Expressions
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:
-
resultCompute the result type of this expression, expressed as an
ExprResult. The result may be either that the expression returns a value of a particular type, or that the expression never returns (e.g. by invokingreturnorthrowon all paths). -
codeCalled by the parent block or expression to generate intermediate code for the expression. The first parameter is a
CodeGenobject that contains information about the code generation state. The second parameterCodeResultcontains the location where the code should place the generated result.If
resultindicates that the expression returns a reference to a value, then thecodemember must be prepared to either generate a reference to a value, or to produce a non-reference. Expressions that report that they return a non-reference can assume that they will not be asked to return a reference. -
castPenaltyCalled to determine the penalty to cast the result of the expression into the type passed as a parameter. This interface is used when an exact match is not found, and some automatic type casting is necessary. Expressions may use this function to determine what type to return depending on the context in which they are used. For example, the
nullliteral reports a result ofvoid, since the exact type is not known. However, if it is used in a context where aMaybe<T>is expected, it will return a suitable type.Should return a non-negative value if the cast is possible. This means that the
codefunction will properly handle receiving a result of the type passed tocastPenalty. A negative result means that the cast is not possible.
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.