crafting-interpreters/ruby/lib/lox/parser.rb

require_relative "error"
require_relative "expr"

module Lox
  class Parser
    class State < Struct.new(:tokens, :current)
      def initialize(tokens)
        super(tokens, 0)
      end

      def expression = equality

      def equality
        expr = comparison

        while match?(:BANG_EQUAL, :EQUAL_EQUAL)
          op = prev
          right = comparison
          expr = Expr::Binary.new(expr, op, right)
        end

        expr
      end

      def comparison
        expr = term

        while match?(:GREATER, :GREATER_EQUAL, :LESS, :LESS_EQUAL)
          op = prev
          right = term
          expr = Expr::Binary.new(expr, op, right)
        end

        expr
      end


      def term
        expr = factor

        while match?(:MINUS, :PLUS)
          op = prev
          right = factor
          expr = Expr::Binary.new(expr, op, right)
        end

        expr
      end

      def factor
        expr = unary

        while match?(:SLASH, :STAR)
          op = prev
          right = unary
          expr = Expr::Binary.new(expr, op, right)
        end

        expr
      end

      def unary
        return primary unless match?(:BANG, :MINUS)

        op = prev
        right = unary
        Expr::Unary.new(op, right)
      end

      def primary
        return Expr::Literal.new(false) if match?(:FALSE)
        return Expr::Literal.new(true) if match?(:TRUE)
        return Expr::Literal.new(nil) if match?(:NIL)
        return Expr::Literal.new(prev.literal) if match?(:NUMBER, :STRING)

        if match?(:LEFT_PAREN)
          expr = expression
          consume!(:RIGHT_PAREN, "Expect ')' after expression.")
          return Expr::Grouping.new(expr)
        end

        raise ParseError.new(peek, "Expect expression.")
      end

      private

      def match?(*types)
        return false unless check?(*types)

        advance!
        return true
      end

      def consume!(type, message)
        raise ParseError.new(peek, message) unless check?(type)

        advance!
      end

      def check?(*types)
        return false if eot?

        types.include?(peek.type)
      end

      def advance!
        self.current += 1 unless eot?
        prev
      end

      def eot? = peek.type === :EOF
      def peek = tokens.fetch(current)
      def prev = tokens.fetch(current - 1)

      def synchronize!
        advance!

        until eot?
          return if prev.type == :SEMICOLON
          return if %i[CLASS FUN VAR FOR IF WHILE PRINT RETURN].include?(peek.type)

          advance!
        end
      end
    end

    # In the book, this returns nil when there's an error, but
    # that feels weird so let's move that error handling up the
    # stack for now.
    def parse(tokens)
      state = State.new(tokens)
      state.expression
    end
  end
end