jashkenas--coffeescript/lib/coffee_script/nodes.rb

module CoffeeScript

  # The abstract base class for all CoffeeScript nodes.
  # All nodes are implement a "compile_node" method, which performs the
  # code generation for that node. To compile a node, call the "compile"
  # method, which wraps "compile_node" in some extra smarts, to know when the
  # generated code should be wrapped up in a closure. An options hash is passed
  # and cloned throughout, containing messages from higher in the AST,
  # information about the current scope, and indentation level.
  class Node
    # Tabs are two spaces for pretty-printing.
    TAB = '  '

    # Tag this node as a statement, meaning that it can't be used directly as
    # the result of an expression.
    def self.statement
      class_eval "def statement?; true; end"
    end

    # Tag this node as a statement that cannot be transformed into an expression.
    # (break, continue, etc.) It doesn't make sense to try to transform it.
    def self.statement_only
      statement
      class_eval "def statement_only?; true; end"
    end

    # This node needs to know if it's being compiled as a top-level statement,
    # in order to compile without special expression conversion.
    def self.top_sensitive
      class_eval "def top_sensitive?; true; end"
    end

    # Provide a quick implementation of a children method.
    def self.children(*attributes)
      attr_reader(*attributes)
      attrs = attributes.map {|a| "[@#{a}]" }.join(', ')
      class_eval "def children; [#{attrs}].flatten.compact; end"
    end

    def write(code)
      puts "#{self.class.to_s}:\n#{@options.inspect}\n#{code}\n\n" if ENV['VERBOSE']
      code
    end

    # This is extremely important -- we convert JS statements into expressions
    # by wrapping them in a closure, only if it's possible, and we're not at
    # the top level of a block (which would be unnecessary), and we haven't
    # already been asked to return the result.
    def compile(o={})
      @options = o.dup
      @indent  = o[:indent]
      top      = self.top_sensitive? ? @options[:top] : @options.delete(:top)
      closure  = statement? && !statement_only? && !top && !@options[:return] && !self.is_a?(CommentNode)
      closure  &&= !contains? {|n| n.statement_only? }
      closure  ? compile_closure(@options) : compile_node(@options)
    end

    # Statements converted into expressions share scope with their parent
    # closure, to preserve JavaScript-style lexical scope.
    def compile_closure(o={})
      @indent = o[:indent]
      ClosureNode.wrap(self).compile(o.merge(:shared_scope => o[:scope]))
    end

    # Quick short method for the current indentation level, plus tabbing in.
    def idt(tabs=0)
      @indent + (TAB * tabs)
    end

    # Does this node, or any of its children, contain a node of a certain kind?
    def contains?(&block)
      children.each do |node|
        return true if yield(node)
        return true if node.is_a?(Node) && node.contains?(&block)
      end
      false
    end

    # Default implementations of the common node methods.
    def unwrap;           self;   end
    def children;         [];     end
    def statement?;       false;  end
    def statement_only?;  false;  end
    def top_sensitive?;   false;  end
  end

  # A collection of nodes, each one representing an expression.
  class Expressions < Node
    statement
    children :expressions
    attr_accessor :function

    TRAILING_WHITESPACE = /\s+$/

    # Wrap up a node as an Expressions, unless it already is.
    def self.wrap(*nodes)
      return nodes[0] if nodes.length == 1 && nodes[0].is_a?(Expressions)
      Expressions.new(*nodes)
    end

    def initialize(*nodes)
      @expressions = nodes.flatten
    end

    # Tack an expression on to the end of this expression list.
    def <<(node)
      @expressions << node
      self
    end

    # Tack an expression on to the beginning of this expression list.
    def unshift(node)
      @expressions.unshift(node)
      self
    end

    # If this Expressions consists of a single node, pull it back out.
    def unwrap
      @expressions.length == 1 ? @expressions.first : self
    end

    # Is this an empty block of code?
    def empty?
      @expressions.empty?
    end

    # Is the node last in this block of expressions?
    def last?(node)
      @last_index ||= @expressions.last.is_a?(CommentNode) ? -2 : -1
      node == @expressions[@last_index]
    end

    def compile(o={})
      o[:scope] ? super(o) : compile_root(o)
    end

    # Compile each expression in the Expressions body.
    def compile_node(o={})
      write(@expressions.map {|n| compile_expression(n, o.dup) }.join("\n"))
    end

    # If this is the top-level Expressions, wrap everything in a safety closure.
    def compile_root(o={})
      indent = o[:no_wrap] ? '' : TAB
      @indent = indent
      o.merge!(:indent => indent, :scope => Scope.new(nil, self, nil))
      code = o[:globals] ? compile_node(o) : compile_with_declarations(o)
      code.gsub!(TRAILING_WHITESPACE, '')
      write(o[:no_wrap] ? code : "(function(){\n#{code}\n})();")
    end

    # Compile the expressions body, with declarations of all inner variables
    # pushed up to the top.
    def compile_with_declarations(o={})
      code  = compile_node(o)
      args  = self.contains? {|n| n.is_a?(ValueNode) && n.arguments? }
      argv  = args && o[:scope].check('arguments') ? '' : 'var '
      code  = "#{idt}#{argv}arguments = Array.prototype.slice.call(arguments, 0);\n#{code}" if args
      code  = "#{idt}var #{o[:scope].compiled_assignments};\n#{code}" if o[:scope].assignments?(self)
      code  = "#{idt}var #{o[:scope].compiled_declarations};\n#{code}" if o[:scope].declarations?(self)
      write(code)
    end

    # Compiles a single expression within the expressions body.
    def compile_expression(node, o)
      @indent = o[:indent]
      stmt    = node.statement?
      # We need to return the result if this is the last node in the expressions body.
      returns = o.delete(:return) && last?(node) && !node.statement_only?
      # Return the regular compile of the node, unless we need to return the result.
      return "#{stmt ? '' : idt}#{node.compile(o.merge(:top => true))}#{stmt ? '' : ';'}" unless returns
      # If it's a statement, the node knows how to return itself.
      return node.compile(o.merge(:return => true)) if node.statement?
      # Otherwise, we can just return the value of the expression.
      return "#{idt}return #{node.compile(o)};"
    end

  end

  # Literals are static values that can be passed through directly into
  # JavaScript without translation, eg.: strings, numbers, true, false, null...
  class LiteralNode < Node
    children :value

    # Values of a literal node that much be treated as a statement -- no
    # sense returning or assigning them.
    STATEMENTS = ['break', 'continue']

    # Wrap up a compiler-generated string as a LiteralNode.
    def self.wrap(string)
      self.new(Value.new(string))
    end

    def initialize(value)
      @value = value
    end

    def statement?
      STATEMENTS.include?(@value.to_s)
    end
    alias_method :statement_only?, :statement?

    def compile_node(o)
      indent = statement? ? idt : ''
      ending = statement? ? ';' : ''
      "#{indent}#{@value}#{ending}"
    end
  end

  # Return an expression, or wrap it in a closure and return it.
  class ReturnNode < Node
    statement_only
    children :expression

    def initialize(expression)
      @expression = expression
    end

    def compile_node(o)
      return write(@expression.compile(o.merge(:return => true))) if @expression.statement?
      compiled = @expression.compile(o)
      write("#{idt}return #{compiled};")
    end
  end

  # A value, indexed or dotted into, or vanilla.
  class ValueNode < Node
    children :base, :properties
    attr_reader :last, :source

    # Soak up undefined properties and call attempts.
    SOAK = " == undefined ? undefined : "

    def initialize(base, properties=[])
      @base, @properties = base, [properties].flatten
    end

    def <<(other)
      @properties << other
      self
    end

    def properties?
      return !@properties.empty? || @base.is_a?(ThisNode)
    end

    def array?
      @base.is_a?(ArrayNode) && !properties?
    end

    def object?
      @base.is_a?(ObjectNode) && !properties?
    end

    def splice?
      properties? && @properties.last.is_a?(SliceNode)
    end

    def arguments?
      @base.to_s == 'arguments'
    end

    def unwrap
      @properties.empty? ? @base : self
    end

    # Values are statements if their base is a statement.
    def statement?
      @base.is_a?(Node) && @base.statement? && !properties?
    end

    def compile_node(o)
      soaked    = false
      only      = o.delete(:only_first)
      props     = only ? @properties[0...-1] : @properties
      baseline  = @base.compile(o)
      parts     = [baseline.dup]
      props.each do |prop|
        if prop.is_a?(AccessorNode) && prop.soak
          soaked = true
          if @base.is_a?(CallNode) && prop == props.first
            temp = o[:scope].free_variable
            parts[-1] = "(#{temp} = #{baseline})#{SOAK}#{baseline = temp.to_s + prop.compile(o)}"
          else
            parts[-1] << "#{SOAK}#{baseline += prop.compile(o)}"
          end
        else
          part = prop.compile(o)
          baseline += part
          parts << part
        end
      end
      @last = parts.last
      @source = parts.length > 1 ? parts[0...-1].join('') : nil
      code = parts.join('').gsub(')())', '()))')
      write(soaked ? "(#{code})" : code)
    end
  end

  # Pass through CoffeeScript comments into JavaScript comments at the
  # same position.
  class CommentNode < Node
    statement

    def initialize(lines)
      @lines = lines.value
    end

    def compile_node(o={})
      delimiter = "\n#{idt}//"
      write("#{delimiter}#{@lines.join(delimiter)}")
    end

  end

  # Node for a function invocation. Takes care of converting super() calls into
  # calls against the prototype's function of the same name.
  class CallNode < Node
    children :variable, :arguments

    def initialize(variable, arguments=[])
      @variable, @arguments = variable, arguments
      @prefix = ''
    end

    def new_instance
      @prefix = "new "
      self
    end

    def <<(argument)
      @arguments << argument
      self
    end

    # Compile a vanilla function call.
    def compile_node(o)
      return write(compile_splat(o)) if @arguments.any? {|a| a.is_a?(SplatNode) }
      args = @arguments.map{|a| a.compile(o) }.join(', ')
      return write(compile_super(args, o)) if @variable == 'super'
      write("#{@prefix}#{@variable.compile(o)}(#{args})")
    end

    # Compile a call against the superclass's implementation of the current function.
    def compile_super(args, o)
      methname = o[:scope].function.name
      arg_part = args.empty? ? '' : ", #{args}"
      meth     = o[:scope].function.proto ?
                    "#{o[:scope].function.proto}.__superClass__.#{methname}" :
                    "#{methname}.__superClass__.constructor"
      "#{meth}.call(this#{arg_part})"
    end

    # Compile a function call being passed variable arguments.
    def compile_splat(o)
      meth = @variable.compile(o)
      obj  = @variable.source || 'this'
      args = @arguments.map do |arg|
        code = arg.compile(o)
        code = arg.is_a?(SplatNode) ? code : "[#{code}]"
        arg.equal?(@arguments.first) ? code : ".concat(#{code})"
      end
      "#{@prefix}#{meth}.apply(#{obj}, #{args.join('')})"
    end

    # If the code generation wished to use the result of a function call
    # in multiple places, ensure that the function is only ever called once.
    def compile_reference(o)
      reference = o[:scope].free_variable
      call = ParentheticalNode.new(AssignNode.new(reference, self))
      return call, reference
    end
  end

  # Node to extend an object's prototype with an ancestor object.
  # After goog.inherits from the Closure Library.
  class ExtendsNode < Node
    children :sub_object, :super_object
    statement

    def initialize(sub_object, super_object)
      @sub_object, @super_object = sub_object, super_object
    end

    # Hooking one constructor into another's prototype chain.
    def compile_node(o={})
      constructor = o[:scope].free_variable
      sub, sup = @sub_object.compile(o), @super_object.compile(o)
      "#{idt}#{constructor} = function(){};\n#{idt}" +
      "#{constructor}.prototype = #{sup}.prototype;\n#{idt}" +
      "#{sub}.__superClass__ = #{sup}.prototype;\n#{idt}" +
      "#{sub}.prototype = new #{constructor}();\n#{idt}" +
      "#{sub}.prototype.constructor = #{sub};"
    end

  end

  # A dotted accessor into a part of a value, or the :: shorthand for
  # an accessor into the object's prototype.
  class AccessorNode < Node
    children :name
    attr_reader :soak

    def initialize(name, tag=nil)
      @name       = name
      @prototype  = tag == :prototype
      @soak       = tag == :soak
    end

    def compile_node(o)
      proto = @prototype ? "prototype." : ''
      write(".#{proto}#{@name}")
    end
  end

  # An indexed accessor into a part of an array or object.
  class IndexNode < Node
    children :index

    def initialize(index)
      @index = index
    end

    def compile_node(o)
      write("[#{@index.compile(o)}]")
    end
  end

  # A this-reference, using '@'.
  class ThisNode < Node
    def initialize(property=nil)
      @property = property
    end

    def compile_node(o)
      prop = @property ? ".#{@property}" : ''
      write("this#{prop}")
    end
  end

  # A range literal. Ranges can be used to extract portions (slices) of arrays,
  # or to specify a range for list comprehensions.
  class RangeNode < Node
    children :from, :to

    def initialize(from, to, exclusive=false)
      @from, @to, @exclusive = from, to, exclusive
    end

    def exclusive?
      @exclusive
    end

    def compile_variables(o)
      @indent = o[:indent]
      @from_var, @to_var = o[:scope].free_variable, o[:scope].free_variable
      from_val,  to_val  = @from.compile(o), @to.compile(o)
      write("#{@from_var} = #{from_val}; #{@to_var} = #{to_val};\n#{idt}")
    end

    def compile_node(o)
      return compile_array(o) unless o[:index]
      idx, step = o.delete(:index), o.delete(:step)
      vars      = "#{idx}=#{@from_var}"
      step      = step ? step.compile(o) : '1'
      equals    = @exclusive ? '' : '='
      intro     = "(#{@from_var} <= #{@to_var} ? #{idx}"
      compare   = "#{intro} <#{equals} #{@to_var} : #{idx} >#{equals} #{@to_var})"
      incr      = "#{intro} += #{step} : #{idx} -= #{step})"
      write("#{vars}; #{compare}; #{incr}")
    end

    # Expand the range into the equivalent array, if it's not being used as
    # part of a comprehension, slice, or splice.
    # TODO: This generates pretty ugly code ... shrink it.
    def compile_array(o)
      body = Expressions.wrap(LiteralNode.wrap('i'))
      arr  = Expressions.wrap(ForNode.new(body, {:source => ValueNode.new(self)}, Value.new('i')))
      ParentheticalNode.new(CallNode.new(CodeNode.new([], arr))).compile(o)
    end

  end

  # An array slice literal. Unlike JavaScript's Array#slice, the second parameter
  # specifies the index of the end of the slice (just like the first parameter)
  # is the index of the beginning.
  class SliceNode < Node
    children :range

    def initialize(range)
      @range = range
    end

    def compile_node(o)
      from      = @range.from.compile(o)
      to        = @range.to.compile(o)
      plus_part = @range.exclusive? ? '' : ' + 1'
      write(".slice(#{from}, #{to}#{plus_part})")
    end
  end

  # An object literal.
  class ObjectNode < Node
    children :properties
    alias_method :objects, :properties

    def initialize(properties = [])
      @properties = properties
    end

    # All the mucking about with commas is to make sure that CommentNodes and
    # AssignNodes get interleaved correctly, with no trailing commas or
    # commas affixed to comments. TODO: Extract this and add it to ArrayNode.
    def compile_node(o)
      o[:indent] = idt(1)
      joins = Hash.new("\n")
      non_comments = @properties.select {|p| !p.is_a?(CommentNode) }
      non_comments.each {|p| joins[p] = p == non_comments.last ? "\n" : ",\n" }
      props  = @properties.map { |prop|
        join = joins[prop]
        join = '' if prop == @properties.last
        indent = prop.is_a?(CommentNode) ? '' : idt(1)
        "#{indent}#{prop.compile(o)}#{join}"
      }.join('')
      write("{\n#{props}\n#{idt}}")
    end
  end

  # An array literal.
  class ArrayNode < Node
    children :objects

    def initialize(objects=[])
      @objects = objects
    end

    def compile_node(o)
      o[:indent] = idt(1)
      objects = @objects.map { |obj|
        code = obj.compile(o)
        obj.is_a?(CommentNode) ? "\n#{code}\n#{o[:indent]}" :
        obj == @objects.last   ? code : "#{code}, "
      }.join('')
      ending = objects.include?("\n") ? "\n#{idt}]" : ']'
      write("[#{objects}#{ending}")
    end
  end

  # A faux-node that is never created by the grammar, but is used during
  # code generation to generate a quick "array.push(value)" tree of nodes.
  class PushNode
    def self.wrap(array, expressions)
      expr = expressions.unwrap
      return expressions if expr.statement_only? || expr.contains? {|n| n.statement_only? }
      Expressions.wrap(CallNode.new(
        ValueNode.new(LiteralNode.new(array), [AccessorNode.new(Value.new('push'))]),
        [expr]
      ))
    end
  end

  # A faux-node used to wrap an expressions body in a closure.
  class ClosureNode
    def self.wrap(expressions, statement=false)
      func = ParentheticalNode.new(CodeNode.new([], Expressions.wrap(expressions)))
      call = CallNode.new(ValueNode.new(func, AccessorNode.new(Value.new('call'))), [Value.new('this')])
      statement ? Expressions.wrap(call) : call
    end
  end

  # Setting the value of a local variable, or the value of an object property.
  class AssignNode < Node
    top_sensitive
    children :variable, :value

    PROTO_ASSIGN = /\A(\S+)\.prototype/
    LEADING_DOT = /\A\.(prototype\.)?/

    def initialize(variable, value, context=nil)
      @variable, @value, @context = variable, value, context
    end

    def value?
      @variable.is_a?(ValueNode)
    end

    def statement?
      value? && (@variable.array? || @variable.object?)
    end

    def compile_node(o)
      top   = o.delete(:top)
      return  compile_pattern_match(o) if statement?
      return  compile_splice(o) if value? && @variable.splice?
      stmt  = o.delete(:as_statement)
      name  = @variable.compile(o)
      last  = value? ? @variable.last.to_s.sub(LEADING_DOT, '') : name
      proto = name[PROTO_ASSIGN, 1]
      if @value.is_a?(CodeNode)
        @value.name  = last  if last.match(Lexer::IDENTIFIER)
        @value.proto = proto if proto
      end
      return write("#{name}: #{@value.compile(o)}") if @context == :object
      o[:scope].find(name) unless value? && @variable.properties?
      val = "#{name} = #{@value.compile(o)}"
      return write("#{idt}#{val};") if stmt
      val = "(#{val})" if !top || o[:return]
      val = "#{idt}return #{val}" if o[:return]
      write(val)
    end

    # Implementation of recursive pattern matching, when assigning array or
    # object literals to a value. Peeks at their properties to assign inner names.
    # See: http://wiki.ecmascript.org/doku.php?id=harmony:destructuring
    def compile_pattern_match(o)
      val_var = o[:scope].free_variable
      assigns = ["#{idt}#{val_var} = #{@value.compile(o)};"]
      o.merge!(:top => true, :as_statement => true)
      @variable.base.objects.each_with_index do |obj, i|
        obj, i = obj.value, obj.variable.base if @variable.object?
        access_class = @variable.array? ? IndexNode : AccessorNode
        if obj.is_a?(SplatNode)
          val = LiteralNode.wrap(obj.compile_value(o, val_var, @variable.base.objects.index(obj)))
        else
          val = ValueNode.new(val_var, [access_class.new(Value.new(i.to_s))])
        end
        assigns << AssignNode.new(obj, val).compile(o)
      end
      write(assigns.join("\n"))
    end

    def compile_splice(o)
      var   = @variable.compile(o.merge(:only_first => true))
      range = @variable.properties.last.range
      plus  = range.exclusive? ? '' : ' + 1'
      from  = range.from.compile(o)
      to    = "#{range.to.compile(o)} - #{from}#{plus}"
      write("#{var}.splice.apply(#{var}, [#{from}, #{to}].concat(#{@value.compile(o)}))")
    end
  end

  # A function definition. The only node that creates a new Scope.
  # A CodeNode does not have any children -- they're within the new scope.
  class CodeNode < Node
    top_sensitive
    attr_reader :params, :body, :bound
    attr_accessor :name, :proto

    def initialize(params, body, tag=nil)
      @params = params
      @body   = body
      @bound  = tag == :boundfunc
    end

    def compile_node(o)
      shared_scope = o.delete(:shared_scope)
      top          = o.delete(:top)
      o[:scope]    = shared_scope || Scope.new(o[:scope], @body, self)
      o[:return]   = true
      o[:top]      = true
      o[:indent]   = idt(@bound ? 2 : 1)
      o.delete(:no_wrap)
      o.delete(:globals)
      if @params.last.is_a?(SplatNode)
        splat = @params.pop
        splat.index = @params.length
        @body.unshift(splat)
      end
      @params.each {|id| o[:scope].parameter(id.to_s) }
      code  = @body.empty? ? "" : "\n#{@body.compile_with_declarations(o)}\n"
      name_part = @name ? " #{@name}" : ''
      func  = "function#{@bound ? '' : name_part}(#{@params.join(', ')}) {#{code}#{idt(@bound ? 1 : 0)}}"
      func  = "(#{func})" if top && !@bound
      return write(func) unless @bound
      inner = "(function#{name_part}() {\n#{idt(2)}return __func.apply(__this, arguments);\n#{idt(1)}});"
      write("(function(__this) {\n#{idt(1)}var __func = #{func};\n#{idt(1)}return #{inner}\n#{idt}})(this)")
    end
  end

  # A splat, either as a parameter to a function, an argument to a call,
  # or in a destructuring assignment.
  class SplatNode < Node
    children :name
    attr_accessor :index

    def initialize(name)
      @name = name
    end

    def compile_node(o={})
      write(@index ? compile_param(o) : @name.compile(o))
    end

    def compile_param(o)
      o[:scope].find(@name)
      "#{@name} = Array.prototype.slice.call(arguments, #{@index})"
    end

    def compile_value(o, name, index)
      "Array.prototype.slice.call(#{name}, #{index})"
    end

  end

  # A while loop, the only sort of low-level loop exposed by CoffeeScript. From
  # it, all other loops can be manufactured.
  class WhileNode < Node
    top_sensitive
    children :condition, :body
    statement

    def initialize(condition, body)
      @condition, @body = condition, body
    end

    def compile_node(o)
      returns     = o.delete(:return)
      top         = o.delete(:top) && !returns
      o[:indent]  = idt(1)
      o[:top]     = true
      cond        = @condition.compile(o)
      set         = ''
      if !top
        rvar      = o[:scope].free_variable
        set       = "#{idt}#{rvar} = [];\n"
        @body     = PushNode.wrap(rvar, @body)
      end
      post        = returns ? "\n#{idt}return #{rvar};" : ''
      return write("#{set}#{idt}while (#{cond}) null;#{post}") if @body.nil?
      write("#{set}#{idt}while (#{cond}) {\n#{@body.compile(o)}\n#{idt}}#{post}")
    end
  end

  # Simple Arithmetic and logical operations. Performs some conversion from
  # CoffeeScript operations into their JavaScript equivalents.
  class OpNode < Node
    children :first, :second
    attr_reader :operator
    attr_accessor :second

    CONVERSIONS = {
      :==     => "===",
      :'!='   => "!==",
      :and    => '&&',
      :or     => '||',
      :is     => '===',
      :isnt   => "!==",
      :not    => '!'
    }
    CHAINABLE        = [:<, :>, :>=, :<=, :===, :'!==']
    ASSIGNMENT       = [:'||=', :'&&=', :'?=']
    PREFIX_OPERATORS = [:typeof, :delete]

    def initialize(operator, first, second=nil, flip=false)
      @first, @second, @flip = first, second, flip
      @operator = CONVERSIONS[operator.to_sym] || operator
    end

    def unary?
      @second.nil?
    end

    def chainable?
      CHAINABLE.include?(operator.to_sym)
    end

    def compile_node(o)
      return write(compile_chain(o)) if chainable? && @first.unwrap.is_a?(OpNode) && @first.unwrap.chainable?
      return write(compile_assignment(o)) if ASSIGNMENT.include?(@operator.to_sym)
      return write(compile_unary(o)) if unary?
      return write(compile_existence(o)) if @operator == '?'
      write("#{@first.compile(o)} #{@operator} #{@second.compile(o)}")
    end

    # Mimic Python's chained comparisons. See:
    # http://docs.python.org/reference/expressions.html#notin
    def compile_chain(o)
      shared = @first.unwrap.second
      @first.second, shared = *shared.compile_reference(o) if shared.is_a?(CallNode)
      "(#{@first.compile(o)}) && (#{shared.compile(o)} #{@operator} #{@second.compile(o)})"
    end

    def compile_assignment(o)
      first, second = @first.compile(o), @second.compile(o)
      o[:scope].find(first) if @first.unwrap.is_a?(Value)
      return "#{first} = #{ExistenceNode.compile_test(o, @first)} ? #{first} : #{second}" if @operator == '?='
      "#{first} = #{first} #{@operator[0..1]} #{second}"
    end

    def compile_existence(o)
      first, second = @first.compile(o), @second.compile(o)
      "#{ExistenceNode.compile_test(o, @first)} ? #{first} : #{second}"
    end

    def compile_unary(o)
      space = PREFIX_OPERATORS.include?(@operator.to_sym) ? ' ' : ''
      parts = [@operator.to_s, space, @first.compile(o)]
      parts.reverse! if @flip
      parts.join('')
    end
  end

  # The replacement for the for loop is an array comprehension (that compiles)
  # into a for loop. Also acts as an expression, able to return the result
  # of the comprehenion. Unlike Python array comprehensions, it's able to pass
  # the current index of the loop as a second parameter.
  class ForNode < Node
    top_sensitive
    children :body, :source, :filter
    attr_reader :name, :index, :step
    statement

    def initialize(body, source, name, index=nil)
      @body, @name, @index = body, name, index
      @source = source[:source]
      @filter = source[:filter]
      @step   = source[:step]
      @object = !!source[:object]
      @name, @index = @index, @name if @object
    end

    def compile_node(o)
      top_level     = o.delete(:top) && !o[:return]
      range         = @source.is_a?(ValueNode) && @source.base.is_a?(RangeNode) && @source.properties.empty?
      source        = range ? @source.base : @source
      scope         = o[:scope]
      name_found    = @name  && scope.find(@name)
      index_found   = @index && scope.find(@index)
      body_dent     = idt(1)
      rvar          = scope.free_variable unless top_level
      svar          = scope.free_variable
      ivar          = range ? name : @index ? @index : scope.free_variable
      var_part      = ''
      body          = Expressions.wrap(@body)
      if range
        index_var   = scope.free_variable
        source_part = source.compile_variables(o)
        for_part    = "#{index_var}=0, #{source.compile(o.merge(:index => ivar, :step => @step))}, #{index_var}++"
      else
        index_var   = nil
        source_part = "#{svar} = #{@source.compile(o)};\n#{idt}"
        step_part   = @step ? "#{ivar} += #{@step.compile(o)}" : "#{ivar}++"
        for_part    = @object ? "#{ivar} in #{svar}" : "#{ivar} = 0; #{ivar} < #{svar}.length; #{step_part}"
        var_part    = "#{body_dent}#{@name} = #{svar}[#{ivar}];\n" if @name
        # body.unshift(AssignNode.new(@name, ValueNode.new(svar, [IndexNode.new(ivar)]))) if @name
      end
      set_result    = rvar ? "#{idt}#{rvar} = []; " : idt
      return_result = rvar || ''
      body = ClosureNode.wrap(body, true) if top_level && contains? {|n| n.is_a? CodeNode }
      body = PushNode.wrap(rvar, body) unless top_level
      if o[:return]
        return_result = "return #{return_result}" if o[:return]
        o.delete(:return)
        body = IfNode.new(@filter, body, nil, :statement => true) if @filter
      elsif @filter
        body = Expressions.wrap(IfNode.new(@filter, body))
      end
      if @object
        o[:scope].assign("__hasProp", "Object.prototype.hasOwnProperty", true)
        body = Expressions.wrap(IfNode.new(
          CallNode.new(
            ValueNode.new(LiteralNode.wrap("__hasProp"), [AccessorNode.new(Value.new('call'))]),
            [LiteralNode.wrap(svar), LiteralNode.wrap(ivar)]
          ),
          Expressions.wrap(body), nil, {:statement => true}
        ))
      end

      return_result = "\n#{idt}#{return_result};" unless top_level
      body = body.compile(o.merge(:indent => body_dent, :top => true))
      vars = range ? @name : "#{@name}, #{ivar}"
      return write(set_result + source_part + "for (#{for_part}) {\n#{var_part}#{body}\n#{idt}}\n#{idt}#{return_result}")
    end
  end

  # A try/catch/finally block.
  class TryNode < Node
    children :try, :recovery, :finally
    attr_reader :error
    statement

    def initialize(try, error, recovery, finally=nil)
      @try, @error, @recovery, @finally = try, error, recovery, finally
    end

    def compile_node(o)
      o[:indent] = idt(1)
      o[:top] = true
      error_part = @error ? " (#{@error}) " : ' '
      catch_part = @recovery &&  " catch#{error_part}{\n#{@recovery.compile(o)}\n#{idt}}"
      finally_part = @finally && " finally {\n#{@finally.compile(o.merge(:return => nil))}\n#{idt}}"
      write("#{idt}try {\n#{@try.compile(o)}\n#{idt}}#{catch_part}#{finally_part}")
    end
  end

  # Throw an exception.
  class ThrowNode < Node
    children :expression
    statement_only

    def initialize(expression)
      @expression = expression
    end

    def compile_node(o)
      write("#{idt}throw #{@expression.compile(o)};")
    end
  end

  # Check an expression for existence (meaning not null or undefined).
  class ExistenceNode < Node
    children :expression

    def self.compile_test(o, variable)
      first, second = variable, variable
      first, second = *variable.compile_reference(o) if variable.is_a?(CallNode)
      "(typeof #{first.compile(o)} !== \"undefined\" && #{second.compile(o)} !== null)"
    end

    def initialize(expression)
      @expression = expression
    end

    def compile_node(o)
      write(ExistenceNode.compile_test(o, @expression))
    end
  end

  # An extra set of parentheses, supplied by the script source.
  # You can't wrap parentheses around bits that get compiled into JS statements,
  # unfortunately.
  class ParentheticalNode < Node
    children :expressions

    def initialize(expressions, line=nil)
      @expressions = expressions.unwrap
      @line = line
    end

    def compile_node(o)
      compiled = @expressions.compile(o)
      compiled = compiled[0...-1] if compiled[-1..-1] == ';'
      write("(#{compiled})")
    end
  end

  # If/else statements. Switch/whens get compiled into these. Acts as an
  # expression by pushing down requested returns to the expression bodies.
  # Single-expression IfNodes are compiled into ternary operators if possible,
  # because ternaries are first-class returnable assignable expressions.
  class IfNode < Node
    children :condition, :body, :else_body

    def initialize(condition, body, else_body=nil, tags={})
      @condition = condition
      @body      = body && body.unwrap
      @else_body = else_body && else_body.unwrap
      @tags      = tags
      @multiple  = true if @condition.is_a?(Array)
      @condition = OpNode.new("!", ParentheticalNode.new(@condition)) if @tags[:invert]
    end

    def <<(else_body)
      eb = else_body.unwrap
      @else_body ? @else_body << eb : @else_body = eb
      self
    end

    def add_comment(comment)
      @comment = comment
      self
    end

    def force_statement
      @tags[:statement] = true
      self
    end

    # Rewrite a chain of IfNodes with their switch condition for equality.
    def rewrite_condition(expression)
      @condition = @multiple ? @condition.map {|c| OpNode.new("is", expression, c) } :
                               OpNode.new("is", expression, @condition)
      @else_body.rewrite_condition(expression) if chain?
      self
    end

    # Rewrite a chain of IfNodes to add a default case as the final else.
    def add_else(exprs)
      chain? ? @else_body.add_else(exprs) : @else_body = (exprs && exprs.unwrap)
      self
    end

    # If the else_body is an IfNode itself, then we've got an if-else chain.
    def chain?
      @chain ||= @else_body && @else_body.is_a?(IfNode)
    end

    # The IfNode only compiles into a statement if either of the bodies needs
    # to be a statement.
    def statement?
      @is_statement ||= !!(@comment || @tags[:statement] || @body.statement? || (@else_body && @else_body.statement?))
    end

    def compile_condition(o)
      [@condition].flatten.map {|c| c.compile(o) }.join(' || ')
    end

    def compile_node(o)
      write(statement? ? compile_statement(o) : compile_ternary(o))
    end

    # Compile the IfNode as a regular if-else statement. Flattened chains
    # force sub-else bodies into statement form.
    def compile_statement(o)
      child       = o.delete(:chain_child)
      cond_o      = o.dup
      cond_o.delete(:return)
      o[:indent]  = idt(1)
      o[:top]     = true
      if_dent     = child ? '' : idt
      com_dent    = child ? idt : ''
      prefix      = @comment ? @comment.compile(cond_o) + "\n#{com_dent}" : ''
      body        = Expressions.wrap(@body).compile(o)
      if_part     = "#{prefix}#{if_dent}if (#{compile_condition(cond_o)}) {\n#{body}\n#{idt}}"
      return if_part unless @else_body
      else_part = chain? ?
        " else #{@else_body.compile(o.merge(:indent => idt, :chain_child => true))}" :
        " else {\n#{Expressions.wrap(@else_body).compile(o)}\n#{idt}}"
      if_part + else_part
    end

    # Compile the IfNode into a ternary operator.
    def compile_ternary(o)
      if_part   = "#{@condition.compile(o)} ? #{@body.compile(o)}"
      else_part = @else_body ? "#{@else_body.compile(o)}" : 'null'
      "#{if_part} : #{else_part}"
    end
  end

end