Dhaka::Evaluator

Abstract base class for evaluators.

Defining an evaluator is an easy way to perform syntax-directed evaluation without having to generate an abstract syntax tree representation of the input.

An evaluation rule for a given production named bar is defined by calling for_bar with a block that performs the evaluation. For detailed examples, see the evaluators in the test suite.

The following is an evaluator for arithmetic expressions. When a parse tree node is encountered that corresponds to the production named addition, the block passed to for_addition is invoked. The evaluate method is then recursively called on the child nodes, in this case the operands to the addition operation. The result is obtained by adding the evaluation results of the child nodes.

class ArithmeticPrecedenceEvaluator < Dhaka::Evaluator

  self.grammar = ArithmeticPrecedenceGrammar

  define_evaluation_rules do

    for_subtraction do 
      evaluate(child_nodes[0]) - evaluate(child_nodes[2])
    end

    for_addition do
      evaluate(child_nodes[0]) + evaluate(child_nodes[2])
    end

    for_division do
      evaluate(child_nodes[0]).to_f/evaluate(child_nodes[2]) 
    end

    for_multiplication do
      evaluate(child_nodes[0]) * evaluate(child_nodes[2])
    end

    for_literal do
      child_nodes[0].token.value.to_i
    end

    for_parenthetized_expression do
      evaluate(child_nodes[1])
    end

    for_negated_expression do
      -evaluate(child_nodes[1])
    end

    for_power do
      evaluate(child_nodes[0])**evaluate(child_nodes[2])
    end

  end

end