A Syntactic Model of Mutation and Aliasing

Traditionally, semantic models of imperative languages use an auxiliary structure which mimics memory. In this way, ownership and other encapsulation properties need to be reconstructed from the graph structure of such global memory. We present an alternative "syntactic" model where memory is encoded as part of the program rather than as a separate resource. This means that execution can be modelled by just rewriting source code terms, as in semantic models for functional programs. Formally, this is achieved by the block construct, introducing local variable declarations, which play the role of memory when their initializing expressions have been evaluated. In this way, we obtain a language semantics which directly represents at the syntactic level constraints on aliasing, allowing simpler reasoning about related properties. To illustrate this advantage, we consider the issue, widely studied in the literature, of characterizing an isolated portion of memory, which cannot be reached through external references. In the syntactic model, closed block values, called "capsules", provide a simple representation of isolated portions of memory, and capsules can be safely moved to another location in the memory, without introducing sharing, by means of "affine' variables. We prove that the syntactic model can be encoded in the conventional one, hence efficiently implemented.Comment: In Proceedings DCM 2018 and ITRS 2018 , arXiv:1904.0956

Aliasing control in an imperative pure calculus

We present an imperative object calculus where types are annotated with two modifiers for aliasing control. The lent modifier prevents objects to be aliased, whereas the capsule modifier characterizes expressions that will reduce to isolated portions of store. There are two key novelties w.r.t. similar proposals. First, the expressivity of the type system is greatly enhanced by promotion and swapping rules. The former recognizes as capsule an expression which only uses external references as lent. The latter allows a lent reference to be freely aliased, if all the other references are regarded as lent. Second, execution is modeled in a pure setting, where it is simpler to understand alias control. That is, properties of modifiers can be directly expressed on source terms, rather than as invariants on an auxiliary structure which mimics physical memory. Formally, this is achieved by the block construct, introducing local variable declarations, which play the role of store when evaluated