________________________________________________________________________

This file is part of Logtalk <https://logtalk.org/>  
SPDX-FileCopyrightText: 1998-2024 Paulo Moura <pmoura@logtalk.org>
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
________________________________________________________________________


`packs`
=======

This tool provides predicates for downloading, installing, upgrading, and
uninstalling third-party libraries and applications, generically known as
_packs_. Collections of pack specifications are made available using
_registries_. Registries can be local to a system, publicly shared, or
private to a company (e.g., only available in a VPN). There is no concept
of a central registry. Users can decide which registries they trust and want
to use and add them using their published URLs. The tool supports both pack
checksums and signatures and takes several steps to sanitize registry and
pack specifications. As with other Logtalk developer tools, portability is
a main goal. This tool can be used with any supported Prolog backend and run
on both POSIX and Windows systems. Moreover, this tool can be used not only
for handling Logtalk packs but also for Prolog only packs, thus providing a
solution for sharing portable resources between multiple systems.

A list of public Logtalk and Prolog pack registries is available at:

https://github.com/LogtalkDotOrg/pack-registries

This tool is in the beta stage of development. Feedback is most welcome.


Requirements
------------

On POSIX systems (Linux, macOS, ...), the following shell commands are
required:

- `sha256sum` (provided by GNU `coreutils`)
- `curl` (default file downloader)
- `wget` (alternative file downloader)
- `bsdtar` (provided by `libarchive` or `libarchive-tools`)
- `gpg` (provided by `gnupg2`)
- `git`
- `direnv` (when using virtual environments)

The tool uses `bsdtar` instead of GNU `tar` so that it can uncompress
`.zip` archives (`unzip` doesn't provide the desired options that allow
a simple and reliable solution for ignoring the non-predictable name of
the wrapper directory).

On Windows systems, the following shell commands are required:

- `certutil.exe`
- `curl.exe` (default file downloader)
- `wget.exe` (alternative file downloader)
- `tar.exe`
- `gpg.exe`
- `git.exe`
- `Set-PsEnv` (when using virtual environments)

In recent Windows 10 builds, only `wget`, `gpg`, `git`, and `Set-PsEnv`
should require installation. You can download the GnuPG software from:

https://www.gnupg.org/

You can download Git from:

https://gitforwindows.org

You can download Wget from:

https://eternallybored.org/misc/wget/

You can also use Chocolatey to install the commands above:

	> choco install gnupg git wget

To install [Set-PsEnv](https://github.com/rajivharris/Set-PsEnv) from
the PowerShell Gallery:

	PS> Install-Module -Name Set-PsEnv

On macOS systems, Apple bundles both `curl` and BSD `tar` (under the name
`tar`; you can simply create a `bsdtar` alias or install a more recent
version). The required commands can be easily installed using MacPorts:

	$ sudo port install coreutils wget libarchive gnupg2 git direnv

Or using Homebrew:

	$ brew install coreutils wget libarchive gnupg2 git direnv

On Linux systems, use the distribution's own package manager to install any
missing command. For example, in recent Ubuntu versions:

	$ sudo apt update
	$ sudo apt install coreutils curl wget libarchive-tools gnupg2 git direnv


API documentation
-----------------

This tool API documentation is available at:

[../../docs/library_index.html#packs](../../docs/library_index.html#packs)


Loading
-------

This tool can be loaded using the query:

	| ?- logtalk_load(packs(loader)).


Testing
-------

To run the tool tests, use the query:

	| ?- logtalk_load(packs(tester)).

The tests can be run without interfering with the user packs setup.


Usage
-----

The `packs` tool loads at startup all the currently defined registry and pack
specifications (from the registries/packs storage directory; see below). When
no registry/pack setup exists, a new one is automatically created.

The tool provides two main objects, `registries` and `packs`, for handling,
respectively, registries and packs. Both objects accept a `help/0` message
that describes the most common queries.


Registries and packs storage
----------------------------

The tool uses a directory specified using the `logtalk_packs` library alias
when defined (in a settings file or in a backend Prolog initialization file).
When this library alias is not defined, the tool uses the value of the
`LOGTALKPACKS` environment variable when defined. Otherwise, it defaults to
the `~/logtalk_packs` directory. The actual directory can be retrieved by
the query:

	| ?- packs::logtalk_packs(Directory).
	...

This directory holds sub-directories for registries, packs, and archives.
These sub-directories are automatically created when loading the `packs`
tool if they don't exist. Users should not manually modify the contents
of these directories. Multiple and independent registry/pack setups are
possible using _virtual environments_ as explained next.

Your registries and packs setup can be saved and restored (e.g., in a different
system) by using the `packs::save/1-2` and `packs::restore/1-2` predicates, as
explained in the next section about virtual environments. If necessary, before
restoring, the `packs::reset/0` predicate can be called to delete any defined
registries and installed packs.


Virtual environments
--------------------

An application may require specific pack versions. These requirements may
differ between applications. Different applications may also have conflicting
requirements. Therefore, a _virtual environment_ where an application
requirements are fulfilled may be required to develop and/or run it. A virtual
environment is essentially a registries/packs storage directory.

Defining the `logtalk_packs` library alias in a settings file or defining
the `LOGTALKPACKS` environment variable before starting Logtalk allows easy
creation and switching between virtual environments. By using a per-application
settings file (or a per-application environment variable definition), each
application can thus use its own virtual environment. The `settings.lgt` file
can define the `logtalk_packs` library alias using code such as:

	:- initialization((
		logtalk_load_context(directory, Directory),
		assertz(logtalk_library_path(logtalk_packs, Directory))
	)).

The definition of the `logtalk_packs` library alias **must** always be an
atom and thus never use library notation (i.e., it must never depend on other
library aliases).

When a virtual environment also requires a specific Logtalk version (e.g., the
version used to test and certify it), this can be installed as a pack from
the official [talkshow](https://github.com/LogtalkDotOrg/talkshow) registry
and used by (re)defining the `LOGTALKHOME` and `LOGTALKUSER` environment
variables to point to its pack directory (which can be queried by using the
`packs::directory/2` message).

Experimental `lgtenv.sh` and `lgtenv.ps1` scripts are included to simplify
creating virtual environments. For example:

	$ lgtenv -d ~/my_venv -c -p logtalk_packs
	$ cd ~/my_venv
	direnv: loading ~/my_venv/.envrc
	direnv: export +LOGTALKPACKS

Type `lgtenv -h` for details on the script options.

These scripts require, respectively, [direnv](https://github.com/direnv/direnv)
and [Set-PsEnv](https://github.com/rajivharris/Set-PsEnv) to be installed.
These utilities load and unload environment variables when changing the current
directory. On Windows systems, when using the `lgtenv.ps1` script, you also
need to redefine the PowerShell prompt in a profile file
(e.g., `$HOME\Documents\PowerShell\Profile.ps1`) to mimic the functionality of
`direnv` of automatically loading an existing `.env` file when changing to its
directory. For example:

	function prompt {
	    Set-PsEnv
	    'PS ' + $(Get-Location) + '> '
	}

A virtual environment setup (i.e., the currently defined registries and
installed packs) can be saved into a file (e.g., `requirements.lgt`) using
the `packs::save/1` predicate:

	| ?- packs::save('requirements.lgt').
	...

This query saves a listing of all the installed packs and their registries.
Using the saved file, the virtual environment setup can then be restored
using the `packs::restore/1-2` predicates. The file uses a simple format with
`registry/2`, `pack/3`, `pinned_registry/1`, and `pinned_pack/1` facts (in
this order) and can be manually created or edited if necessary. For example:

	registry(talkshow, 'https://github.com/LogtalkDotOrg/talkshow.git').
	pack(talkshow, logtalk, 3:45:0).
	pack(talkshow, lflat, 2:1:0).

These files can be distributed with applications so that users can easily
fulfill application requirements by running the query once:

	| ?- packs::restore('requirements.lgt').

Subsequently, the application `loader.lgt` file can then load the required
packs using their loader files:

	:- initialization((
		% load required packs
		logtalk_load(foo(loader)),
		logtalk_load(bar(loader)),
		...
		% load application files
		...
	)).

Note that restoring encrypted registries or encrypted packs requires entering
the required passphrases. Although the `restore/2` predicate accepts a list of
options that include the `gpg/1` option, this only allows specifying a single
and common passphrase when interactive entering of passphrases is not
convenient or possible.


Registry specification
----------------------

A registry is a git remote repo that can be cloned, a downloadable archive,
or a local directory containing a Logtalk loader file that loads source files
defining the registry itself and the packs it provides. The registry name is
ideally a valid unquoted atom. The registry directory must contain at least
two Logtalk source files:

- A file defining an object named after the registry with a `_registry`
suffix, implementing the `registry_protocol`. This naming convention helps
prevent name conflicts.

- A loader file (named `loader.lgt` or `loader.logtalk`) that loads the
registry object file and all pack object files.

An example of a registry specification object would be:

	:- object(jdoe_awesome_packs_registry,
		implements(registry_protocol)).

		:- info([
			version is 1:0:0,
			author is 'John Doe',
			date is 2021-10-18,
			comment is 'John Doe awesome packs registry spec.'
		]).

		name(jdoe_awesome_packs).

		description('John Doe awesome packs').

		home('https://example.com/jdoe_awesome_packs').

		clone('https://github.com/jdoe/jdoe_awesome_packs.git').

		archive('https://github.com/jdoe/jdoe_awesome_packs/archive/main.zip').

	:- end_object.

Optionally, the registry object can also define a `note(Action, Note)`
predicate. The `Action` argument is an atom: `add`, `update`, or
`delete`. The `Note` argument is also an atom. The tool will print
any available notes when executing one of the registry actions. See the
`registry_protocol` documentation for more details.

The registry directory should also contain `LICENSE` and `README.md` files
(individual packs can use a different license, however). The path to the
`README.md` file is printed when the registry is added. It can also be
queried using the `registries::directory/2` predicate. The `NOTES.md` file
name can also be used in alternative to the recommended `README.md` file name.

Summarizing the required directory structure using the above example (note
that the registry and pack specification files are named after the objects):

	jdoe_awesome_packs
		LICENSE
		README.md
		jdoe_awesome_packs_registry.lgt
		loader.lgt
		foo_pack.lgt
		bar_pack.lgt
		...

With the contents of the `loader.lgt` file being:

	:- initialization((
		logtalk_load(jdoe_awesome_packs_registry),
		logtalk_load(foo_pack),
		logtalk_load(bar_pack),
		...
	)).

It would be, of course, possible to have all objects in a single source file.
But having a file per-object and a loader file helps maintenance, and it's
also a tool requirement for applying safety procedures to the source file
contents and thus successfully loading the registry and pack specs.

As registries are git repos in the most common case, and thus adding them
performs a git repo cloning, they should only contain the strictly required
files.


Registry handling
-----------------

Registries can be added using the `registries::add/1-3` predicates, which take
a registry URL. Using the example above:

	| ?- registries::add('https://github.com/jdoe/jdoe_awesome_packs.git').

HTTPS URLs must end with either a `.git` extension or an archive extension
(same valid extensions as for pack archives, including `gpg` encrypted).
Git cloning URLs are preferred as they simplify updating registries. But a
registry can also be made available via a local directory (using a `file://`
URL) or a downloadable archive (using a `https://` URL).

For registries made available using an archive, the `registries::add/2-3`
predicates **must** be used as the registry name cannot in general be inferred
from the URL basename or from the archived directory name. The registry
argument must also be the declared registry name in the registry specification
object. For example:

	| ?- registries::add(
			jdoe_awesome_packs,
			'https://github.com/jdoe/jdoe_awesome_packs/archive/main.zip'
	     ).

When a registry may be already defined, you can use the `update(true)` option
to ensure that the registry will be updated to its latest definition:

	| ?- registries::add(
			jdoe_awesome_packs,
			'https://github.com/jdoe/jdoe_awesome_packs/archive/main.zip',
			[update(true)]
	     ).

The added registries can be listed using the `registries::list/0` predicate:

	| ?- registries::list.

	% Defined registries:
	%   jdoe_awesome_packs (git)
	%   ...

The `registries::describe/1` predicate can be used to print the details of a
registry:

	| ?- registries::describe(jdoe_awesome_packs).

	% Registry:    jdoe_awesome_packs
	% Description: John Doe awesome packs
	% Home:        https://example.com/jdoe_awesome_packs
	% Cloning URL: https://github.com/jdoe/jdoe_awesome_packs.git
	% Archive URL: https://github.com/jdoe/jdoe_awesome_packs/archive/main.zip

To update all registries, use the `registries::update/0` predicate. To update
a single registry, use the `registries::update/1-2` predicates. After updating,
you can use the `packs::outdated/0-1` predicates to list any outdated packs.

Registries can also be deleted using the `registries::delete/1-2` predicate.
By default, any registries with installed packs cannot be deleted. If you
force deletion (by using the `force(true)` option), you can use the
`packs::orphaned/0` predicate to list any orphaned packs that are installed.

See the tool API documentation on the [registries](../../docs/registries_0.html)
object for other useful predicates.


Registry development
--------------------

To simplify registry development and testing, use a local directory and a
`file://` URL when calling the `registries::add/1` predicate. For example:

	| ?- registries::add('file:///home/jdoe/work/my_pack_collection').

If the directory is a git repo, the tool will clone it when adding it.
Otherwise, the files in the directory are copied to the registry definition
directory. This allows the registry to be added and deleted without
consequences for the original registry source files.

To check your registry specifications, use the `registries::lint/0-1`
predicates after adding the registry.


Pack specification
------------------

A pack is specified using a Logtalk source file defining an object that
implements the `pack_protocol`. The source file should be named after the
pack with a `_pack` suffix. This naming convention helps prevent name
conflicts, notably with the pack's own objects. The file must be available
from a declared pack registry (by having the registry loader file loading
it). The pack name is preferably a valid unquoted atom. An example of a
pack specification object would be:

	:- object(lflat_pack,
		implements(pack_protocol)).

		:- info([
			version is 1:0:0,
			author is 'Paulo Moura',
			date is 2021-10-18,
			comment is 'L-FLAT - Logtalk Formal Language and Automata Toolkit pack spec.'
		]).

		name(lflat).

		description('L-FLAT - Logtalk Formal Language and Automata Toolkit').

		license('MIT').

		home('https://github.com/l-flat/lflat').

		version(
			2:1:0,
			stable,
			'https://github.com/l-flat/lflat/archive/refs/tags/v2.1.0.tar.gz',
			sha256 - '9c298c2a08c4e2a1972c14720ef1498e7f116c7cd8bf7702c8d22d8ff549b6a1',
			[logtalk @>= 3:42:0],
			all
		).

		version(
			2:0:2,
			stable,
			'https://github.com/l-flat/lflat/archive/refs/tags/v2.0.2.tar.gz',
			sha256 - '8774b3863efc03bb6c284935885dcf34f69f115656d2496a33a446b6199f3e19',
			[logtalk @>= 3:36:0],
			all
		).

	:- end_object.

The `license/1` argument must be an atom and should, whenever possible, be
a license identifier as specified in the [SPDX standard](https://spdx.org/licenses/).

Optionally, the pack object can also define a `note(Action, Version, Note)`
predicate. The `Action` argument is an atom: `install`, `update`, or
`uninstall`. The `Note` argument is also an atom. The tool will print
any available notes when executing one of the registry actions. See the
`pack_protocol` documentation for more details.

The pack sources must be available either as a local directory (when using
a `file://` URL) or for downloading as a supported archive. The checksum for
the archive must use the SHA-256 hash algorithm (`sha256`). The pack may
optionally be signed. Supported archive formats and extensions are:

- `.zip`
- `.tgz`, `.tar.gz`
- `.tbz2`, `.tar.bz2`

Also, for encrypted packs, all the extensions above with a `.gpg` suffix
(e.g., `.zip.gpg`).

The pack sources should contain `LICENSE`, `README.md` (or `NOTES.md`), and
`loader.lgt` (or `loader.logtalk`) files. Ideally, it should also contain a
`tester.lgt` (`tester.logtalk`) file. The path to the `README.md` file is
printed when the pack is installed or updated. It can also be queried using
the `packs::directory/2` predicate.


Encrypted packs
---------------

Packs can be `gpg` encrypted, with a choice of passphrase-based encryption,
key-based encryption, or both. Encrypted pack archives must always have a
`.gpg` extension. For example, to encrypt a pack archive with a symmetric
cipher using a passphrase:

	$ tar -cvzf - my_pack | gpg -c --cipher-algo AES256 > v1.2.1.tar.gz.gpg

In this case, the passphrase would need to be securely communicated to any
users installing or updating the pack.

See the `gpg` documentation for full details on encrypting and decrypting
archives. If you get a "gpg: problem with the agent: Inappropriate ioctl
for device" error message with the command above, try:

	$ export GPG_TTY=$(tty)


Signed packs
------------

Packs can be `gpg` signed. Detached signature files are assumed and expected
to share the name of the archive and use `.asc` or `.sig` extensions. For
example, if the pack archive name is `v1.0.0.tar.gz`, the signature file must
be named `v1.0.0.tar.gz.asc` or `v1.0.0.tar.gz.sig`. When the `checksig(true)`
option is used, the signature file is automatically downloaded using a URL
constructed from the pack archive URL. When both `.asc` and `.sig` files
exist, the `.asc` file is used. An example of signing a pack and creating the
`.asc` file (assuming the default signing key) is:

	$ gpg --armor --detach-sign v1.0.0.tar.gz

To create instead a `.sig` file:

	$ gpg --detach-sign v1.0.0.tar.gz

See the `gpg` documentation for full details on signing archives and sharing
the public keys required to verify the signatures.


Pack URLs and Single Sign-On
----------------------------

Typically, pack archive download URLs are HTTPS URLs and handled using `curl`.
It's also possible to use `git archive` to download pack archives, provided
that the server supports it (as of this writing, Bitbucket and GitLab public
hosting services support it but not GitHub). Using `git archive` is specially
useful when the packs registry is hosted on a server using Single Sign-On (SSO)
for authentication. In this case, HTTPS URLs can only be handled by `curl` by
passing a token (see below for an example). When the user has setup SSH keys
to authenticate to the packs registry server, `git archive` simplifies pack
installation, providing a better user experience. For example:

	version(
		1:0:1,
		stable,
		'git@gitlab.com:me/foo.git/v1.0.1.zip',
		sha256 - '0894c7cdb8968b6bbcf00e3673c1c16cfa98232573af30ceddda207b20a7a207',
		[logtalk @>= 3:36:0],
		all
	).

The pseudo-URL must be the concatenation of the SSH repo cloning URL with the
archive name. The archive name must be the concatenation of a valid tag with
a supported archive extension. SSH repo cloning URLs use the format:

	git@<hostname>:path/to/project.git

They can usually be easily copied from the hosting service repo webpage. To
compute the checksum, you must first download the archive. For example:

	$ git archive --output=foo-v1.0.1.zip --remote=git@gitlab.com:me/foo.git v1.0.1
	$ openssl sha256 foo-v1.0.1.zip

Be sure to use a format that is supported by both the `packs` tool and the
`git archive` command (the format is inferred from the `--output` option).
Do not download the archive from the web interface of the git hosting
service in order to compute the checksum. Different implementations
of the archiving and compressing algorithms may be used, resulting in
mismatched checksums.

Users installing packs available using `git archive` URLs are advised to
run a SSH agent to avoid being prompted for passwords when installing
or updating packs. They must also upload their SSH public keys to the
pack provider hosts.


Multiple pack versions
----------------------

A pack may specify multiple versions. Each version is described using a
`version/6` predicate clause as illustrated in the example above. The
versions must be listed in order from newest to oldest. For details, see
the `pack_protocol` API documentation.

Listing multiple versions allows the pack specification to be updated (by
updating its registry) without forcing existing users into installing (or
updating to) the latest version of the pack.


Pack dependencies
-----------------

Pack dependencies on other packs can be specified using a list of
`Registry::Pack Operator Version` terms where `Operator` is a standard term
comparison operator:

- `Registry::Pack @>= Version` - the pack requires a dependency with a version
equal or above the specified one. For example, `logtalk @>= 3:36:0` means that
the pack requires Logtalk 3.36.0 or a later version.

- `Registry::Pack @=< Version` - the pack requires a dependency with a version
up to the specified one. For example, `common::bits @=< 2:1` means that the pack
requires a `common::bits` pack up to 2.1. This includes all previous versions
and also all patches for version 2.1 (e.g., 2.1.7, 2.1.8, ...) but not version
2.2 or newer.

- `Registry::Pack @< Version` - the pack requires a dependency with version
older than the specified one. For example, `common::bits @< 3` means that the
pack requires a `common::bits` 2.x or older version.

- `Registry::Pack @> Version` - the pack requires a dependency with version
newer than the specified one. For example, `common::bits @> 2:4` means that
the pack requires a `common::bits` 2.5 or newer version.

- `Registry::Pack == Version` - the pack requires a dependency with a specific
version. For example, `common::bits == 2:1` means that the pack requires a
`common::bits` pack version 2.1.x (thus, from version 2.1.0 to the latest patch
for version 2.1).

- `Registry::Pack \== Version` - the pack requires a dependency with any version
other than the one specified. For example, `common::bits \== 2.1` means that
the pack requires a `common::bits` pack version other than any 2.1.x version.

To specify _range_ dependencies by using two consecutive elements with the lower
bound followed by the upper bound. For example, `common::bits @>= 2, common::bits @< 3`
means all `common::bits` 2.x versions but not older or newer major versions.

It's also possible to specify _alternative_ dependencies using the `(;)/2`
operator. For example, `(common::bits == 1:9; common::bits @>= 2:3)` means
either `common::bits` 1.9.x versions or 2.3.x and later versions. Alternatives
should be listed in decreasing order of preference.

When a pack also depends on a Logtalk or backend version, the name `logtalk`
or the backend identifier atom can be used in place of `Registry::Pack` (see
below for the table of backend specifiers). For example, `logtalk @>= 3.36.0`.

When a pack also depends on an operating-system version (e.g., a pack
containing shared libraries with executable code), the `os(Name,Machine)`
compound term can also be used in place of `Registry::Pack`. For example,
`os('Darwin',x86_64) @>= '23.0.0'`. Note that, in this case, the release
is an atom. The operating-system data (name, machine, and release) is
queried using the corresponding `os` library predicates (see the library
documentation for details).


Pack portability
----------------

Ideally, packs are fully portable and can be used with all Logtalk-supported
Prolog backends. This can be declared by using the atom `all` in the last
argument of the `version/6` predicate (see example above).

When a pack can only be used with a subset of the Prolog backends, the last
argument of the `version/6` predicate is a list of backend identifiers
(atoms):

- B-Prolog: `b`
- Ciao Prolog: `ciao`
- CxProlog: `cx`
- ECLiPSe: `eclipse`
- GNU Prolog: `gnu`
- JIProlog: `ji`
- XVM: `xvm`
- Quintus Prolog: `quintus`
- SICStus Prolog: `sicstus`
- SWI-Prolog: `swi`
- Tau Prolog: `tau`
- Trealla Prolog: `trealla`
- XSB: `xsb`
- YAP: `yap`


Pack development
----------------

To simplify pack development and testing, define a local registry and add to
it a pack specification with the development version available from a local
directory. For example:

	version(
		0:11:0,
		beta,
		'file:///home/jdoe/work/my_awesome_library',
		none,
		[],
		all
	).

If the directory is a git repo, the tool will clone it when installing
the pack. Otherwise, the files in the directory are copied to the pack
installation directory. This allows the pack to be installed, updated,
and uninstalled without consequences for the pack source files.

You can also use a local archive instead of a directory. For example:

	version(
		1:0:0,
		stable,
		'file:///home/jdoe/work/my_awesome_library/v1.0.0.tar.gz',
		sha256 - '1944773afba1908cc6194297ff6b5ac649a844ef69a69b2bcdf267cfa8bfce1e',
		[],
		all
	).

Packs that are expected to be fully portable should always be checked by
loading them with the `portability` flag set to `warning`.

To check your pack manifest files, use the `packs::lint/0-2` predicates
after adding the registry that provides the packs.


Pack handling
-------------

Packs must be available from a defined registry. To list all packs that are
available for installation, use the `packs::available/0` predicate:

	| ?- packs::available.

To list all installed packs, call the `packs::installed/0` predicate:

	| ?- packs::installed.

To list only the installed packs from a specific registry, call instead the
`packs::installed/1` predicate. For example:

	| ?- packs::installed(talkshow).

To know more about a specific pack, use the `packs::describe/1-2` predicates.
For example:

	| ?- packs::describe(bar).

The `packs::describe/2` predicate can be used when two or more registries
provide packs with the same name. For example:

	| ?- packs::describe(reg, bar).

To install the latest version of a pack, we can use the `packs::install/1-4`
predicates. In the most simple case, when a pack name is unique among
registries, we can use the `packs::install/1` predicate. For example:

	| ?- packs::install(bar).

Any pack dependencies are also checked and installed or updated if necessary.
Other install predicates are available to disambiguate between registries and
to install a specific pack version.

Packs become available for loading immediately after successful installation
(no restarting of the Logtalk session is required). For example, after the
pack `bar` is installed, you can load it at the top-level by typing:

	| ?- {bar(loader)}.

or load it from a loader file using the goal `logtalk_load(bar(loader))`.

After updating the defined registries, outdated packs can be listed using
the `packs::outdated/0` predicate. You can update all outdated packs by
calling the `packs::update/0` predicate or update a single pack using the
`packs::update/1-2` predicates. For example:

	| ?- packs::update(bar).

By default, updating a pack fails if it would break any dependent pack
(the `force(true)` option, described below, can be used to force updating
in this case).

The tool provides versions of the pack install, update, and uninstall
predicates that accept a list of options:

- `verbose(Boolean)` (default is `false`)
- `clean(Boolean)` (default is `false`)
- `update(Boolean)` (default is `false`)
- `force(Boolean)` (default is `false`)
- `compatible(Boolean)` (default is `true`)
- `checksum(Boolean)` (default is `true`)
- `checksig(Boolean)` (default is `false`)
- `git(Atom)` (extra command-line options; default is `''`)
- `downloader(Atom)` (downloader utility; default is `curl`)
- `curl(Atom)` (extra command-line options; default is `''`)
- `wget(Atom)` (extra command-line options; default is `''`)
- `gpg(Atom)` (extra command-line options; default is `''`)
- `tar(Atom)` (extra command-line options; default is `''`)

Note that, by default, only compatible packs can be installed. To install
a pack that is incompatible with the current Logtalk version, backend
version, or operating-system version, use the `install/4` or `update/3`
predicates with the option `compatible(false)`.

When installing large packs over unreliable network conditions, you may
try switching the default downloader utility from `curl` to `wget`.

When a pack may already be installed, you can use the `update(true)` option
to ensure that the installation will be updated to the specified version:

	| ?- packs::install(reg, bar, 1:1:2, [update(true)]).

When using a `checksig(true)` option to check a pack signature, it is strongly
advised that you also use the `verbose(true)` option. For example:

	| ?- packs::install(reg, bar, 1:1:2, [verbose(true), checksig(true)]).

Note that the public key used to sign the pack archive must already be present
in your local system.

Downloading pack archives may require passing extra command-line options to
`curl` for authentication. A common solution is to use a personal access token.
The details depend on the server software. An example when using GitHub:

	| ?- packs::install(reg, bar, 1:1:2, [curl('--header "Authorization: token foo42"')]).

Another example when using GitLab:

	| ?- packs::install(reg, bar, 1:1:2, [curl('--header "PRIVATE-TOKEN: foo42"')]).

Pack archives may be `gpg` encrypted. Encryption can be passphrase-based,
key-based, or both. When using only passphrase-based encryption, the archive
passphrase must be entered (if not cached) when installing or updating a pack.
In this case, the passphrase can be entered interactively or using the `gpg/1`
option. For example:

	| ?- packs::install(reg, bar, 1:1:2, [gpg('--batch --passphrase test123')]).

See the `gpg` documentation for details. When using the `gpg/1` option, you
should be careful to not leak passphrases in, e.g., the query history.

To uninstall a pack that you no longer need, use the `packs::uninstall/1-2`
predicates. By default, only packs with no dependent packs can be uninstalled.
You can print or get a list of the packs that depend on a given pack by using
the `packs::dependents/1-3` predicates. For example:

	| ?- packs::dependents(reg, bar, Dependents).

See the tool API documentation on the [packs](../../docs/packs_0.html) object
for other useful predicates.


Pack documentation
------------------

The path to the pack `README.md` file is printed when the pack is installed
or updated. It can also be retrieved at any time by using the `readme/2`
predicate. For example:

	| ?- packs::readme(lflat, Path).

Additional documentation may also be available from the pack home page, which
can be printed by using the `describe/1-2` predicates. For example:

	| ?- packs::describe(lflat).

	% Registry:    ...
	% Pack:        lflat
	% Description: L-FLAT - Logtalk Formal Language and Automata Toolkit
	% License:     MIT
	% Home:        https://github.com/l-flat/lflat
	% Versions:
	...

The pack API documentation can be generated using the `lgtdoc` tool library
and directory predicates (depending on the pack source files organization).
For example:

	| ?- {lflat(loader)},
	     {lgtdoc(loader)},
	     logtalk::expand_library_path(lflat, Path),
	     lgtdoc::rdirectory(Path).
	...

This query creates a `xml_docs` directory in the current directory. The XML
documentation files can then be converted into a final format, e.g., HTML,
using one of the scripts provided by the `lgtdoc` tool. For example:

	$ cd xml_docs
	$ lgt2html

For more details and alternatives, see the `lgtdoc` tool documentation.

It is also possible to add API documentation and diagrams for all the installed
packs to the Logtalk distribution API documentation and diagrams by calling
the `update_html_docs` and `update_svg_diagrams` scripts with the `-i` option.
See the scripts documentation for more details.

Pinning registries and packs
----------------------------

Registries and packs can be _pinned_ after installation to prevent accidental
updating or deleting, e.g., when using the batch `update/0` predicate. This is
useful when your application requires a specific version or for security
considerations (see below). For example, if we want the `bar` pack to stay at
its current installed version:

	| ?- packs::pin(bar).
	yes

After, any attempt to update or uninstall the pack will fail with an error
message:

	| ?- packs::update(bar).
	!     Cannot update pinned pack: bar
	no

	| ?- packs::uninstall(bar).
	!     Cannot uninstall pinned pack: bar
	no

To enable the pack to be updated or uninstalled, the pack must first be
unpinned. Alternatively, the `force(true)` option can be used. Note that
if you force update a pinned pack, the new version will be unpinned.

It's also possible to pin (or unpin) all defined registries or installed
packs at once by using the `pin/0` (or `unpin/0`) predicates. But note that
registries added after or packs installed after will not be automatically
pinned.


Testing packs
-------------

Logtalk packs (as most Logtalk libraries, tools, and examples) are expected
to have a `tester.lgt` or `tester.logtalk` tests driver file at the root of
their directory, which can be used for both automated and manual testing.
For example, after installing the `foo` pack:

	| ?- {foo(tester)}.

To test all installed packs, you can use the `logtalk_tester` automation
script from the installed packs directory, which you can query using the
goal:

	| ?- packs::prefix(Directory).

Note that running the packs tests, like simply loading the pack, can result
in calling arbitrary code, which can potentially harm your system. Always
take into account the security considerations discussed below.


Security considerations
-----------------------

New pack registries should be examined before being added, specially if
public and from a previously unknown source. The same precautions should
be taken when adding or updating a pack. Note that a registry can always
index third-party packs.

Pack checksums are checked by default. But pack signatures are only checked
if requested, as packs are often unsigned. Care should be taken when adding
public keys for pack signers to your local system.

Registry and pack spec files, plus the registry loader file, are compiled
by term-expanding them so that only expected terms are actually loaded and
only expected `logtalk_load/2` goals with expected relative file paths are
allowed. Predicates defining URLs are discarded if the URLs are neither
`https://` nor `file://` URLs or if they contain non-allowed characters
(currently, only alpha-numeric ASCII characters plus the ASCII `/`, `.`,
`-`, and `_` characters are accepted). But note that this tool makes no
attempt to audit pack source files themselves.

Registries and packs can always be pinned so that they are not accidentally
updated to a version that you may not have had the chance to audit.


Best practices
--------------

- Make available a new pack registry as a git repo. This simplifies updating
the registry and rolling back to a previous version.

- Use registry and pack names that are valid unquoted atoms, thus simplifying
usage. Use descriptive names with underscores if necessary to link words.

- Name the registry and pack specification objects after their names with a
`_registry` or `_pack` suffix. Save the objects in files named after the
objects.

- Create new pack versions from git tags.

- If the sources of a pack are available from a git repo, consider using
signed commits and signed tags for increased security.

- When a new pack version breaks backwards compatibility, list both the old
and the new versions on the pack specification file.

- Pin registries and packs when specific versions are critical for your work
so that you can still easily batch update the remaining packs and registries.

- Include the `$LOGTALKPACKS` directory (or the default `~/logtalk_packs`
directory) on your regular backups.


Installing Prolog packs
-----------------------

This tool can also be used to install Prolog packs that don't use Logtalk.
After installing a `pl_pack` Prolog pack from a `pl_reg` registry, it can
be found in the `$LOGTALKPACKS/packs/pl_reg/pl_pack` directory. When the
`LOGTALKPACKS` environment variable is not defined, the pack directory is
by default `~/logtalk_packs/packs/pl_reg/pl_pack`.

Different Prolog systems provide different solutions for locating Prolog
code. For example, several Prolog systems adopted the Quintus Prolog
`file_search_path/2` hook predicate. For these systems, a solution could
be to add a fact to this predicate for each installed Prolog pack. For
example, assuming a `pl_pack` Prolog pack:

	:- multifile(file_search_path/2).
	:- dynamic(file_search_path/2).

	file_search_path(library, '$LOGTALKPACKS/packs/pl_pack').

If the Prolog system also supports reading an initialization file at
startup, the above definition could be added there.


Help with warnings
------------------

Load the `tutor` tool to get help with selected warnings printed by the
`packs` tool.


Known issues
------------

Using the `verbose(true)` option on Windows systems may not provide the
shell commands output depending on the backend.

On Windows systems, the reset, delete, and uninstall predicates may fail
to delete all affected folders and files due to a operating-system bug.
Depending on the backend, this bug may cause some of the tests to fail.
For details on this bug, see:

https://github.com/microsoft/terminal/issues/309

The workaround is to use the Windows File Explorer to delete the leftover
folders and files.

When using Ciao Prolog 1.20.0, a workaround is used for this system
non-standard support for multifile predicates.

When using GNU Prolog 1.5.0 as the backend on Windows, you may get an
error on `directory_files/2` calls. For details and a workaround, see:

https://github.com/didoudiaz/gprolog/issues/4

This issue is fixed in the latest GNU Prolog git version.

Using SICStus Prolog as the backend on Windows doesn't currently work
in version 4.7.0 and earlier versions. The underlying issues are fixed
in the SICStus Prolog 4.7.1 version.

XSB has an odd bug (likely in its parser) when reading files that may
cause a pack installed version to be reported as the `end_of_file` atom.

Some tests fail on Windows when using ECLiPSe or XSB due to file path
representation issues. 
