ConTeXt: Letterspacing in style alternative

Question

I want to create a new style alternative, which would allow me to change the text and font (to uppercase bold in this example) while also increasing the letterspacing. However, trying to do this leads to a crash:

\setuppapersize[S3]

\definecharacterkerning [mykerning] [factor=0.16]
\definealternativestyle [mystyle]
  [
    \WORDS\bfx\setcharacterkerning[mykerning]
  ]


\starttext
  {\WORDS\bfx\setcharacterkerning[mykerning] It works here.}
  
  {\mystyle It crashes here.}
\stoptext

tex error       > tex error on line 13 in file /home/…/test.tex: ! Argument of \setcharacterkerning has an extra }

Why does this crash occur? (\WORDS works, why not \setcharacterkerning?) Is there a way to fix it? I want to be able to use it as an argument for style=.

Answer 1

_{Sorry for the long answer. The bracket issue was discussed elsewhere and I think it's explained in a manual, but I don't remember which, so I'll include a brief note here.}

Your problem is: ConTeXt is closing a pair of brackets [] at the first occurrence of the closing bracket. Thus, something like

\setupsomething[key=\value[]]

will read [key=\value[] and crash. For such situations ConTeXt provides \setups to include all your complicated macros:

\startsetups mystyle 
\WORDS\bfx\setcharacterkerning[mykerning]
\stopsetups

Both \setups[mystyle] and \setup{mystyle} work, so we'll choose the latter in order to avoid the bracket issue above.

\setuppapersize[S3]
\definecharacterkerning [mykerning] [factor=0.16]
\startsetups mystyle
\WORDS\bfx\setcharacterkerning[mykerning]
\stopsetups
\definealternativestyle[mystyle][\setups{mystyle}]
\setuphead[title][style=\mystyle]
%Test example
\starttext
\starttitle[title=Hello]
\input jojomayer
\stoptitle
\stoptext