How not to write a YAML parser (for https://github.com/KeeperPat/music-yaml-parsing)

zomgparser.rb

Dash=(Dedent=-(Indent=1.0/0)).to_s[0...1]

def scan(file)
  File.readlines(file).inject([[],[]]) do |(tokens,levels),line|
    case (levels.last || 0) <=> (level = line[/^\s*(-\s*)?/].length)
    when -1 then tokens << Indent; levels << level
    when  1 then while (levels.last || 0) > level; levels.pop; tokens << Dedent; end
    end
    if matches = line.match(/^\s*(-\s*)?(\w+):\s*(.*)$/)
      tokens << Dash if matches[1]
      tokens << matches[2].intern
      tokens << matches[3] unless matches[3].empty?
    end
    next tokens, levels
  end.first
end

def compile(tokens)
  "".tap do |str|
    case token = tokens.shift
    when Symbol, Dash
      dash = token == Dash
      str << "#{dash ? '' : 'H'}["
      begin str << (dash ? '' : "#{token}:") << compile(tokens); token = tokens.shift
      end while (dash ? token == Dash : token.is_a?(Symbol)) && str << ","
      tokens.unshift(token)
      str << "]"
    when Indent
      while tokens.first != Dedent && !tokens.empty?; str << compile(tokens); end
      tokens.shift
    when String then str << token.dump
    end
  end
end

class H < Hash
  def [](key) super(key.intern) end
  def method_missing(key) self[key] end
end

data = eval compile scan "music.yaml"

p data["genres"].last["artists"].first["albums"].first["tracks"].last["name"],
  data.genres.last.artists.first.albums.first.tracks.last.name

immediately reminded me of "how not to write a YAML emitter" I came across a while back
https://github.com/spree/spree_i18n/blob/00cd5ed517fd608d8df41964383726035bed2203/lib/spree/i18n_utils.rb#L34-43