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3.4 Storage model

Variables and objects such as pairs, strings, vectors, and bytevectors implicitly denote locations or sequences of locations. A string, for example, denotes as many locations as there are characters in the string. A new value can be stored into one of these locations using the string-set! procedure, but the string continues to denote the same locations as before.

An object fetched from a location, by a variable reference or by a procedure such as car, vector-ref, or string-ref, is equivalent in the sense of eqv? (Equivalence predicates) to the object last stored in the location before the fetch.

Every location is marked to show whether it is in use. No variable or object ever refers to a location that is not in use.

Whenever this report speaks of storage being newly allocated for a variable or object, what is meant is that an appropriate number of locations are chosen from the set of locations that are not in use, and the chosen locations are marked to indicate that they are now in use before the variable or object is made to denote them. Notwithstanding this, it is understood that the empty list cannot be newly allocated, because it is a unique object. It is also understood that empty strings, empty vectors, and empty bytevectors, which contain no locations, may or may not be newly allocated.

Every object that denotes locations is either mutable or immutable. Literal constants, the strings returned by symbol->string, and possibly the environment returned by scheme-report-environment are immutable objects. All objects created by the other procedures listed in this report are mutable. It is an error to attempt to store a new value into a location that is denoted by an immutable object.

These locations are to be understood as conceptual, not physical. Hence, they do not necessarily correspond to memory addresses, and even if they do, the memory address might not be constant.

Rationale:

In many systems it is desirable for constants (i.e. the values of literal expressions) to reside in read-only memory. Making it an error to alter constants permits this implementation strategy, while not requiring other systems to distinguish between mutable and immutable objects.