chervil/lib/chervil/parser.rb

module Chervil
  class Parser
    def initialize(lexer)
      @lexer = lexer
      @tree = Array.new
      @current_token = @lexer.get_next_token
      @error = nil
    end

    def parse
      until @current_token.type == :eof
        @tree << expr
        return @error unless @error.nil?
      end
      @tree
    end

    def expr
      if [:string, :number, :boolean].include?(@current_token.type)
        constant
      elsif @current_token.type == :identifier
        identifier
      elsif @current_token.type == :lparen
        list
      elsif @current_token.type == :quote
        quotation
      else
        @error = Error.new("Unexpected token #{@current_token.type}: #{@current_token.value}")
      end
    end

    def quotation
      eat(:quote)
      value = expr
      AST::Quotation.new(value)
    end

    def list
      eat(:lparen)

      if @current_token.type == :identifier
        if @current_token.value == 'define'
          return definition
        elsif @current_token.value == 'if'
          return conditional
        elsif @current_token.value == 'lambda'
          return lambda_
        end
      end

      elements = Array.new
      until @current_token.type == :rparen || @current_token.type == :eof
        elements << expr
      end

      eat(:rparen)

      AST::List.new(elements)
    end

    def definition
      eat(:identifier) # the `define`
      if @current_token.type == :identifier
        name = identifier
        value = expr
        eat(:rparen)
        AST::Definition.new(name, value)
      elsif @current_token.type == :lparen
        eat(:lparen)
        name = identifier
        params = Array.new
        until [:rparen, :eof].include?(@current_token.type)
          params << expr
        end
        eat(:rparen)

        body = Array.new
        until [:rparen, :eof].include?(@current_token.type)
          body << expr
        end

        eat(:rparen)

        AST::Definition.new(name, AST::Function.new(name, params, body))
      end
    end

    def conditional
      eat(:identifier) # the `if`
      predicate = expr
      true_branch = expr
      false_branch = expr
      eat(:rparen)
      AST::Conditional.new(predicate, true_branch, false_branch)
    end

    def lambda_
      eat(:identifier) # the `lambda`

      name = AST::Identifier.new("lambda")
      params = Array.new

      eat(:lparen)
      while @current_token.type == :identifier
        params.push(identifier)
      end
      eat(:rparen)

      body = Array.new
      until [:rparen, :eof].include?(@current_token.type)
        body << expr
      end

      eat(:rparen)

      AST::Function.new(name, params, body)
    end

    def identifier
      identifier = eat(:identifier)
      AST::Identifier.new(identifier.value)
    end

    def constant
      case @current_token.type
      when :number
        token = eat(:number)
        AST::Number.new(token.value.to_f)
      when :string
        token = eat(:string)
        AST::String.new(token.value)
      when :boolean
        token = eat(:boolean)
        AST::Boolean.new(token.value)
      end
    end

    def eat(type)
      if @current_token.type == type
        token = @current_token
        @current_token = @lexer.get_next_token
        token
      else
        @error = Error.new("Expected #{type} but got #{@current_token.type}")
      end
    end
  end
end