Data model

Values and objects

Values are a primitive building block of Alore programs. A value is a reference to an object, i.e. it names a single object. A value always refers to an object — there is no notion of empty values. Alore programs can access objects only through values: an object is only visible to the program if there is a value that refers to it. Any number of values may refer to the same object.

Each instance of a variable is represented by a single value. Objects typically contain values as well. A Pair object, for example, contains two values. Not all objects are composite objects. An integer object, for example, is not a composite object.

Objects and types

All data processed by an Alore program is stored in objects or referenced by objects. Each object has a specific type that is constant across the life-time of the object, and usually some state, that may be mutable or immutable (constant). The state of an object differentiates the object from other objects of the same type. All types are represented by objects as well.

The type of nil is the only exception: it does not have a type object associated with it. The nil object behaves otherwise like a std::Constant object and supports the same operations.

For example, the type of the object 15 is std::Int and its state is the value fifteen. This object, like all integers, is immutable. Therefore the object 15 cannot be modified to value 7 (or any other value).

In contrast, the type of the object [1, 2] is std::Array and its state is composed of its length (2) and two references to Int objects (1 and 2). Array objects are mutable: their length and the objects they contain can be changed, but only by using the methods provided by the Array type.

Types

Each type is represented by a class. Classes and types are sometimes used as synonyms, but note that a type may also refer to all subclasses of the related class.

Objects that have the class X as their type may be referred to in several more or less equivalent ways:

• instances of X
• X objects
• X instances
• objects of type X (this can also refer to objects of subtypes)

Class objects have several uses:

• A class specifies the structure of all instances of the class. For example, it specifies which methods and members are included in the public interface of the instances, and it also specifies their behavior. Additionally, it defines how the state of the instances is represented.
• Instances of a class can (usually) be constructed by calling the class object.
• The is operator can be used with the class object to test if a object is an instance of the class or any subclass of the class.

Classes can also be inherited from to form subtypes. A subclass inherits the behavior of its superclass, but it may add or override some functionality.

The following types are called primitive types: Int, Str, Float, Boolean, Type, Function, Constant, Tuple, Pair and Range. They are all defined in the std module, and they cannot be superclasses of other classes. All other classes can act as superclasses.

Global definitions

Global definitions include all global variables, global constants, function definitions and class definitions in every module of a program. Each global definition refers to a single value. Function definitions, class definitions and uninitialized global constant definitions are automatically initialized before the program is run. Other global definitions refer to the nil object when the program is started.

Local variables

Local variables are defined within functions or methods. New instances of local variables are created every time a function or a method is called, and they are garbage collected some time after they can no longer be referenced within the invocation that defined them or any anonymous function created during the invocation. As a special case, if a local variable is not referenced by any anonymous function, it will be destroyed, at the latest, when the function invocation that holds the variable is exited. Note that only the value is destroyed; the object referred to by the value may not be freed immediately, even if the local variable was the last reference to this object.

Each instance of a local variable is represented by a separate, independent value.

Member variables

Each class may define one or more explicit member variables (or constants). For each explicit member variable or constant, there is a corresponding value slot in every instance of the classes that contain the definition. A value slot behaves like a variable, but unlike local variables and global definitions, member value slots cannot be accessed directly, but are always accessed through accessor methods: getters and setters (see Class definitions for details).

Garbage collection

The root set of the object graph contains global definitions, local variables and any temporary values needed while evaluating statements. The live object set contains all objects reachable from the root set. The live object set contains

1. objects directly referenced by the root set
2. all objects referenced by other objects in the live set.

All objects not within the live object set are destroyed and freed automatically by the run-time system. It is undefined when this happens, but the run-time should, in general, try to free them as early as possible.

Other objects that can be shown not to affect the execution of a program can be freed automatically as well.