface predicate whose prototype is `Template` according to the options given by `Options`. Refer to the foreign code interface for more information (section 8.1).
`foreign(Template)` is equivalent to `foreign(Template, [])`.
Portability
GNU Prolog directive.

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