Commands with KeyVal arguments

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< Inside ConTeXt

(a post on the mailing list by Taco Hoekwater from [2004-06-28]:)

The 'key' to the keyval functionality in ConTeXt are two macros called \getparameters and \processaction.

Here is a 'quickstart', assuming you want to define \MyZigzag. The code example is interrupted with explantory running text.

\unprotect % enable exclamations in macro names

\def\MyZigzag#1[#2]{% % This brace is closed below, after the \expandafter block
  % the #1 makes sure we allow a space before the bracket


Now you have a set of new macros that all start with ZZ. At least there are \ZZDir,\ZZLinewidth, \ZZColor and \ZZWidth (these have default values) but possibly others as well, depending on user input. In the next lines you make use of these variables, for example as follows:


If you want some of the variables to accept keyword values, then you also need to use \processaction on the ZZ variable in question to map the keywords onto actual values.

Say you want "Dir" to be mandatory and that it accepts 4 directional keywords, as well as a direct angle specification. I've used all mixed case keywords, because otherwise you might run into conflicts with the multilingual interface:

	[Down       =>\def\Dir{270},
	 Left       =>\def\Dir{180},
   	 Up         =>\def\Dir{90},
 	 Right      =>\def\Dir{0},
	 \s!default =>\errorDir, 
	 \s!unknown =>\checkDir{\ZZDir}]
} % this brace belongs to \def!

\s!default may be triggered because \ZZDir's expansion is empty unless the user supplied something.

The first argument to \processaction has to be expanded, so you need the \expandafters.

for completeness, here is an example definition of \checkDir and \errorDir:

   \def\Dir{0}% error recovery
   \message{Please supply "Dir" argument}%

  \doifnumberelse {#1}
                  {\message{Invalid "Dir" argument! (#1)}}

\protect % end of definitions