This blog is updated daily.
A general description is here.
Interrupts can be suspended while evaluating an expression using ‘suspendInterrupts’. Subexpression can be evaluated with interrupts enabled using ‘allowInterrupts’. These functions can be used to make sure cleanup handlers cannon be interrupted.
R 3.5.0 includes a framework that allows packages to provide alternate representations of basic R objects (‘ALTREP’). The framework is still experimental and may undergo changes in future R releases as more experience is gained. For now, documentation is provided in <URL: https://svn.r-project.org/R/branches/ALTREP/ALTREP.html>.