Pedigree Types

Pedigree Types are an intuitive ownership type system requiring minimal programmer annotations. Reusing the vocabulary of human genealogy, Pedigree Types programmers can qualify any object reference with a pedigree – a child, sibling, parent, grandparent, etc – to indicate what relationship the object being referred to has with the referant on the standard ownership tree, following the owners-as-dominators convention. Such a qualifier serves as a heap shape constraint that must hold at run time and is enforced statically. Pedigree child captures the intention of encapsulation, i.e. ownership: the modified object reference is ensured not to escape the boundary of its parent. Among existing ownership type systems, Pedigree Types are closest to Universe Types. The former can be viewed as extending the latter with a more general form of pedigree modifiers, so that the relationship between any pair of objects on the aforementioned ownership tree can be named and – more importantly – inferred. We use a constraint-based type system which is proved sound via subject reduction. Other technical originalities include a polymorphic treatment of pedigrees not explicitly specified by programmers, and use of linear diophantine equations in type constraints to enforce the hierarchy.