Basic Storm is an extensible language, and it is thus possible to develop language extensions for it. This is done by creating a production that extends a suitable rule of the Basic Storm syntax. To create new expressions, extend the rule lang:bs:SExpr. To create new statements (i.e. expressions that do not need to end with a semicolon), extend the rule lang:bs:SStmt. Both of these rules are expected to return a value of type lang:bs:Expr, which is the central class that represents an expression in Basic Storm. As such, an extension may either create an abstract syntax tree that consists of existing classes in Basic Storm, or create a new abstract syntax tree node that implements the desired behavior. The latter is done by simply inheriting from the Expr class (or any of its subclasses).

The Expr class have the following members that are relevant when developing an extension. Both when implementing new expression classes, and when inspecting the abstract syntax tree:

The class contains a few additional members that are used internally. The ones mentioned above are, however, the ones that are most relevant when developing extensions.