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Date:         Wed, 25 Mar 2015 05:15:08 +0000
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From: Brent Longborough <brent@LONGBOROUGH.ORG>
Subject: Re: [l3keys] Suggestion: Add a key property for specifying key that /has/ to be used
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Status: R

On 2014-09-29 10:37, Denis Bitouzé wrote:
> Hi,
> 
> among others, `.value_forbidden:` and `.value_required:` key properties
> from `l3keys` package are useful: they let us specify that a given key,
> when used, either cannot or must receive a value.
> 
> What could be useful as well is a key property, say `.required:`, that would
> specify that the corresponding key /has/ to be used. This could be helpful for
> instance in the case of a document command `\MyModuleSetup` for setting up
> a module `mymodule`:
> 
>     \DeclareDocumentCommand \MyModuleSetup { m }
>       { \keys_set:nn { mymodule } { #1 } }
> 
> where some module property /has/ to be specified.
> 
> I know this can be achieved with something like test of existence of
> some tokenlist and message emitted in case of nonexistence but a high
> level key property for this could be nice.
> 
> May I ask what is your opinion about that?
> 
> Thanks in anticipation.
> 

Just a couple of thoughts, humbly offered with some uncertainty:

1. A compulsory key seems to me, on the face of it, to imply a
less-than-friendly user experience.

2. A simple solution might be to provide a default (such as "nil"), and,
'later in the package', test the key for "nil" and complain if it has
not been overridden.

Brent Longborough