Is there a way to add optional and/or named argument to command

Question

I'm writing a handbook for my students and I want to create a command which should draw a task definition, its type and possible features, like source and link to the solution on YouTube.

In my plan all of these definitions should work (of course I use pseudo syntax here on purpose, just to show you my intention):

\task{definition=$x=3$}
\task{definition=$x=3$, source='ABC'}
\task{definition=$x=3$, link='https://youtube.ru'}
\task{definition=$x=3$, source='ABC', link='https://youtube.ru'}

But I cannot find any tool similar to optional and named arguments.

Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. — Community, Aug 16 '23 at 00:32
Check How to create a command with key values? and A big list of every keyval package — Dai Bowen, Aug 16 '23 at 00:36
Welcome to TeX.SE. Ignore the Community Bot. Sometimes it doesn't know what it's talking about. — Teepeemm, Aug 16 '23 at 01:08
Welcome. // Using the keyvalue approach is certainly the most flexible. If you want to stick with the standard \newcommand, you may want to provide default values. See e.g. here: https://en.wikibooks.org/wiki/LaTeX/Macros#New_commands . // Hint: you may also find the column to the right helpful (Linked, Related) — MS-SPO, Aug 16 '23 at 06:49
@campa I wouldn't have deleted your answer! (voted to undelete) — David Carlisle, Aug 16 '23 at 09:18
Seconding @DavidCarlisle's undelete. Aside from anything else, keyval has backwards compatibility with older LaTeX in its favour. — cfr, Aug 16 '23 at 13:02
@campa My comment was not meant as criticism, but merely as a discussion about the best solutions in 2023, and the pros and cons of pre-expl3 packages such as keyval. I certainly didn’t intend for you to delete the answer. — Gaussler, Aug 16 '23 at 13:22
@Gaussler The advantage of pre-expl3 packages is that I understand them (Grumpy Old Man talking here ;-)) — campa, Aug 16 '23 at 15:44

score 10 · Accepted Answer · edited Aug 16 '23 at 19:26

Classically the LaTeX2e solution was the keyval package (as used by \includegraphics[height=2cm, scale=3]{...}) but recent LaTeX releases include a keyval parser built in, based on l3keys.

@Campa's example modified to use the newer system:

\documentclass{article}
\makeatletter
\DeclareKeys[task]{
definition .store = \task@definition,
source     .store = \task@source,
link       .store = \task@link
}
\NewDocumentCommand\task{m}{%
   \begingroup
   \SetKeys[task]{#1}%
   This is a task with definition \task@definition.%
   \ifx\task@source@empty\else\space You can find its source in \task@source.\fi
   \ifx\task@link@empty\else\space (See \task@link)\fi
   \endgroup
}
\makeatother
\begin{document}
\parindent0pt
\task{definition={$x=3$}}\par % you must "hide" the =
\task{definition={$x=3$}, source=ABC}\par
\task{definition={$x=3$}, link=https://youtube.ru}\par
\task{definition={$x=3$}, source=ABC, link=https://youtube.ru}
\task{definition={$x=3$}} % you must "hide" the =
\end{document}

score 4 · Answer 2 · answered Aug 16 '23 at 06:53

This might be a start using keyval

\documentclass{article}
\usepackage{keyval}
\makeatletter
\newcommand{\task@definition}{}
\newcommand{\task@source}{}
\newcommand{\task@link}{}
\define@key{task}{definition}{\def\task@definition{#1}}
\define@key{task}{source}{\def\task@source{#1}}
\define@key{task}{link}{\def\task@link{#1}}
\newcommand{\task}[1]{%
   \begingroup
   \setkeys{task}{#1}%
   This is a task with definition \task@definition.%
   \ifx\task@source@empty\else\space You can find its source in \task@source.\fi
   \ifx\task@link@empty\else\space (See \task@link)\fi
   \endgroup
}
\makeatother
\begin{document}
\parindent0pt
\task{definition={$x=3$}}\par % you must "hide" the =
\task{definition={$x=3$}, source=ABC}\par
\task{definition={$x=3$}, link=https://youtube.ru}\par
\task{definition={$x=3$}, source=ABC, link=https://youtube.ru}
\task{definition={$x=3$}} % you must "hide" the =
\end{document}

Not sure the keyval package is the right solution to recommend in 2023. — Gaussler, Aug 16 '23 at 08:41

Skillmon · Answer 3 · 2023-08-16T19:57:27.263

For a simple task, imho¹, nothing beats the simplicity of expkv-cs and \ekvcSplit (this only works for up to 9 keys, for more it's most likely better to use methods from expkv-def which work similar to \DeclareKeys -- or another key=value implementation like the one built into LaTeX).

_{¹I'm the author of expkv and family.}

To have an easy way to add formatting instructions and extra text to a key one can use a front facing key that decorates some internal key. So every time you say link = www.example.com this becomes link-internal={ (See www.example.com)}.

Also in expkv you don't have to hide an additional =, but you'd have to hide a ,.

This borrows the example created by @campa and modifies it to use expkv-cs:

\documentclass{article}
\usepackage{expkv-cs}
\ekvcSplit\task
  {
    % set up primary keys, all defaulting to an empty value
     definition = {}
    ,source-internal = {}
    ,link-internal = {}
  }
  {This is a task with definition #1.#2#3}
% set up decorators
\ekvcSecondaryKeys\task
  {
     meta source = {source-internal = { You can find its source in #1.}}
    ,meta link   = {link-internal   = { (See #1)}}
  }
\begin{document}
\parindent0pt
\task{definition=$x=3$}\par
\task{definition=$x=3$, source=ABC}\par
\task{definition=$x=3$, link=https://youtube.ru}\par
\task{definition=$x=3$, source=ABC, link=https://youtube.ru}
\task{definition=$x=3$}
\end{document}

Alternatively to the \ekvcSplit version, you could also use the \ekvcHash version, in which you can access the key-values by name (the entire list will be forwarded inside of #1, and \ekvcValue can be used to access items from that list -- this works for arbitrary many keys).

\documentclass{article}
\usepackage{expkv-cs}
\ekvcHash\task
  {
    % set up primary keys, all defaulting to an empty value
     definition = {}
    ,source-internal = {}
    ,link-internal = {}
  }
  {%
    This is a task with definition \ekvcValue{definition}{#1}.%
    \ekvcValue{source-internal}{#1}%
    \ekvcValue{link-internal}{#1}%
  }
% set up decorators
\ekvcSecondaryKeys\task
  {
     meta source = {source-internal = { You can find its source in #1.}}
    ,meta link   = {link-internal   = { (See #1)}}
  }
\begin{document}
\parindent0pt
\task{definition=$x=3$}\par
\task{definition=$x=3$, source=ABC}\par
\task{definition=$x=3$, link=https://youtube.ru}\par
\task{definition=$x=3$, source=ABC, link=https://youtube.ru}
\task{definition=$x=3$}
\end{document}

If you want an interface closer to the one provided by \DeclareKeys you could use expkv-def. In addition to a store-handler it also provides dataT. The instruction dataT link = \taskLink will define \taskLink such that by default it'll gobble the next token or brace-group, but if the key was used it'll put the value as {<value>} behind the next token or brace-group (and would strip the braces from that following brace-group).

\documentclass{article}
\usepackage{expkv-def}
\ekvdefinekeys{task}
  {
     store definition = \taskDefinition
    ,dataT source     = \taskSource
    ,dataT link       = \taskLink
  }
\newcommand\taskSourceFormatter[1]{ You can find its source in #1.}
\newcommand\taskLinkFormatter[1]{ (See #1)}
\newcommand\task[1]
  {%
    \begingroup
      \ekvset{task}{#1}%
      This is a task with definition \taskDefinition.%
      \taskSource\taskSourceFormatter
      \taskLink\taskLinkFormatter
    \endgroup
  }
\begin{document}
\parindent0pt
\task{definition=$x=3$}\par
\task{definition=$x=3$, source=ABC}\par
\task{definition=$x=3$, link=https://youtube.ru}\par
\task{definition=$x=3$, source=ABC, link=https://youtube.ru}
\task{definition=$x=3$}
\end{document}

And if you really want to, you could use basically the same syntax as for keyval by using the basic expkv without any of the extension packages:

\documentclass{article}
\usepackage{expkv}
\newcommand\taskDefinition{}
\newcommand\taskSource{}
\newcommand*\taskLink{}
\ekvdef{task}{definition}{\def\taskDefinition{#1}}
\ekvdef{task}{source}{\def\taskSource{#1}}
\ekvdef{task}{link}{\def\taskLink{#1}}
\newcommand\task[1]
  {%
    \begingroup
      \ekvset{task}{#1}%
      This is a task with definition \taskDefinition.%
      \ifx\taskSource\empty\else\space You can find its source in \taskSource.\fi
      \ifx\taskLink\empty\else\space (See \taskLink)\fi
    \endgroup
  }
\begin{document}
\parindent0pt
\task{definition=$x=3$}\par
\task{definition=$x=3$, source=ABC}\par
\task{definition=$x=3$, link=https://youtube.ru}\par
\task{definition=$x=3$, source=ABC, link=https://youtube.ru}
\task{definition=$x=3$}
\end{document}

Result of all code blocks:

Is there a way to add optional and/or named argument to command

3 Answers3