# [NTG-context] Defining command with optional and mandatory arguments

Christoph Reller christoph.reller at gmail.com
Fri May 25 07:08:06 CEST 2018

On Thu, 24 May 2018 14:50:36 +0200, Hans Hagen <j.hagen at xs4all.nl> wrote:
> On 5/24/2018 11:21 AM, Christoph Reller wrote:
>> On Wed, 23 May 2018 16:01:05 +0200, Hans Hagen <j.hagen at xs4all.nl> wrote:
>>> On 5/23/2018 3:39 PM, Christoph Reller wrote:
>>>>
>>>> What is the right way to define a command with both mandatory and
>>>> optional arguments, e.g:
>>>>
>>>> \MyCommand[optional][mandatory]
>>>>
>>>> Consider the following MWE:
>>>>
>>>> \unexpanded\def\MyCommand[#1]{
>>>>     \dosingleempty{\doMyCommand[#1]}}
>>>> \def\doMyCommand[#1][#2]{
>>>>     \doifsomething{#1}{number 1: #1\par}
>>>>     \doifsomething{#2}{number 2: #2}\blank[big]}
>>>> \starttext
>>>> \MyCommand[A][B]
>>>> \MyCommand[A]
>>>> \stoptext
>>>>
>>>> In last year's versions of ConTeXt the output was
>>>>
>>>> number 1: A
>>>> number 2: B
>>>> number 1: A
>>>>
>>>>
>>>> number 1: A
>>>> number 2: B
>>>> number 2: A
>>>>
>>>> Is this behavior intended? How can I make a definition whose behavior
>>>> does not change in new versions of ConTeXt?
>>> i'm not sure wht happens at your end but this is the best way:
>>>
>>> \unexpanded\def\MyCommand
>>>     {\dodoubleempty\doMyCommand}
>>>
>>> \def\doMyCommand[#1][#2]%
>>>     {\iffirstargument
>>>        number 1: #1%
>>>        \par
>>>      \fi
>>>      \ifsecondargument
>>>        number 2: #2%
>>>      \fi
>>>      \blank[big]}
>>>
>>> \starttext
>>>       \MyCommand[A][B]
>>>       \MyCommand[A]
>>> \stoptext
>>
>> Thank you Hans for this information. My question is rather about error
>> handling. I want:
>>
>> \MyCommand[A][B] % <- succeeds with #1->A, #2->B
>> \MyCommand[A] % <- succeeds with #1->A
>> \MyCommand % <- fails with "! Use of \MyCommand doesn't match its definition"
>>
>> I just wanted to ask whether there is a standard way to achieve this
>> with \do<whatever>empty. If not, then this is also okay.

Thank you for your hint, Hans. \dodoubleempty renders both arguments
optional and hence the command can be used without any argument, which
is not what I intended. Of course I can still test in the command's
definition whether at least one argument is given and generate an
error myself.

In i-context.pdf, many arguments are documented as being optional and
I assume that all the others are mandatory. A quick test has, however,
shown that when omitting mandatory arguments, either no error is
generated (e.g., nothing is setup or defined) or an obscure error
emerges other than "! Use of \<command> doesn't match its definition".

So I take it as a design decision that reporting missing mandatory
arguments as errors is not part of the interface implementation, and
that is a perfectly acceptable decision for me. It is just good to
know.

Thank you all for your highly valued feedback!

Cheers,

Christoph