An Imperative Language with Read/Write Type Modes

Reading and writing of data is fundamental in computing, as is its control. However control of reading and writing has traditionally only been available at the level of le systems, and not programming language data structures. In this paper a simple imperative language is described which uses type modes to control reading and writing of data. A type may be labelled read-write or read-only; a read-only type is guaranteed by the type system not to be written. Furthermore a read-write type may be treated read-only in a sub-context. To achieve this implicit aliasing is prevented and the program heap is partitioned into collections. Collections form a unit of read-write control of heap allocated data, by isolating diierent heap regions. Collections were originally introduced in the Euclid and Turing programming languages for aliasing control; however this was rather restrictive and not strictly enforced. Controlling aliasing is beneecial in its own right since aliasing is a common source of programming errors.