## `help` -- display a help page

#### Introduction

`help(`"word"`)` or `?word` displays the online help page related to `word`.

#### Call(s)

`help("word")`
`?word`

#### Parameters

`word` | - | any keyword

#### Returns

the void object `null()` of type `DOM_NULL`.

#### Related Functions

`info`, `Pref::ansi`

#### Details

- `help(`"word"`)` displays a help page with information about the keyword `"word"`.

- The exact form of the output depends on the platform. In the terminal version, information is displayed as ASCII text. In XMuPAD and on MacOS or Windows platforms, the help page is displayed in a separate help window with hypertext functionality. Highlighted words are hypertext links, which can be followed by clicking on them with the mouse.

- The call `?word` is a short form of `help(`"word"`)`. The `?` command is not a MuPAD function and cannot be used in expressions. It can only be entered interactively and in a line of its own. Note that the search word `word` must neither be put in quotation marks nor followed by a terminating semicolon.

- The keyword may contain the wildcards `?` and `*`. `?` represents any single character, and `*` represents an arbitrarily long, possibly empty, sequence of characters. There are three exceptions: `?*` and `?`*`` lead directly to the help page for `_mult`, and `?`**`` leads to the help page for `_power`. Cf. example 1.

- If there is no help page for the specified keyword, then a list of keywords with similar spelling is displayed. Cf. example 2.

- The command `anames(All)` returns a set with the names of all currently loaded system functions. The command `?**` returns a list of all available help pages.
- The documentation for modules is not included in this help system. Example 3 demonstrates the calls for obtaining information about modules and their functions.

- `help` is a function of the system kernel.

#### Example 1

`help` expands wildcards:

    >> ?*type

          Try:  domtype hastype testtype type Type Type::AnyType

An exception: `?*` leads directly to the help page for `_mult`:

    >> ?*

          * -- multiply expressions

          Introduction

           a * b respectively _mult(a, b) computes the product a*b.

          Call(s)

          o a * b _mult( <a, b...>)

          Parameters

           a, b  -  arithmetical expressions

          [...]

#### Example 2

There is no information on the non-existent function `worm`:

    >> ?worm

          Sorry, no help page available for 'worm' !

          Try:  norm

#### Example 3

MuPAD supports `C++` compiled kernel extensions, called dynamic modules. The documentation of a dynamic module is not integrated into the MuPAD hypertext help system, but is provided as plain text online documentation, which can be displayed via the `"doc"` method of the corresponding module, e.g., `util::doc`:

    >> module(util): util::doc()

          MODULE
            util - A collection of utility functions

          INTRODUCTION
            The module provides a collection of useful utility functions.

          INTERFACE
            util::busyWaiting, util::date,      util::doc,
            util::kernelPath,  util::kernelPid, util::sleep,
            util::time,        util::userName

    >> util::doc("kernelPath")

          NAME
            util::kernelPath - Returns the pathname of the MuPAD kernel

          SYNOPSIS
            util::kernelPath()

          DESCRIPTION
            This function returns the pathname of the MuPAD kernel.

          EXAMPLES
            >> util::kernelPath()

               "C:\\PROGRA~1\\SCIFACE\\MUPADP~1.5\\BIN\\MUPKERN.EXE"

            >> util::kernelPath()

               "/usr/local/mupad/linux/bin/mupad"

          SEE ALSO
            util::kernelPid, util::userName

#### Background

- In the terminal version, the viewer called to display the help pages in ASCII format is given by the system variable `PAGER`. See `Pref::ansi` on how to control the format of this output.

#### Changes

- `?*` no longer lists all available help pages, but instead leads to the help page of `_mult`.

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