  In type-and-coeffect systems, contexts are enriched by coeffects modeling how
they are actually used, typically through annotations on single variables.
Coeffects are computed bottom-up, combining, for each term, the coeffects of
its subterms, through a fixed set of algebraic operators. We show that this
principled approach can be adopted to track sharing in the imperative paradigm,
that is, links among variables possibly introduced by the execution. This
provides a significant example of non-structural coeffects, which cannot be
computed by-variable, since the way a given variable is used can affect the
coeffects of other variables. To illustrate the effectiveness of the approach,
we enhance the type system tracking sharing to model a sophisticated set of
features related to uniqueness and immutability. Thanks to the coeffect-based
approach, we can express such features in a simple way and prove related
properties with standard techniques.
