## A.45 library(prolog_xref): Prolog cross-referencer data collection

See also
  Where this library analyses source text, `library(prolog_codewalk)` may be used to analyse loaded code. The `library(check)` exploits `library(prolog_codewalk)` to report on e.g., undefined predicates.
bug
  meta_predicate/1 declarations take the module into consideration. Predicates that are both available as meta-predicate and normal (in different modules) are handled as meta-predicate in all places.

This library collects information on defined and used objects in Prolog source files. Typically these are predicates, but we expect the library to deal with other types of objects in the future. The library is a building block for tools doing dependency tracking in applications. Dependency tracking is useful to reveal the structure of an unknown program or detect missing components at compile time, but also for program transformation or minimising a program saved state by only saving the reachable objects.

The library is exploited by two graphical tools in the SWI-Prolog environment: the XPCE front-end started by gxref/0, and `library(prolog_colour)`, which exploits this library for its syntax highlighting.

For all predicates described below, Source is the source that is processed. This is normally a filename in any notation acceptable to the file loading predicates (see load_files/2). Input handling is done by the `library(prolog_source)`, which may be hooked to process any source that can be translated into a Prolog stream holding Prolog source text. Callable is a callable term (see callable/1). Callables do not carry a module qualifier unless the referred predicate is not in the module defined by Source.

[semidet,multifile]prolog:called_by(+Goal, +Module, +Context, -Called)
  True when Called is a list of callable terms called from Goal, handled by the predicate Module:Goal and executed in the context of the module Context. Elements of Called may be qualified. If not, they are called in the context of the module Context.
[multifile]prolog:called_by(+Goal, -ListOfCalled)
  If this succeeds, the cross-referencer assumes Goal may call any of the goals in ListOfCalled. If this call fails, default meta-goal analysis is used to determine additional called goals.
  deprecated
    New code should use prolog:called_by/4
[multifile]prolog:meta_goal(+Goal, -Pattern)
  Define meta-predicates. See the examples in this file for details.
[multifile]prolog:hook(Goal)
  True if Goal is a hook that is called spontaneously (e.g., from foreign code).
[det]xref_source(+Source)
[det]xref_source(+Source, +Options)
  Generate the cross-reference data for Source if not already done and the source is not modified. Checking for modifications is only done for files. Options processed:
  silent(+Boolean)
    If `true` (default `false`), emit warning messages.
  module(+Module)
    Define the initial context module to work in.
  register_called(+Which)
    Determines which calls are registerd. Which is one of `all`, `non_iso` or `non_built_in` (default).
  comments(+CommentHandling)
    How to handle comments. If `store`, comments are stored into the database as if the file was compiled. If `collect`, comments are entered to the xref database and made available through xref_mode/2 and xref_comment/4. If `ignore`, comments are simply ignored. Default is to `collect` comments.
  process_include(+Boolean)
    Process the content of included files (default is `true`).
  stream(+Stream)
    Process the input from Stream rather than opening Source.

  Source | File specification or XPCE buffer

[det]xref_clean(+Source)
  Reset the database for the given source.
xref_current_source(?Source)
  Check what sources have been analysed.
[det]xref_done(+Source, -Time)
  Cross-reference executed at Time
[nondet]xref_called(?Source, ?Called, ?By)
[nondet]xref_called(?Source, ?Called, ?By, ?Cond)
[nondet]xref_called(?Source, ?Called, ?By, ?Cond, ?Line)
  True when By is called from Called in Source. Note that xref_called/3 and xref_called/4 use distinct/2 to return only distinct `Called-By` pairs. The xref_called/5 version may return duplicate `Called-By` if Called is called from multiple clauses in By, but at most one call per clause.

  By | is a head term or one of the reserved terms `'<directive>'(Line)` or `'<public>'(Line)`, indicating the call is from an (often initialization/1) directive or there is a public/1 directive that claims the predicate is called from in some untractable way.
  Cond | is the (accumulated) condition as defined by `:- if(Cond)` under which the calling code is compiled.
  Line | is the start line of the calling clause.

[nondet]xref_defined(?Source, +Goal, ?How)
  Test if Goal is accessible in Source. If this is the case, How specifies the reason why the predicate is accessible. Note that this predicate does not deal with built-in or global predicates, just locally defined and imported ones. How is one of of the terms below. Location is one of Line (an integer) or File:Line if the definition comes from an included (using `:-` `include(File)`) directive.

  - `dynamic(Location)`
  - `thread_local(Location)`
  - `multifile(Location)`
  - `public(Location)`
  - `local(Location)`
  - `foreign(Location)`
  - `constraint(Location)`
  - `imported(From)`
  - dcg
xref_definition_line(+How, -Line)
  If the 3th argument of xref_defined contains line info, return this in Line.
[nondet]xref_exported(?Source, ?Head)
  True when Source exports Head.
[nondet]xref_module(?Source, ?Module)
  True if Module is defined in Source.
[nondet]xref_uses_file(?Source, ?Spec, ?Path)
  True when Source tries to load a file using Spec.

  Spec | is a specification for absolute_file_name/3
  Path | is either an absolute file name of the target file or the atom `<not_found>`.

[nondet]xref_op(?Source, Op)
  Give the operators active inside the module. This is intended to setup the environment for incremental parsing of a term from the source-file.

  Op | Term of the form `op(Priority, Type, Name)`

[nondet]xref_prolog_flag(?Source, ?Flag, ?Value, ?Line)
  True when Flag is set to Value at Line in Source. This is intended to support incremental parsing of a term from the source-file.
[nondet]xref_comment(?Source, ?Title, ?Comment)
  Is true when Source has a section comment with Title and Comment
[nondet]xref_comment(?Source, ?Head, ?Summary, ?Comment)
  Is true when Head in Source has the given PlDoc comment.
[nondet]xref_mode(?Source, ?Mode, ?Det)
  Is true when Source provides a predicate with Mode and determinism.
[nondet]xref_option(?Source, ?Option)
  True when Source was processed using Option. Options are defined with xref_source/2.
[semidet]xref_meta(+Source, +Head, -Called)
  True when Head calls Called in Source.

  Called | is a list of called terms, terms of the form Term+Extra or terms of the form `//`(Term).

[semidet]xref_meta(+Head, -Called)
[semidet]xref_meta_src(+Head, -Called, +Src)
  True when Called is a list of terms called from Head. Each element in Called can be of the form Term+Int, which means that Term must be extended with Int additional arguments. The variant xref_meta/3 first queries the local context.
  deprecated
    New code should use xref_meta/3.
  To be done
    - Split predifined in several categories. E.g., the ISO predicates cannot be redefined.
    - Rely on the meta_predicate property for many predicates.
xref_hook(?Callable)
  Definition of known hooks. Hooks that can be called in any module are unqualified. Other hooks are qualified with the module where they are called.
[semidet]xref_public_list(+Spec, +Source, +Options)
  Find meta-information about File. If Spec resolves to a Prolog source file, this predicate reads all terms upto the first term that is not a directive. If Spec resolves to a SWI-Prolog‘.qlf` file, it extracts part of the information from the QLF file. It uses the module and meta_predicate directives to assemble the information in Options. Options processed:
  path(-Path)
    Path is the full path name of the referenced file. If Spec resolves to a .qlf file, Path is the name of the embedded Prolog file.
  module(-Module)
    Module is the module defines in Spec.
  exports(-Exports)
    Exports is a list of predicate indicators and operators collected from the module/2 term and reexport declarations.
  public `Public-`
    Public declarations of the file. Currently always `[]` for .qlf files.
  meta(-Meta)
    Meta is a list of heads as they appear in meta_predicate/1 declarations. Currently always `[]` for .qlf files.
  silent(+Boolean)
    Do not print any messages or raise exceptions on errors.

  The information collected by this predicate is cached. The cached data is considered valid as long as the modification time of the file does not change.

  Source | is the file from which Spec is referenced.

[semidet]xref_public_list(+File, -Path, -Export, +Src)
[semidet]xref_public_list(+File, -Path, -Module, -Export, -Meta, +Src)
[semidet]xref_public_list(+File, -Path, -Module, -Export, -Public, -Meta, +Src)
  Find meta-information about File. This predicate reads all terms upto the first term that is not a directive. It uses the module and meta_predicate directives to assemble the information described below.

  These predicates fail if File is not a module-file.

  Path | is the canonical path to File
  Module | is the module defined in Path
  Export | is a list of predicate indicators.
  Meta | is a list of heads as they appear in meta_predicate/1 declarations.
  Src | is the place from which File is referenced.

  deprecated
    New code should use xref_public_list/3, which unifies all variations using an option list.
[semidet]xref_source_file(+Spec, -File, +Src)
[semidet]xref_source_file(+Spec, -File, +Src, +Options)
  Find named source file from Spec, relative to Src.