This is a product, not a sum. It's just a pair of Reader and Writer. Sum types would also give you the ability to say "reader or writer" and use pattern matchin

This is a product, not a sum. It's just a pair of Reader and Writer. Sum types would also give you the ability to say "reader or writer" and use pattern matching to find out which one you were given. C-like languages traditionally give only the 'product' half of types (C also gives union but they're super low level, and Go just eschewed them). A _modern_ language would give both products and sums.