UK TeX FAQ -- question label keyval

### Key-value input for macros and package options

When we discussed extending the number of arguments to a macro, we suggested that large numbers of arguments, distinguished only by their position, aren’t very kind to the user, and that a package such as keyval offers a more attractive user interface. We now consider the packages that the macro programmer might use, to create such a user interface.

The simplest key-value processor remains keyval; it has a command `\``define@key` to declare a key and a handler to process it, and a macro `\``setkeys` to offer values to the handler of one or more keys. Thus:

>     \define@key{my}{foo}{Foo is #1\par}
>     \define@key{my}{bar}[99]{Bar is #1\par}
>     ...
>     \setkeys{my}{foo=3,bar}

will produce output saying:

> Foo is 3
> Bar is 99

This has defined two keys ‘`foo`’ and ‘`bar`’ in family ‘`my`’, and then executed them, the first with argument ‘`3`’ and the second with no argument, so that the default value of ‘`99`’ is picked up. In effect, the two calls to `\``define@key` are simply defining commands, as (for example):

>     \newcommand{\KV@my@foo}[1]{Foo is #1}

(the definition of `\``KV@my@bar` is similar, but trickier). The command `\``setkeys` knows how to find those commands when it needs to process each key — it is easy to regurgitate the structure of the command name, with family name (‘`my`’, here) after the first ‘`@`’, and the key name after the second ‘`@`’. (The ‘`KV`’ part is fixed, in keyval.)

These simple commands are enough, in fact, to process the botanical example offered as replacement for multi-argument commands in the question mentioned above, or the optional arguments of the `\``includegraphics` command of the graphicx package. (The last is, in fact, what keyval was designed to do.)

However, we need more if we’re to to have package options in ‘key-value’ form. Packages like hyperref have enormously complicated package options which need key-value processing at `\``ProcessOptions` time: keyval can’t do that on its own.

Heiko Oberdiek’s kvoptions package comes to our help: it enables the programmer to declare class or package options that operate as key and value pairs. The package defines commands `\``DeclareBoolOption` for options whose value should be either true or false, and `\``DeclareStringOption` for all other options that have a value. Keys are declared using keyval and may remain available for use within the document, or may be ‘cancelled’ to avoid confusion. If you have loaded kvoptions, the LaTeX kernel’s `\``DeclareOption` becomes `\``DeclareVoidOption` (it’s an option with no value), and `\``DeclareOption*` becomes `\``DeclareDefaultOption`.

Heiko also provides kvsetkeys which is a more robust version of setkeys, with some of the rough edges made smoother.

Hendri Adriaens’ xkeyval offers more flexibility than the original keyval and is more robust than the original, too. Like kvoptions, the package also has mechanisms to allow class and package options in key-value form (macros `\``DeclareOptionX`, `\``ExecuteOptionsX` and `\``ProcessOptionsX`. Pstricks bundle packages use a xkeyval derivative called pst-xkey for their own key-value manipulation.

The (widely-respected) pgf graphics package has its own key-value package called pgfkeys. The documentation of the package (part of the huge pgf manual, in part 5, “utilities”) contains a useful comparison with other key-value systems; some notable differences are:

- key organisation: pgfkeys uses a tree structure, while keyval and xkeyval both associate keys with a family;
- pgfkeys supports multi-argument key code; and
- pgfkeys can support call-backs when an unknown key appears (these things are called handlers.

Keys are organized in a tree that is reminiscent of the Unix fille tree. A typical key might be, /tikz/coordinate system/x or just /x. When you specify keys you can provide the complete path of the key, but you usually just provide the name of the key (corresponding to the file name without any path) and the path is added automatically. So a `\``pgfkeys` command might be:

>     \pgfkeys{/my key=hello,/your keys/main key=something\strange,
>       key name without path=something else}

and for each key mentioned, the associated code will be executed. … and that code is also set up using `\``pgfkeys`:

>     \pgfkeys{/my key/.code=The value is '#1'.}

after which

>     \pgfkeys{/my key=hi!}

will produce just

> The value is ’hi!’.

The manual goes on, showing how to define a key with two arguments, how to provide default value for a key, and how to define aliases for particular key sequences (which are called “styles”). All in all, it seems a well thought-out system, offering a lot of flexibility that isn’t available with the other keys packages. However, there seems to be no mechanism for using pgfkeys keys as part of the options of another package, in the way that kvoptions does.

Another key-value system that’s part of larger set of macros is scrbase, which uses the facilities of keyval to build a larger set of facilities, originally for use within the KOMA-script bundle. For English-speaking authors, there are difficulties from the German-only documentation; however, from a partial translation available to the author of this answer, a summary is possible. The package may build on the facilities either of kyeval or of xkeyval, and builds its functionality on the structure of the ‘key family’. The user may define family ‘members’ and keys are defined relative to the members. (For example, the package scrbase is part of the KOMA-script bundle; so its keys are all members of the scrbase.sty family within the KOMA family. The function `\``FamilyProcessOptions` allows the programmer to decode the options of the package in terms of the package’s key family. Note that there is no special provision made for “traditional” package options, as in the kvoptions package.

This brief summary was guided by input from two sources: a draft article for TUGboat by Joseph Wright, and the partial translation of the documentation of package scrbase prepared by Philipp Stephani. At Joseph’s suggestion, discussion of his keys3 package, which is designed for use with the experimental LaTeX3, has been omitted.

`keyval.sty`
  Distributed as part of macros/latex/required/graphics (or browse the directory)
`kvoptions.sty`
  Distributed as part of macros/latex/contrib/oberdiek (or browse the directory)
`kvsetkeys.sty`
  Distributed as part of macros/latex/contrib/oberdiek (or browse the directory)
`pgfkeys.sty`
  Distributed as part of graphics/pgf (or browse the directory)
`scrbase.sty`
  Distributed as part of macros/latex/contrib/koma-script (or browse the directory)
`xkeyval.sty`
  macros/latex/contrib/xkeyval (or browse the directory)

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