
Entities

The entities used in modular structural operational semantics are classified according to how they flow during program execution:

Contextual
A contextual entity remains fixed for successive steps in the computation of a term, but can be different for the computations of sub-terms.
Mutable
Changes to a mutable entity are threaded sequentially through the computation of a term and its sub-terms.
Input
An input entity is a sequence of values, and the sequences input by successive steps are independent.
Output
An output entity is a sequence of values, and the sequences output by successive steps are independent.
Control
A control entity is a value that can optionally be signalled by a step. The corresponding step of an enclosing term may inspect the value, and signal the same value, signal a different value, or not signal.

There are no funcons for reifying entities as values, nor for reflecting values as entities! This is because the way that particular entities flow during funcon execution is inherent and characteristic; reification and reflection would allow the flow to be circumvented.

The following entities are used in the definitions of funcons for computations. The execution of a term is unaffected by entities that are not involved in the funcons used in it.

• The given-value is a contextual entity that supports references to a previously computed value. Funcon computation arguments that refer to an unknown given value correspond to ‘lambdas’.

• The environment is a contextual entity that represents the currently visible bindings. Some funcons affect the environment during the execution of computation arguments, but the entity implicitly reverts back to the previous value afterwards (‘push’ and ‘pop’ steps are not required).

• The used-atom-set is a mutable entity supporting the generation of fresh atomic values.

• The store is a mutable entity representing the currently allocated variables and their current values. It supports uninitialised variables, and recycling of variables. Single-assignment variables are used to represent cyclic bindings.

• The standard-in is an input entity representing a sequences of interactive input values. Inputting a value removes it from the sequence.

• The standard-out is an output entity representing sequences of interactive output values. Outputting a value adds it to the sequence.

• The abrupted` entity is a control entity which signals abrupt termination of a computation.