.. _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), XVM, 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.

When browsing the Texinfo versions of the Handbook and the APIs
documentation generated with a recent version of Sphinx and using a
recent version of Texinfo, the Texinfo search feature often displays the
previous nodes of the searched nodes.

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``.
