Exploring how to parse `word space word` with a student.

1.rb

# Parses word lists without doing anything fancy
require 'treetop'                   # => true
Treetop.load_from_string <<GRAMMAR  # => AParser
grammar A
  rule word_list
    word (' ' word)* {
      def eval
        head = elements[0].eval
        tail = elements[1].elements.map { |space_word| space_word.elements[1].eval }
        [head] + tail
      end
    }
  end
  
  rule word
    [a-z]+ {
      def eval
        "EVALUATED(\#{text_value})"
      end
    }
  end
end
GRAMMAR


# word list
AParser.new.parse('hello world how are you')  # => SyntaxNode+WordList2+WordList1 offset=0, "...lo world how are you" (eval,word):\n  SyntaxNode+Word0 offset=0, "hello" (eval):\n    SyntaxNode offset=0, "h"\n    SyntaxNode offset=1, "e"\n    SyntaxNode offset=2, "l"\n    SyntaxNode offset=3, "l"\n    SyntaxNode offset=4, "o"\n  SyntaxNode offset=5, " world how are you":\n    SyntaxNode+WordList0 offset=5, " world" (word):\n      SyntaxNode...
           .eval                              # => ["EVALUATED(hello)", "EVALUATED(world)", "EVALUATED(how)", "EVALUATED(are)", "EVALUATED(you)"]

# many words
AParser.new.parse('hello world ')  # => nil

2.rb

# same thing, but we name some of the elements and use some fancy Ruby syntax
require 'treetop'                   # => true
Treetop.load_from_string <<GRAMMAR  # => AParser
grammar A
  rule word_list
    head:word tail_words:(' ' word)* {
      def eval
        [head.eval, *tail.map(&:eval)]
      end

      def tail
        tail_words.elements.map(&:word)
      end
    }
  end

  rule word
    [a-z]+ {
      def eval
        "EVALUATED(\#{text_value})"
      end
    }
  end
end
GRAMMAR


# word list
AParser.new.parse('hello world how are you')  # => SyntaxNode+WordList2+WordList1 offset=0, "...lo world how are you" (eval,tail,head,tail_words):\n  SyntaxNode+Word0 offset=0, "hello" (eval):\n    SyntaxNode offset=0, "h"\n    SyntaxNode offset=1, "e"\n    SyntaxNode offset=2, "l"\n    SyntaxNode offset=3, "l"\n    SyntaxNode offset=4, "o"\n  SyntaxNode offset=5, " world how are you":\n    SyntaxNode+WordList0 offset=5, " world" (word):\n...
           .eval                              # => ["EVALUATED(hello)", "EVALUATED(world)", "EVALUATED(how)", "EVALUATED(are)", "EVALUATED(you)"]

# many words
AParser.new.parse('hello world ')  # => nil