A functional, lazily-evaluated solution to https://github.com/KeeperPat/music-yaml-parsing for a "coding challenged" competition

music.rb

Proc.module_eval { def method_missing(m,*a); call(m.to_s,*a); end }
Array.module_eval { def rest; self[1..-1]; end }
Object.module_eval { def let; yield self; end }

Music = Module.new do
  def self.load(filename)
    depth      = ->(l) { %r(^([\s-]*)\w).match(l)[1].length }
    ldepth     = ->(l) { %r(^(\s*)\S).match(l)[1].length }
    nonzero    = ->(n) { n > 0 }
    empty      = ->(c) { c.empty? }
    take_while = ->(p, c) {
      c.empty? ? [] : p.(c.first) ? [c.first] + take_while[p, c.rest] : []
    }
    drop_while     = ->(p, c) {
      c.empty? ? [] : p.(c.first) ? drop_while[p, c.rest] : c
    }
    list = ->(*a) {}
    kv  = ->(ls) { ->(f, ls, k) {
      if l = ls.first
        ck = ->(l) { %r((\w+):).match(l)[1] }.(l)
        d = depth[l]
        case ck
        when k
          v = ->(l) { %r(:\s+(.*)).match(l) }.(l)
          case v
          when nil then list.(take_while.(->(l){depth[l] > d}, ls.rest))
          else v[1]
          end
        else f.(f, drop_while.(->(l){depth[l] > d}, ls.rest), k)
        end
      end
    }.let { |f| f.curry.(f, ls) } }
    list = ->(ls) { ->(f, ls, n) {
      if l = ls.first
        d = ldepth[l]
        v = kv.( [l] + take_while[->(l){ ldepth[l] > d }, ls.rest] )
        r = drop_while.(->(l){ldepth[l] > d}, ls.rest)
        case n
        when "last"
          case r
          when empty then v
          else f.(f, r, n)
          end
        when 0, "first" then v
        when nonzero then f.(f, r, n-1)
        else nil
        end
      end
    }.let { |f| f.curry.(f, ls) } }
    kv[ File.read(filename).split("\n") ]
  end
end

data = Music.load "music.yaml"
require "pp"
puts data["genres"].last["artists"].first["albums"].first["tracks"].last["name"] #=> "But Not For Me"
puts data.genres.last.artists.first.albums.first.tracks.last.name #=> "But Not For Me"
puts data.genres.first.name #=> Classic Rock
puts data.genres[0].artists[0].name #=> Led Zeppelin
puts data.genres[0].artists[0].albums[1].name #=> II

music.yaml

genres:
  - name: Classic Rock
    artists:
      - name: Led Zeppelin
        albums:
        - name: I
          tracks:
            - name: Babe, I'm Gonna Leave You
            - name: Black Mountain Side
            - name: Communication Breakdown
            - name: Dazed And Confused
            - name: Good Times, Bad Times
            - name: How Many More Times
            - name: I Can't Quit You Baby
            - name: You Shook Me
            - name: Your Time Is Gonna Come
        - name: II
          tracks:
            - name: Bring It On Home
            - name: Heartbreaker
            - name: The Lemon Song
            - name: Living Loving Maid
            - name: Moby Dick
            - name: Ramble On
            - name: Thank You
            - name: What Is And What Should Never Be
            - name: Whole Lotta Love
  - name: Jazz
    artists:
      - name: John Coltrane
        albums:
        - name: My Favorite Things
          tracks:
            - name: My Favorite Things
            - name: Every Time We Say Goodbye
            - name: Summertime
            - name: But Not For Me