.. _library_help:

``help``
========

This tool provides basic on-line help for Logtalk features and libraries
when running in most operating-systems. For help on the Logtalk compiler
error and warning messages, see the ``tutor`` tool.

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

This tool API documentation is available at:

`../../docs/library_index.html#help <../../docs/library_index.html#help>`__

For sample queries, please see the ``SCRIPT.txt`` file in the tool
directory.

Loading
-------

::

   | ?- logtalk_load(help(loader)).

Testing
-------

To test this tool, load the ``tester.lgt`` file:

::

   | ?- logtalk_load(help(tester)).

Supported operating-systems
---------------------------

Currently, support is limited to Linux, macOS, and Windows.

On Windows, the ``start`` command must be available. On Linux, the
``xdg-open`` command must be available. On macOS, the command ``open``
is used.

This tool relies on the library portable operating-system access
abstraction.

Usage
-----

After loading the tool, use the query ``help::help`` to get started.

Experimental features
---------------------

On POSIX systems, when using Ciao Prolog, ECLiPSe, GNU Prolog (1.5.1 or
later version), LVM, SICStus Prolog, SWI-Prolog, Trealla Prolog, or XSB
as the backend, ``apis/1`` and ``handbook/0-1`` predicates are made
available. These predicates open inline at the top-level interpreter the
Texinfo versions of the Handbook and the APIs documentation. The
optional argument is a starting node, which can be an atom, a predicate
indicator, or a non-terminal indicator. When there are several nodes for
the same argument (e.g. multiple implementations of the ``member/2``
predicate), one of them will be displayed. Some examples:

::

   | ?- help::handbook.

   | ?- help::handbook(base64).

   | ?- help::handbook(logtalk_load/2).

   | ?- help::apis.

   | ?- help::apis(check/2).

   | ?- help::apis(message_tokens//2).

Although less useful, you can also browse the ``man`` pages of Logtalk
scripts. For example:

::

   | ?- help::man(logtalk_tester).

When you finish consult the documentation and quit the ``info`` process,
you will be back to the top-level prompt (if you find that the top-level
have scrolled from its last position, try to set your terminal terminfo
to ``xterm-256colour``).

If you're running Logtalk from a git clone of its repo, you will need to
run the ``scripts/update_html_docs.sh`` or
``scripts/update_html_docs.ps1`` scripts to generate the APIs
documentation ``.info`` file and also run the
``manuals/sources/build_manuals.sh`` or
``manuals/sources/build_manuals.ps1`` scripts to generated the Handbook
``.info`` file. In alternative, you can download the ``.info`` files for
the latest stable release from the Logtalk website and save them to the
``docs`` and ``manuals`` directories.

The required ``info`` command is provided by the third-party ``texinfo``
package (tested with version 6.8). On macOS, this package can be
installed with either MacPorts:

::

   $ sudo port install texinfo

Or using Homebrew:

::

   $ brew install texinfo

On Linux systems, use the distribution own package manager to install
the ``texinfo`` package. For example, in Ubuntu systems:

::

   $ sudo apt install info

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

The open commands used to open documentation URLs drop the fragment
part, thus preventing navigating to the specified position on the
documentation page.

ECLiPSe defines a ``help`` prefix operator that forces wrapping this
atom between parenthesis when sending messages to the tool. E.g. use
``(help)::help`` instead of ``help::help``.
