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Commands with optional arguments (view source)
Revision as of 20:38, 14 August 2021
, 20:38, 14 August 2021Add pitfall with nested macro calls
< [[Inside ConTeXt]] | [[Commands with KeyVal arguments]] >
In ConTeXt, the optional argument processing is handled as a two-step process. First, we write the command for the end-user as a wrapper command, which calls {{cmd|dosingleempty}}, {{cmd|dodoubleempty}} , {{cmd|dotripleempty}}, ... (from {{src|syst-aux.mkiv}} or {{src|syst-gen.mkii}}) to handle the arguments properly -- including the optional ones -- and then calls a "private" command that contains the internals of the macro. Note that this function call does not explicitly refer to the arguments at all.
For a command with two optional arguments, we use:
<texcode>
\def\MyCommand{\dodoubleempty\doMyCommand}
=== Examples ===
To define <ttcode>\mycommand[.#1.][.2.]{.3.}</tt>, i.e., with curly braces around a non-optional third argument, you just define <texcode>\def\mycommand{\dodoubleempty\doMycommand}\def\doMycommand[#1][#2]#3{whatever}</texcode> To define <code>\mycommand[with one optional]{text}</code>argument and one mandatory argument, do the following<texcodecontext source="yes">
\def\mynewcommand{\dosingleempty\doMyNewCommand}
\def\doMyNewCommand[#1]#2{%
\iffirstargument
There is an optional parameter: {\bf #1}\par%
\else
No optional parameter\par%
\fi
This is the mandatory text: {\em #2}%
}
\starttext
\mynewcommand[opt]{Hello People}
\blank
\mynewcommand{Hello People}
\stoptext
</texcodecontext>
To define <code>\def\mynewcommand{\dosingleempty\doMyNewCommand}\def\doMyNewCommandmycommand[#1][#2]{% \iffirstargument There is an optional parameter: {\bf #13}\par \else No </code> with two optional parameter\par \fi This is the arguments and one mandatory text: {\em #2}}argument, do
<texcode>\starttextdef\mynewcommand[opt]mycommand{Hello People\dodoubleempty\doMycommand}\blankdef\mynewcommanddoMycommand[#1][#2]#3{Hello Peoplewhatever}\stoptext</contexttexcode>
=== Pitfalls ===
<context source="yes" text="produces">
\startalignment[center]
\iffirstargument
There is an optional parameter: {\bf #1}\par%
\else
No optional parameter\par%
\fi
This is the mandatory text: {\em #2}%
\stopalignment
}
Use <code>\doifsomethingelse</code> instead:
<context source="yes" text="This time this time is correct:">
\def\mynewcommand{\dosingleempty\doMyNewCommand}
\def\doMyNewCommand[#1]#2{%
\startalignment[center]%
\doifsomethingelse{#1}
{There is an optional parameter: {\bf #1}\par}
{No optional parameter\par}
This is the mandatory text: {\em #2}
\stopalignment%
}
\starttext
</context>
==== Calling the macro from within command arguments (e.g. float title) ====
-- I have no deeper insight yet on the why and neither on which other cases this might apply to, but I hope it may be useful to someone running into errors.
If you wanted to call your macro from within an "argument" such as you would have to with titles or captions in floats:
<texcode>
\startplacefigure[title={We look at \mynewcommand{people} in floats}]
(content)
\stopplacefigure
</texcode>
The above will create a `"\doMyNewCommand doesn't match its definition"` error.
This can be solved by preceding the definition that takes the optional argument with `\unexpanded`:
<texcode>
\unexpanded\def\doMyNewCommand[#1]#2{% ... (etc.)
</texcode>
=== Comparison with LaTeX ===
On a final note, for comparative purposes: in LaTeX, a new command with an optional argument is defined with <code>\newcommand</code>.
http://archive.contextgarden.net/message/20101215.225603.cc903e62.en.html
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