require 'strscan' # Extended Bakus-Nour Form (EBNF), being the W3C variation is # originaly defined in the # [W3C XML 1.0 Spec](http://www.w3.org/TR/REC-xml/#sec-notation). # # This version attempts to be less strict than the strict definition # to allow for coloquial variations (such as in the Turtle syntax). # # A rule takes the following form: # \[1\] symbol ::= expression # # Comments include the content between '/*' and '*/' # # @see http://www.w3.org/2000/10/swap/grammar/ebnf2turtle.py # @see http://www.w3.org/2000/10/swap/grammar/ebnf2bnf.n3 # # Based on bnf2turtle by Dan Connolly. # # Motivation # ---------- # # Many specifications include grammars that look formal but are not # actually checked, by machine, against test data sets. Debugging the # grammar in the XML specification has been a long, tedious manual # process. Only when the loop is closed between a fully formal grammar # and a large test data set can we be confident that we have an accurate # specification of a language (and even then, only the syntax of the language). # # # The grammar in the [N3 design note][] has evolved based on the original # manual transcription into a python recursive-descent parser and # subsequent development of test cases. Rather than maintain the grammar # and the parser independently, our [goal] is to formalize the language # syntax sufficiently to replace the manual implementation with one # derived mechanically from the specification. # # # [N3 design note]: http://www.w3.org/DesignIssues/Notation3 # # Related Work # ------------ # # Sean Palmer's [n3p announcement][] demonstrated the feasibility of the # approach, though that work did not cover some aspects of N3. # # In development of the [SPARQL specification][], Eric Prud'hommeaux # developed [Yacker][], which converts EBNF syntax to perl and C and C++ # yacc grammars. It includes an interactive facility for checking # strings against the resulting grammars. # Yosi Scharf used it in [cwm Release 1.1.0rc1][], which includes # a SPAQRL parser that is *almost* completely mechanically generated. # # The N3/turtle output from yacker is lower level than the EBNF notation # from the XML specification; it has the ?, +, and * operators compiled # down to pure context-free rules, obscuring the grammar # structure. Since that transformation is straightforwardly expressed in # semantic web rules (see [bnf-rules.n3][]), it seems best to keep the RDF # expression of the grammar in terms of the higher level EBNF # constructs. # # [goal]: http://www.w3.org/2002/02/mid/1086902566.21030.1479.camel@dirk;list=public-cwm-bugs # [n3p announcement]: http://lists.w3.org/Archives/Public/public-cwm-talk/2004OctDec/0029.html # [Yacker]: http://www.w3.org/1999/02/26-modules/User/Yacker # [SPARQL specification]: http://www.w3.org/TR/rdf-sparql-query/ # [Cwm Release 1.1.0rc1]: http://lists.w3.org/Archives/Public/public-cwm-announce/2005JulSep/0000.html # [bnf-rules.n3]: http://www.w3.org/2000/10/swap/grammar/bnf-rules.n3 # # Open Issues and Future Work # --------------------------- # # The yacker output also has the terminals compiled to elaborate regular # expressions. The best strategy for dealing with lexical tokens is not # yet clear. Many tokens in SPARQL are case insensitive; this is not yet # captured formally. # # The schema for the EBNF vocabulary used here (``g:seq``, ``g:alt``, ...) # is not yet published; it should be aligned with [swap/grammar/bnf][] # and the [bnf2html.n3][] rules (and/or the style of linked XHTML grammar # in the SPARQL and XML specificiations). # # It would be interesting to corroborate the claim in the SPARQL spec # that the grammar is LL(1) with a mechanical proof based on N3 rules. # # [swap/grammar/bnf]: http://www.w3.org/2000/10/swap/grammar/bnf # [bnf2html.n3]: http://www.w3.org/2000/10/swap/grammar/bnf2html.n3 # # Background # ---------- # # The [N3 Primer] by Tim Berners-Lee introduces RDF and the Semantic # web using N3, a teaching and scribbling language. Turtle is a subset # of N3 that maps directly to (and from) the standard XML syntax for # RDF. # # [N3 Primer]: http://www.w3.org/2000/10/swap/Primer.html # # @author Gregg Kellogg module EBNF class Base include BNF include LL1 include Parser # Abstract syntax tree from parse # @!attribute [r] ast # @return [Array] attr_reader :ast # Grammar errors, or errors found genering parse tables # @!attribute [r] errors # @return [Array] attr_accessor :errors # Parse the string or file input generating an abstract syntax tree # in S-Expressions (similar to SPARQL SSE) # # @param [#read, #to_s] input # @param [Hash{Symbol => Object}] options # @option options [Boolean, Array] :debug # Output debug information to an array or STDOUT. def initialize(input, options = {}) @options = options @lineno, @depth, @errors = 1, 0, [] terminal = false @ast = [] input = input.respond_to?(:read) ? input.read : input.to_s scanner = StringScanner.new(input) eachRule(scanner) do |r| debug("rule string") {r.inspect} case r when /^@terminals/ # Switch mode to parsing terminals terminal = true when /^@pass\s*(.*)$/m # Ignore, as we can't use this for processing the EBNF grammar itself else rule = depth {ruleParts(r)} rule.kind = :terminal if terminal # Override after we've parsed @terminals rule.orig = r @ast << rule end end end # Iterate over each rule or terminal, except empty # @param [:termina, :rule] kind # @yield rule # @yieldparam [Rule] rule def each(kind, &block) ast.each {|r| block.call(r) if r.kind == kind && r.sym != :_empty} end ## # Write out parsed syntax string as an S-Expression # @return [String] def to_sxp SXP::Generator.string(ast.sort_by{|r| r.id.to_f}.map(&:for_sxp)) end def to_s; to_sxp; end def dup new_obj = super new_obj.instance_variable_set(:@ast, @ast.dup) new_obj end ## # Find a rule given a symbol # @param [Symbol] sym # @return [Rule] def find_rule(sym) (@find ||= {})[sym] ||= ast.detect {|r| r.sym == sym} end ## # Write out syntax tree as Turtle # @param [String] prefix for language # @param [String] ns URI for language # @return [String] def to_ttl(prefix, ns) unless ast.empty? [ "@prefix rdf: .", "@prefix rdfs: .", "@prefix #{prefix}: <#{ns}>.", "@prefix : <#{ns}>.", "@prefix re: .", "@prefix g: .", "", ":language rdfs:isDefinedBy <>; g:start :#{ast.first.id}.", "", ] end.join("\n") + ast.sort.map(&:to_ttl).join("\n") end def depth @depth += 1 ret = yield @depth -= 1 ret end # Progress output, less than debugging def progress(*args) return unless @options[:progress] || @options[:debug] options = args.last.is_a?(Hash) ? args.pop : {} depth = options[:depth] || @depth args << yield if block_given? message = "#{args.join(': ')}" str = "[#{@lineno}]#{' ' * depth}#{message}" @options[:debug] << str if @options[:debug].is_a?(Array) $stderr.puts(str) if @options[:progress] || @options[:debug] == true end # Error output def error(*args) options = args.last.is_a?(Hash) ? args.pop : {} depth = options[:depth] || @depth args << yield if block_given? message = "#{args.join(': ')}" @errors << message str = "[#{@lineno}]#{' ' * depth}#{message}" @options[:debug] << str if @options[:debug].is_a?(Array) $stderr.puts(str) end ## # Progress output when debugging # # @overload debug(node, message) # @param [String] node relative location in input # @param [String] message ("") # # @overload debug(message) # @param [String] message ("") # # @yieldreturn [String] added to message def debug(*args) return unless @options[:debug] options = args.last.is_a?(Hash) ? args.pop : {} depth = options[:depth] || @depth args << yield if block_given? message = "#{args.join(': ')}" str = "[#{@lineno}]#{' ' * depth}#{message}" @options[:debug] << str if @options[:debug].is_a?(Array) $stderr.puts(str) if @options[:debug] == true end end end