The syntax for type definitions are similar for values, classes, and actors. Defining a value type is done using the value keyword as follows:

value <name> [: <options>] {

The syntax above defines a value named <name>. <members> is a list of definitions of the members of the value. Refer to definitions for allowed definitions in types. Options will be discussed further below.

A class is defined using the class keyword:

class <name> [: <options>] {

Similarly to the definition of a value, <name> indicates the name of the class, and <members> is a sequence of members of the class.

Actors are also defined using the class keyword as above. A class is an actor if either the on <thread> option is present, or if it inherits from an actor.


Types may contain definitions of functions (that become member functions), constructors, and member variables. The two latter may only appear inside types, and are described below.

Similarly to top-level definitions, it is possible to specify visibility of members either by prepending the visibility modifier to each member, or by specifying them for many members at once (e.g. private:).

Member Variables

A member variable is defined as follows:

<type> <name> [= <initializer>];

Here, <type> is the name of the type of the variable, and <name> is the name of the variable. An initializer may also be specified. If specified, the initializer is used to initialize the variable if it is not explicitly initialized to some other value in the init-block in a constructor. If no initializer is specified, and no value is present in the init block, the default constructor is used if it exists.


Basic Storm attempts to ensure that it is not possible to access uninitialized variables, and thereby accidentally accessing non-nullable types that have the value null. Because of this, constructors are treated specially in Basic Storm. They are defined using a special syntax as follows:

init(<parameters>) {}

Similarly to function declarations, <parameters> is a list of comma-separated parameters to the constructor. If the constructor is inside an actor that requires a thread to its constructor, the first parameter must be of the type Thread.

What makes constructors special is that they need to contain an init block. The init block represents actually creating the instance by initializing all member variables. An initialization block has the following form in general:

init[(<parameters>)] {
    <name> = <initializer>;

The first part, the optional parenthesis with parameters, is used to pass any required parameters to the constructor of the parent class. If no parameters are required, or if parameters have already been passed earlier, this part can be omitted.

The next part contains a list of all variables that should be initialized. These either have the form <name> = <initializer>;, which creates the variable by copying the result from an expression, or the form <name>(<ctor-params>);, which creates the variable by calling the appropriate constructor. Any variables that are not mentioned in the list are initialized using the initializer specified where the member variable was declared. If no initializer was specified, the default constructor is used if it exists, otherwise an error is raised.

It is worth noting that the order in which variables are initialized does not necessarily match the order in which the variables are listed in the init block. Rather, the system initializes them in the order they appear in memory.

Since the init block is special since it represents the creation of the object itself. As such, the this variable is only accessible after the init block has been executed. This is best illustrated with an example:

class MyClass {
    Int a;
    Int b;

    init(Int b) {
        // Here, 'this' does not exist. We can therefore not access any members.
        a = 10; // Invalid, no instance yet.

        init {
            a = 1;
            // This works as expected. The left hand side has to refer to a
            // member variable. The right hand side can not refer to a member
            // variable, as we don't yet have an instance. It must therefore
            // refer to the parameter.
            b = b;

        // Now we can access the members:
        a = 10; // Valid, we have an instance.

In certain situations it might be useful to create the object in two stages. First, call the constructor of a super class, and then initialize the parts of the derived class. This can be done by explicitly calling super(<parameters>) before the init block. If this is done, no parameters may be provided to init, as the parent class' constructor has already been called. This can be illustrated by the following class, that inherits from MyClass above:

class MyDerivedClass extends MyClass {
    Int c;
    Int d;

    init(Int p, Int q) {
        // Here, 'this' does not exist.
        a = 10; // Invalid, no instance yet.
        c = 10; // Invalid, no instance yet.

        // Here, 'this' is of the type MyClass, since the super class is created.
        a = 10; // Valid, MyClass is created.
        c = 10; // Invalid, this is of type MyClass, which does not contain 'c'.

        init {
            c = q;
            d = q + 10;

        // Now we can access all members.
        a = 10; // Valid, we have an instance.
        c = 10; // Valid, 'this' is now of type MyDerivedClass.

In certain situations, multiple constructors perform similar actions. In such situations it is useful to use another constructor to initialize the object instead of implementing the logic once again. This can be achieved using delegating constructors using the keyword self instead of init and super:

class MyClass {
    Int x;

    // The constructor that performs all the work:
    init(Int a) {
        init { x = a; }

    // Constructor that provides a default value:
    init() {
        // Delegate to the constructor above.

Cast Constructors

By default, Basic Storm is restrictive with automatic typecasting. In some cases it is, however, convenient to allow Basic Storm to automatically convert between types. To achieve this, it is possible to declare a constructor as a cast constructor. This is a constructor that takes a single parameter. Since it is marked as a cast constructor, Basic Storm may use it to automatically convert from one type to another. A cast constructor is defined using the keyword cast instead of init. The syntax is otherwise identical to regular constructors.


As with functions, it is possible to specify options to types as well. Options are specified by adding a colon (:) after the type name, and specifying each option separated by commas. The following options are provided by Basic Storm. Extensions may add more options by extending the rule lang:bs:SClassOption, or by declaring a function that accepts a core:lang:Type as a parameter.

Two options are special: extends and on. If these are the only options, they may be specified without the colon.