Class: AST::Node

Inherits:
Object
  • Object
show all
Defined in:
lib/ast/node.rb

Overview

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

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.

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(type, children = [], properties = {}) ⇒ Node

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.


72
73
74
75
76
77
78
79
80
# 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

Instance Attribute Details

#childrenArray (readonly) Also known as: to_a

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)

Returns:

  • (Array)

56
57
58
# File 'lib/ast/node.rb', line 56

def children
  @children
end

#hashFixnum (readonly)

Returns the precomputed hash value for this node

Returns:

  • (Fixnum)

61
62
63
# File 'lib/ast/node.rb', line 61

def hash
  @hash
end

#typeSymbol (readonly)

Returns the type of this node.

Returns:

  • (Symbol)

43
44
45
# File 'lib/ast/node.rb', line 43

def type
  @type
end

Instance Method Details

#==(other) ⇒ Boolean

Compares `self` to `other`, possibly converting with `to_ast`. Only `type` and `children` are compared; metadata is deliberately ignored.

Returns:

  • (Boolean)

153
154
155
156
157
158
159
160
161
162
163
# 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

#append(element) ⇒ AST::Node Also known as: <<

Appends `element` to `children` and returns the resulting node.

Returns:


177
178
179
# File 'lib/ast/node.rb', line 177

def append(element)
  updated(nil, @children + [element])
end

#concat(array) ⇒ AST::Node Also known as: +

Concatenates `array` with `children` and returns the resulting node.

Returns:


168
169
170
# File 'lib/ast/node.rb', line 168

def concat(array)
  updated(nil, @children + array.to_a)
end

#dupObject Also known as: clone

Nodes are already frozen, so there is no harm in returning the current node as opposed to initializing from scratch and freezing another one.

Returns:

  • self


115
116
117
# File 'lib/ast/node.rb', line 115

def dup
  self
end

#eql?(other) ⇒ Boolean

Test if other object is equal to

Parameters:

  • other (Object)

Returns:

  • (Boolean)

85
86
87
88
89
# File 'lib/ast/node.rb', line 85

def eql?(other)
  self.class.eql?(other.class)   &&
  @type.eql?(other.type)         &&
  @children.eql?(other.children)
end

#inspect(indent = 0) ⇒ String

Converts `self` to a s-expression ruby string. The code return will recreate the node, using the sexp module s()

Parameters:

  • indent (Integer) (defaults to: 0)

    Base indentation level.

Returns:

  • (String)

211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
# 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

#to_astAST::Node

Returns self.

Returns:


229
230
231
# File 'lib/ast/node.rb', line 229

def to_ast
  self
end

#to_sexp(indent = 0) ⇒ String Also known as: to_s

Converts `self` to a pretty-printed s-expression.

Parameters:

  • indent (Integer) (defaults to: 0)

    Base indentation level.

Returns:

  • (String)

187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
# 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

#to_sexp_arrayArray<Symbol, [...Array]>

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.

Returns:

  • (Array<Symbol, [...Array]>)

237
238
239
240
241
242
243
244
245
246
247
# 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

#updated(type = nil, children = nil, properties = nil) ⇒ AST::Node

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.

Parameters:

  • type (Symbol, nil) (defaults to: nil)
  • children (Array, nil) (defaults to: nil)
  • properties (Hash, nil) (defaults to: nil)

Returns:


133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
# 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