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A general description is here.
The object header layout has been changed to support merging the ‘ALTREP’ branch. This requires re-installing packages that use compiled code.
‘deparse()’ (and ‘dump()’) are more useful for S4 objects; ‘dput()’ now using the same internal C code (instead of its previous imperfect workaround R code). More user visibly, ‘dput()’, etc now print the ‘names()’ information only once, using the more readable ‘(tag = value)’ syntax. These functions gain a new (partly experimental) control option _via_ ‘.deparseOpts()’, ‘"nice_names"’, which when set (e.g. as part of ‘"all"’) also use the ‘(tag = value)’ syntax for atomic vectors.
‘m:n’ now also deparses nicely when m > n.
Attributes on symbols are now detected and prevented; attempt to add an attribute to a symbol results in an error.