carbon-lang/executable_semantics/ast/README.md

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

The AST is not quite immutable, because some node properties are set during some
phase of static analysis, rather than during parsing. However, AST mutations are
_monotonic_: once set, a node property cannot be changed. Furthermore, if a
property is set after parsing, its documentation specifies what phase is
responsible for setting it. Certain properties have `has_foo()` members for
querying whether they are set, but those are for internal use within the phase
that sets them. As a result, you can think of the AST as if it were immutable,
but with certain parts that you can't yet observe, depending on what phase of
compilation you're in.

All node types in the AST are derived from [`AstNode`](ast_node.h), and use
[LLVM-style RTTI](https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html) 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_cast`ed
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`](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 [ValueNodeView](static_scope.h) for an example of
this pattern.