[XeTeX] Checking for existence of font feature

[Michael Lynch]

On 18/08/2010 06:10, Will Robertson wrote:
> On 2010-08-12 16:16:54 +0930, Khaled Hosny <khaledhosny at eglug.org> said:
>
>>> Ah, thank you. Are these features (relatively) new? Rather
>>> embarrassing not to have spotted them, but anyway…
>>
>> All were introduced in version 2, I think.
>
> Yep, very new.
>
>>> I've got a problem using these commands. Using XeTeX v0.9995.1
>>> texlive svn 15079 on MiKTeX 2.8, fontspec 2010/08/01, none of these
>>> “lower-level” fontspec features I've tried work.
>>>
>>> I've copied the appropriate example from the manual:
>>>
>>> \fontspec_if_feature:nTF {smcp} {True} {False}.
>
>> Since those command use expl3 syntax, you will need \ExplSyntaxOn before
>> using them.
>
> Right; something like this:
>
> \ExplSyntaxOn
> % note that spaces are completely ignored in here!
> \newcommand \testcaps [2] {
>  \fontspec_if_feature:nTF {smcp} {#1}{#2}
> }
> \ExplSyntaxOff
> ...
> \fontspec{Times New Roman}
> \testcaps{Caps}{No Caps}
>
> Hope this helps,
> Will
>
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Hi Will,

Thanks for that – does just what's needed now. I tried something similar 
myself but it didn't quite work.

I put the \ExplSyntaxOn \~Off inside the definition of the command 
originally, which doesn't work, but when it's outside like you've put it 
does. I always thought that when commands were used in documents, they 
were effectively expanded in place (in which case the \ExplSyntax bits 
wouldn't appear), but obviously it's not that simple. Is there a simple 
explanation for this behaviour?

Thanks again,

Mike