Modernized Algol, or MA, is an imperative, block-structured programming language based on the classic language Algol. MA extends PCF with a new syntactic sort of commands that act on assignables by retrieving and altering their contents. Assignables are introduced by declaring them for use within a specified scope; this is the essence of block structure. Commands are combined by sequencing and are iterated using recursion.

MA maintains a careful separation between pure expressions, whose meaning does not depend on any assignables, and impure commands, whose meaning is given in terms of assignables. The segregation of pure from impure ensures that the evaluation order for expressions is not constrained by the presence of assignables in the language, so that they can be manipulated just as in PCF. Commands, on the other hand, have a constrained execution order, because the execution of one may affect the meaning of another.

A distinctive feature of MA is that it adheres to the stack discipline, which means that assignables are allocated on entry to the scope of their declaration, and deallocated on exit, using a conventional stack discipline. Stack allocation avoids the need for more complex forms of storage management, at the cost of reducing the expressive power of the language.

Basic Commands

The syntax of the language MA of modernized Algol distinguishes pure expressions from impure commands. The expressions include those of PCF (as described in Chapter 19), augmented with one construct, and the commands are those of a simple imperative programming language based on assignment. The language maintains a sharp distinction between variables and assignables. Variables are introduced by λ-abstraction and are given meaning by substitution. Assignables are introduced by a declaration and are given meaning by assignment and retrieval of their contents, which is, for the time being, restricted to natural numbers. Expressions evaluate to values, and have no effect on assignables. Commands are executed for their effect on assignables, and return a value. Composition of commands not only sequences their execution order, but also passes the value returned by the first to the second before it is executed. The returned value of a command is, for the time being, restricted to the natural numbers.