Thanks @cderv
The yaml package you mention is https://github.com/vubiostat/r-yaml/ right ?
I lose myself a bit between "r stuff", !r stuff, !expr stuff I think it would benefit better documentation.
It seems !expr stuff is correctly parsed AND evaluated by rmarkdown::yaml_front_matter() (I just tried) as well as yaml::yaml.load, was that what you were saying ?
I think I will just use this going forward
Many thanks for your help