= Variance-position rule
To check a parameter's variance, start in a positive position, reverse the sign when entering a function or method input, and preserve it through covariant outputs and covariant containers. Contravariant containers reverse the sign; invariant positions prohibit variation. Thus an immutable list of pairs exposes both pair types covariantly; a relatives-enumeration method consumes its first type and produces its second; a two-argument predicate consumes both. This derives the safe variance of three common representations of a <binary relation> from the roles of their inputs and outputs.
Back to article page
