Covariance and contravariance: conflict without a cause
In this chapter we expand the discussion started in Section 4.6.4. We argue that covariance and contravariance appropriately characterize two distinct and independent mechanisms. The so-called contravariance rule correctly captures substitutivity, or the subtyping relation (that establishes which sets of codes can replace in every context another given set). Instead, a covariant relation characterizes the specialization of code (i.e., the definition of new code that replaces the old one in some particular cases).
KeywordsCovariance Assure Dispatch
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