“Use-once” variables and linear objects: storage management, reflection and multi-threading

• Authors: Henry G. Baker
• Venue: ACM SIGPLAN Notices
• Year: 1995

Programming languages should have 'use-once' variables in addition to the usual 'multiple-use' variables. 'Use-once' variables are bound to linear (unshared, unaliased, or singly-referenced) objects. Linear objects are cheap to access and manage, because they require no synchronization or tracing garbage collection. Linear objects can elegantly and efficiently solve otherwise difficult problems of functional/mostly-functional systems---e.g., in-place updating and the efficient initialization of functional objects. Use-once variables are ideal for directly manipulating resources which are inherently linear such as freelists and 'engine ticks' in reflective languages.A 'use-once' variable must be dynamically referenced exactly once within its scope. Unreferenced use-once variables must be explicitly killed, and multiply-referenced use-once variables must be explicitly copied; this duplication and deletion is subject to the constraint that some linear datatypes do not support duplication and deletion methods. Use-once variables are bound only to linear objects, which may reference other linear or non-linear objects. Non-linear objects can reference other non-linear objects, but can reference a linear object only in a way that ensures mutual exclusion.Although implementations have long had implicit use-once variables and linear objects, most languages do not provide the programmer any help for their utilization. For example, use-once variables allow for the safe/controlled use of reified language implementation objects like single-use continuations.Linear objects and use-once variables map elegantly into dataflow models of concurrent computation, and the graphical representations of dataflow models make an appealing visual linear programming language.