Computational models begin with a set of values. The values can be separated into two groups, primitive and composite. The primitive values (or types) are usually numbers, boolean values, and characters. The composite values (or types) are usually arrays, records, and recursively defined values. Strings may occur as either primitive or composite values. Lists, stacks, trees, and queues are
examples of recursively defined values. Associated with the primitive values are the usual operations (e.g., arithmetic operations for the numbers). Associated with each composite type are operations to construct the values of that type and operations to access component elements of the type.