# `help`

This tool provides 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.

## Requirements

On Windows, the `start` command must be available. On Linux, the `xdg-open` command must be available. On macOS, the command `open` is used. On POSIX systems `bsdtar` is also required.

Browsing the Handbook and APIs documentation at the top-level requires a POSIX system and one of the following terminal-based browsers installed:

- https://invisible-island.net/lynx/

- https://w3m.sourceforge.net/

- http://links.twibright.com/

- https://sr.ht/~bptato/chawan/

On macOS, these browsers and `bsdtar` can be installed with either MacPorts:

    $ sudo port install lynx
    $ sudo port install w3m
    $ sudo port install links
    $ sudo port install libarchive

Or using Homebrew:

    $ brew install lynx
    $ brew install w3m
    $ brew install links
    $ brew install chawan
    $ brew install libarchive

On Linux systems, use the distribution’s own package manager to install the browsers and `bsdtar`. For example, in Ubuntu systems:

    $ sudo apt install lynx
    $ sudo apt install w3m
    $ sudo apt install links2
    $ sudo apt install libarchive-tools

For RedHat systems:

    $ sudo dnf install lynx
    $ sudo dnf install w3m
    $ sudo dnf install links2
    $ sudo dnf install bsdtar

If you’re running Logtalk from a git clone of its repo, you will need to run the `docs/apis/sources/build.sh` or `docs/apis/sources/build.ps1` scripts to generate APIs documentation HTML files and also run the `docs/handbook/sources/build.sh` or `docs/handbook/sources/build.ps1` scripts to generate the Handbook HTML files. Alternatively, you can download the documentation for the latest stable release from the Logtalk website and save them to the `docs` directories.

## Customization

The preferred browser can be set in a `settings.lgt` file by defining the `help_default_browser` user-defined flag as follows:

    :- initialization(
        create_logtalk_flag(help_default_browser, Browser, [type(atom), keep(true)])
    ).

The valid values for `Browser` are: `lynx` (default value), `w3m`, `links`, `cha`, and `default` (`default` means use the operating-system default browser instead of one of the terminal-based browsers).

We can also select between browsing the HTML files generated by Sphinx for web browsing or the XHTML files generated by Sphinx for the ePub version of the documentation by defining the `help_default_files` user-defined flag as follows:

    :- initialization(
        create_logtalk_flag(help_default_files, Format, [type(atom), keep(true)])
    ).

The valid values for `Format` are `html` (default value) or `xhtml`.

The `samples/settings-sample.lgt` file contains commented out code for defining these flags.

On POSIX systems, one of the supported terminal-based browsers must be installed unless you prefer using the default browser. By default, the tool checks first for `lynx`, second for `w3m`, third for `links`, and finally for `chawan`. When none of these terminal-based browsers is available in their default installation paths, the documentation is open in the operating-system default browser.

On Windows systems, the documentation is open in the operating-system default browser.

When using the terminal-based browsers, after finishing consulting the documentation and quitting the 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`).

## API documentation

This tool API documentation is available at:

../../apis/library_index.html#help

## 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.

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

## Usage

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

    | ?- logtalk_load(help(loader)).
    ...

    | ?- help::help.
    ...

When using ECLiPSe, you will need to write the object name, `help`, between parentheses to avoid a clash with the `help` built-in operator:

    | ?- (help)::help.
    ...

Same sample help queries:

    % get et on-line help for the `logtalk_load/2` built-in predicate:

    | ?- help::logtalk_load/2.
    ...

    % get on-line help for the `eos//0` built-in non-terminal:

    | ?- help::eos//0.
    ...

    % consult the Handbook:

    | ?- help::handbook.
    ...

    % consult the APIs documentation:

    | ?- help::apis.
    ...

    % consult the APIs documentation about a library predicate:

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

    % consult the APIs documentation about a library non-terminal:

    | ?- help::apis(one_or_more//0).
    ...

    % consult the documentation of a library:

    | ?- help::library(random).
    ...

    % consult the documentation of an entity:

    | ?- help::entity(logtalk).
    ...

    % consult the developer tools documentation:

    | ?- help::tools.
    ...

    % consult the documentation of the lgtunit tool:

    | ?- help::tool(lgtunit).
    ...

    % consult the documentation of the logtalk_tester script:

    | ?- help::man(logtalk_tester).
    ...

## Known issues

When using the `lynx` terminal-based browser, the `help_default_files` flag setting is ignored as this browser doesn’t support XHTML.

When using the terminal-based browsers, the Handbook and APIs search boxes are not usable as they require JavaScript support. Use instead the indexes.

Some CSS support is only provided by the Chawan terminal-based browser. This results in formatting issues when browsing the Logtalk documentation. Some of these issues can be avoided by setting the `help_default_files` flag to `xhtml`.

The open commands used to open documentation URLs in the default browser drop the fragment part, thus preventing navigation to the specified position on the documentation page.

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