A method to evaluate static parts of a ParseTree syntax tree.

ParseTree.eval_static_tree.rb

class ParseTree
  
  # Evaluates a static part of the tree that ParseTree.translate returns.
  # You can pas a block that will be used to get a value that is not static.
  # A simple example:
  #
  #   >> code = '{ :array => ["str", 123, 4.5], :dynamic => method_call }'
  #   >> tree = ParseTree.translate(code)
  #   => [:hash, [:lit, :array], [:array, [:str, "str"], [:lit, 123], [:lit, 4.5]], 
  #              [:lit, :dynamic], [:vcall, :method_call]]
  #   >> ParseTree.eval_static_tree(tree)
  #   RuntimeError: tree is not static: :vcall ...
  #   >> ParseTree.eval_static_tree(tree) { nil }
  #   => {:dynamic=>nil, :array=>["str", 123, 4.5]}
  #
  # <tt>tree</tt>::   The (sub)tree to evaluate. Should be (a part of) the
  #                   array ParseTree.translate(str) returns
  # <tt>&block</tt>:: A handler block for non-static elements.
  def self.eval_static_tree(tree, &block)
    case node_type = tree.shift
    when :lit, :str then tree.last
    when :array     then tree.map { |child| eval_static_tree(child, &block) }
    when :hash
      hash = {}
      hash[eval_static_tree(tree.shift, &block)] = eval_static_tree(tree.shift, &block) until tree.empty?
      return hash
    else
      if block_given?
        yield(tree.unshift(node_type))
      else
        raise "Tree node is not static: #{node_type.inspect}"
      end
    end
  end
end