Class: Dhaka::Evaluator

Inherits:
SimpleDelegator
  • Object
show all
Defined in:
lib/dhaka/evaluator/evaluator.rb

Overview

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

Class Method Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize ⇒ Evaluator

Returns a new instance of Evaluator.



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# File 'lib/dhaka/evaluator/evaluator.rb', line 118

def initialize
  
end

Class Method Details

.define_evaluation_rules(options = {}) ⇒ Object

Define evaluation rules within a block passed to this method. The evaluator will define default evaluation rules for pass-through productions (i.e. productions with expansions consisting of exactly one grammar symbol). The default evaluation rule for such productions is to simply return the result of calling evaluate on the unique child node. Setting the :raise_error option to true tells the evaluator to throw an exception if you neglect to define a rule for a non-pass-through production (one where the expansion consists of multiple symbols), listing all the productions that absolutely need to be defined before you can continue.



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# File 'lib/dhaka/evaluator/evaluator.rb', line 68

def define_evaluation_rules(options = {})
  yield
  check_definitions(options)
end

Instance Method Details

#evaluate(node) ⇒ Object

Evaluate a parse tree node.



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# File 'lib/dhaka/evaluator/evaluator.rb', line 108

def evaluate node
  @node_stack ||= []
  @node_stack << node
  __setobj__(@node_stack.last)
  result      = send(node.production.name)
  @node_stack.pop
  __setobj__(@node_stack.last)
  result
end