1.2.3

## Abstract Syntax Trees

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Once the parser has finished calling `process_*` methods an abstract syntax tree is created. The syntax tree represents the abstract syntactic structure of the parsed source code. Each node in the tree denotes a construct in the source code. This tree is composed of instances of classes defined in lib/compiler/ast/ directory. Each of these classes inherits from `Rubinius::AST::Node` class. The `Node` class defines a few methods that are utilized by the subclasses (eg. pos(g) which records the line number for debugging purposes).

The AST classes usually define at least 3 methods:

- initialize(line, args) - called by various `process_*` methods in the previous stage
- bytecode(g) - called by the Generator in the next stage
- to_sexp - used to represent the AST node as a symbolic expression

The easiest way to see the AST that gets created is to call `to_ast` on a string containing the code, eg:

    irb(main):002:0> "a = 1".to_ast
    => #<Rubinius::AST::LocalVariableAssignment:0xf70
       @value=#<Rubinius::AST::FixnumLiteral:0xf74 @value=1 @line=1>
       @name=:a @variable=nil @line=1>

or compile the code and pass the -A option:

    rbx compile -A -e "def hello;end"
    Script
      @name: __script__
      @file: "(snippet)"
      @body: \
      Define
        @name: hello
        @line: 1
        @arguments: \
        FormalArguments
          @defaults: nil
          @names: \
          @block_arg: nil
          @optional: \
          @splat: nil
          @line: 1
          @required: \
        @body: \
        Block
          @line: 1
          @array: \
            NilLiteral
              @line: 1

Similarily it’s possible to get a representation of the syntax tree as s-expressions:

    irb(main):002:0> "a = 1".to_sexp
    => [:lasgn, :a, [:lit, 1]]

or pass the -S option to compile:

    rbx compile -S -e "def hello;end"
    [:script, [:defn, :hello, [:args], [:scope, [:block, [:nil]]]]]

The AST is a nested structure where nodes contain other nodes. For example the `hello` method defined above is composed of a `Script` node containing `Define` in its `@body` which in turns contains `FormalArguments` in its `@arguments` and `Block` in its `@body`. The `Block` node contains only the `NilLiteral` in its `@array`. The `NilLiteral` node is a leaf node, it doesn’t contain other nodes.

Note that the following `if` expression:

    rbx compile -S -e ":foo if :bar"
    [:script, [:if, [:lit, :bar], [:lit, :foo], nil]]

and the same `if` expression but written differently

    rbx compile -S -e "if :bar then :foo; end"
    [:script, [:if, [:lit, :bar], [:lit, :foo], nil]]

produce exactly the same syntax tree. Because the tree doesn’t represent every detail that appears in the real syntax it is called “abstract”.

## Files Referenced

- lib/compiler/ast/: directory containing definitions of all AST classes

## Customization

There are two ways to customize this stage of the compilation process. The easiest way to customize the creation of the AST is through AST Transforms.

You can also subclass the Melbourne processor and define your own handlers for the `process_` methods. This is an advanced topic that is not yet documented.