Comprehending annotations on object-oriented programs using fractional permissions

Fractional permissions are a general system for managing access to mutable state. We show how fractional permis-sions can give semantics to a regimen of annotations includ-ing“unique,”“non-null,”“read-only,”ownership, and method effects. The unification supports new annotations: “unique-write ” and “from”. We also develop a model of object in-variants in the presence of inheritance using “nesting, ” an extension of “adoption.” 1