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35 lines
1.8 KiB
Markdown
35 lines
1.8 KiB
Markdown
<!--
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Part of the Carbon Language project, under the Apache License v2.0 with LLVM
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Exceptions. See /LICENSE for license information.
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SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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-->
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The code in this directory defines the AST that represents Carbon code in the
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rest of `executable-semantics`.
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All node types in the AST are derived from [`AstNode`](ast_node.h), and use
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[LLVM-style RTTI](https://llvm.org/docs/HowToSetUpLLVMStyleRTTI.html) to support
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safe down-casting and similar operations. Each abstract class `Foo` in the
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hierarchy has a `kind` method which returns a enum `FooKind` that identifies the
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concrete type of the object, and a `FooKind` value can be safely `static_cast`ed
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to `BarKind` if that value represents a type that's derived from both `Foo` and
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`Bar`.
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We rely on code generation to help enforce those invariants, so every node type
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must be described in [`ast_rtti.txt`](ast_rtti.txt). See the documentation in
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(`gen_rtti.py`)[../gen_rtti.py], the code generation script, for details about
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the file format and generated code.
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The AST class hierarchy is structured in a fairly unsurprising way, with
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abstract classes such as `Statement` and `Expression`, and concrete classes
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representing individual syntactic constructs, such as `If` for if-statements.
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Sometimes it is useful to work with a subset of node types that "cuts across"
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the primary class hierarchy. Rather than deal with the pitfalls of multiple
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inheritance, we handle these cases using a form of type erasure: we specify a
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notional interface that those types conform to, and then define a "view" class
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that behaves like a pointer to an instance of that interface. Types declare that
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they model an interface `Foo` by defining a public static member named
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`ImplementsCarbonFoo`. See [NamedEntityView](static_scope.h) for an example of
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this pattern.
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