carbon-lang/executable_semantics/ast/README.md

The code in this directory defines the AST that represents Carbon code in the rest of executable-semantics.

All node types in the AST are derived from AstNode, and use LLVM-style RTTI to support safe down-casting and similar operations. Each abstract class Foo in the hierarchy has a kind method which returns a enum FooKind that identifies the concrete type of the object, and a FooKind value can be safely static_casted to BarKind if that value represents a type that's derived from both Foo and Bar.

We rely on code generation to help enforce those invariants, so every node type must be described in ast_rtti.txt. See the documentation in (gen_rtti.py)[../gen_rtti.py], the code generation script, for details about the file format and generated code.

The AST class hierarchy is structured in a fairly unsurprising way, with abstract classes such as Statement and Expression, and concrete classes representing individual syntactic constructs, such as If for if-statements.

Sometimes it is useful to work with a subset of node types that "cuts across" the primary class hierarchy. Rather than deal with the pitfalls of multiple inheritance, we handle these cases using a form of type erasure: we specify a notional interface that those types conform to, and then define a "view" class that behaves like a pointer to an instance of that interface. Types declare that they model an interface Foo by defining a public static member named ImplementsCarbonFoo. See NamedEntityView for an example of this pattern.