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A general description is here.
For now only active when environment variable ‘_R_CLASS_MATRIX_ARRAY_’ is set to something true (i.e., one of ‘TRUE’, ‘True’, ‘true’, or ‘T’), but planned to be the new unconditional behavior when R 4.0.0 is released:
‘matrix’ objects now also inherit from class ‘"array"’, namely, e.g., ‘class(diag(1))’ is ‘c("matrix", "array")’ which invalidates code assuming that ‘length(class(obj)) == 1’, an incorrect assumption that is less frequently fulfilled now.
S3 methods for ‘"array"’, i.e., ‘<someFun>.array()’, are now also dispatched for ‘matrix’ objects.
Where long vectors are available (i.e., almost everywhere), ‘writeBin()’ now allows long vectors, i.e., of length >= 2^31.