class AST::Node
Node
is an immutable class, instances of which represent abstract syntax tree nodes. It combines semantic information (i.e. anything that affects the algorithmic properties of a program) with meta-information (line numbers or compiler intermediates).
Notes on inheritance
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The distinction between semantics and metadata is important. Complete semantic information should be contained within just the {#type} and {#children} of a Node
instance; in other words, if an AST
was to be stripped of all meta-information, it should remain a valid AST
which could be successfully processed to yield a result with the same algorithmic properties.
Thus, Node
should never be inherited in order to define methods which affect or return semantic information, such as getters for `class_name`, `superclass` and `body` in the case of a hypothetical `ClassNode`. The correct solution is to use a generic Node
with a {#type} of `:class` and three children. See also {Processor} for tips on working with such ASTs.
On the other hand, Node
can and should be inherited to define application-specific metadata (see also {#initialize}) or customize the printing format. It is expected that an application would have one or two such classes and use them across the entire codebase.
The rationale for this pattern is extensibility and maintainability. Unlike static ones, dynamic languages do not require the presence of a predefined, rigid structure, nor does it improve dispatch efficiency, and while such a structure can certainly be defined, it does not add any value but incurs a maintaining cost. For example, extending the AST
even with a transformation-local temporary node type requires making globally visible changes to the codebase.
Attributes
Returns the children of this node. The returned value is frozen. The to_a
alias is useful for decomposing nodes concisely. For example:
node = s(:gasgn, :$foo, s(:integer, 1)) var_name, value = *node p var_name # => :$foo p value # => (integer 1)
@return [Array]
Returns the precomputed hash value for this node @return [Fixnum]
Returns the children of this node. The returned value is frozen. The to_a
alias is useful for decomposing nodes concisely. For example:
node = s(:gasgn, :$foo, s(:integer, 1)) var_name, value = *node p var_name # => :$foo p value # => (integer 1)
@return [Array]
Returns the type of this node. @return [Symbol]
Public Class Methods
Constructs a new instance of Node
.
The arguments `type` and `children` are converted with `to_sym` and `to_a` respectively. Additionally, the result of converting `children` is frozen. While mutating the arguments is generally considered harmful, the most common case is to pass an array literal to the constructor. If your code does not expect the argument to be frozen, use `#dup`.
The `properties` hash is passed to {#assign_properties}.
# File lib/ast/node.rb, line 72 def initialize(type, children=[], properties={}) @type, @children = type.to_sym, children.to_a.freeze assign_properties(properties) @hash = [@type, @children, self.class].hash freeze end
Public Instance Methods
Compares `self` to `other`, possibly converting with `to_ast`. Only `type` and `children` are compared; metadata is deliberately ignored.
@return [Boolean]
# File lib/ast/node.rb, line 153 def ==(other) if equal?(other) true elsif other.respond_to? :to_ast other = other.to_ast other.type == self.type && other.children == self.children else false end end
Appends `element` to `children` and returns the resulting node.
@return [AST::Node]
# File lib/ast/node.rb, line 177 def append(element) updated(nil, @children + [element]) end
Concatenates `array` with `children` and returns the resulting node.
@return [AST::Node]
# File lib/ast/node.rb, line 168 def concat(array) updated(nil, @children + array.to_a) end
Enables matching for Node
, where type is the first element and the children are remaining items.
@return [Array]
# File lib/ast/node.rb, line 253 def deconstruct [type, *children] end
Nodes are already frozen, so there is no harm in returning the current node as opposed to initializing from scratch and freezing another one.
@return self
# File lib/ast/node.rb, line 115 def dup self end
Test if other object is equal to @param [Object] other @return [Boolean]
# File lib/ast/node.rb, line 85 def eql?(other) self.class.eql?(other.class) && @type.eql?(other.type) && @children.eql?(other.children) end
Converts `self` to a s-expression ruby string. The code return will recreate the node, using the sexp module s()
@param [Integer] indent Base indentation level. @return [String]
# File lib/ast/node.rb, line 211 def inspect(indent=0) indented = " " * indent sexp = "#{indented}s(:#{@type}" children.each do |child| if child.is_a?(Node) sexp += ",\n#{child.inspect(indent + 1)}" else sexp += ", #{child.inspect}" end end sexp += ")" sexp end
@return [AST::Node] self
# File lib/ast/node.rb, line 229 def to_ast self end
Converts `self` to a pretty-printed s-expression.
@param [Integer] indent Base indentation level. @return [String]
# File lib/ast/node.rb, line 187 def to_sexp(indent=0) indented = " " * indent sexp = "#{indented}(#{fancy_type}" children.each do |child| if child.is_a?(Node) sexp += "\n#{child.to_sexp(indent + 1)}" else sexp += " #{child.inspect}" end end sexp += ")" sexp end
Converts `self` to an Array where the first element is the type as a Symbol, and subsequent elements are the same representation of its children.
@return [Array<Symbol, […Array]>]
# File lib/ast/node.rb, line 237 def to_sexp_array children_sexp_arrs = children.map do |child| if child.is_a?(Node) child.to_sexp_array else child end end [type, *children_sexp_arrs] end
Returns a new instance of Node
where non-nil arguments replace the corresponding fields of `self`.
For example, `Node.new(:foo, [ 1, 2 ]).updated(:bar)` would yield `(bar 1 2)`, and `Node.new(:foo, [ 1, 2 ]).updated(nil, [])` would yield `(foo)`.
If the resulting node would be identical to `self`, does nothing.
@param [Symbol, nil] type @param [Array, nil] children @param [Hash, nil] properties @return [AST::Node]
# File lib/ast/node.rb, line 133 def updated(type=nil, children=nil, properties=nil) new_type = type || @type new_children = children || @children new_properties = properties || {} if @type == new_type && @children == new_children && properties.nil? self else copy = original_dup copy.send :initialize, new_type, new_children, new_properties copy end end
Protected Instance Methods
By default, each entry in the `properties` hash is assigned to an instance variable in this instance of Node
. A subclass should define attribute readers for such variables. The values passed in the hash are not frozen or whitelisted; such behavior can also be implemented by subclassing Node
and overriding this method.
@return [nil]
# File lib/ast/node.rb, line 98 def assign_properties(properties) properties.each do |name, value| instance_variable_set :"@#{name}", value end nil end
Returns `@type` with all underscores replaced by dashes. This allows to write symbol literals without quotes in Ruby sources and yet have nicely looking s-expressions.
@return [String]
# File lib/ast/node.rb, line 264 def fancy_type @type.to_s.gsub('_', '-') end