Procedurally generated quasi-poetry

grammar

POEM: <LINE> <LINE> <LINE> <LINE> <LINE>
LINE: <NOUN>|<PREPOSITION>|<PRONOUN> $LINEBREAK
ADJECTIVE: black|white|dark|light|bright|murky|muddy|clear <NOUN>|<ADJECTIVE>|$END
NOUN: heart|sun|moon|thunder|fire|time|wind|sea|river|flavor|wave|willow|rain|tree|flower|field|meadow|pasture|harvest|water|father|mother|brother|sister <VERB>|<PREPOSITION>|$END
PRONOUN: my|your|his|her <NOUN>|<ADJECTIVE>
VERB: runs|walks|stands|climbs|crawls|flows|flies|transcends|ascends|descends|sinks <PREPOSITION>|<PRONOUN>|$END
PREPOSITION: above|across|against|along|among|around|before|behind|beneath|beside|between|beyond|during|inside|onto|outside|under|underneath|upon|with|without|through <NOUN>|<PRONOUN>|<ADJECTIVE>

poetaster.rb

#!/usr/bin/ruby

unless ARGV[0]
  $stderr.puts 'No input grammar specified'
  exit 1
end
unless File.readable? ARGV[0]
  $stderr.puts 'Specified grammar file does not exist or cannot be read'
  exit 2
end

@productions = {}

File.new(ARGV[0]).each_with_index do |rule, i|
  name, body = rule.split /:\s*/, 2
  ROOT_PRODUCTION = name if i == 0
  @productions[name] = body.split(/\s+/).map { |opts| opts.split '|' }
end

def satisfy(production)
  @productions[production].each { |opts| handle_opt opts.sample }
end

def handle_opt(opt)
  case opt
  when /^<([A-Z]+)>$/
    satisfy $~[1]
  when '$LINEBREAK'
    puts
  when '$END'
    return
  else
    print "#{opt} "
  end
end

satisfy ROOT_PRODUCTION