Nested Lists

• Suppose we wanted to represent story-telling in Scheme data. A natural representation would be a list of symbols, each symbol being a word of the story. But often part of a story is people saying things to one another, which may themselves be stories; accordingly, we might decide that any utterance by one of the characters should be a single element of the story, made up in turn of a list of symbols or whatever.

  First example:

  This is a very short and silly story.

  would translate into Scheme as

  (list 'This 'is 'a 'very 'short 'and 'silly 'story)
  

  or perhaps, using Scheme's list-building abbreviations,

  '(This is a very short and silly story)
  

  For a slightly more complex example, consider the story

  Jill said, "Jack, come help me with this pail of water."

  This could be represented (using list-building abbreviations) as

  '(Jill said (Jack come help me with this pail of water.)) 
  

  (I've omitted the commas, because they mean something special in Scheme.)

  Finally, consider the somewhat contrived story

  In the immortal words of J.F. Hooke,

  Bill asked, "Will you tell me a story?" Jane replied, "My grandfather used to tell lots of stories. One evening before I went to bed I asked "Grandpa, tell me a story," and he said

  "It was a dark and stormy night. Three men sat in a small hut, shivering with cold and fear. To keep their minds off these problems, the eldest of the three decided to tell a story. He started,

  "It was a dark and stormy night. Three men sat in a small hut, shivering with cold and fear. To keep their minds off these problems, the eldest of the three decided to tell a story. He started,

  "It was a dark and stormy night. Three men ---"

  but before he could finish the sentence, he was interrupted by a tidal wave that killed all three of them instantly. The end."

  The second man said "That is not much of a story. Here is a better one:" but before he could even get started, the monster ate the hut and all three men in it. The end."

  That was one of my favorites of Grandpa's stories. I hope you liked it," but Bill had fallen asleep."

  This passage illustrates the self-referential and deeply-nested nature of Hooke's later work, perhaps influenced by his ill-fated exposure to the Scheme programming language.

  This might translate into

  '(In the immortal words of J.F. Hooke,
     (Bill asked (Will you tell me a story?)
     Jane replied (My grandfather used to tell lots of stories.  One
     evening before I went to bed I asked (Grandpa tell me a story,)
     and he said
       (It was a dark and stormy night.  Three men sat in a small hut,
       shivering with cold and fear.  To keep their minds off these
       problems the eldest of the three decided to tell a story.  He
       started
         (It was a dark and stormy night.  Three men sat in a small hut,
         shivering with cold and fear.  To keep their minds off these
         problems the eldest of the three decided to tell a story.  He
         started
  	 (It was a dark and stormy night.  Three men ---)
         but before he could finish the sentence he was interrupted by a
         tidal wave that killed all three of them instantly.  The end.)
       The second man said (That is not much of a story.  Here is a better
       one:) but before he could even get started the monster ate the hut
       and all three men in it.  The end.)
     That was one of my favorites of Grandpas stories.  I hope you liked
     it) but Bill had fallen asleep.)
  This passage illustrates the self-referential and deeply-nested nature
  of Hooke's later work, perhaps influenced by his ill-fated exposure to
  the Scheme programming language.)
  

• Some time in 1999, the Godiva chocolate company introduced a line of ice cream. One flavor in particular, the Chocolate Hazelnut Praliné, caught my attention because the ingredients list was so deeply nested. Here it is in Scheme:

  (define milk-chocolate (list 'sugar 'cocoa-butter 'milk 'chocolate
  	'natural-flavor 'soy-lecithin))
  (define bittersweet-chocolate (list 'sugar 'chocolate 'cocoa-butter
  	'butter-oil 'natural-flavor 'soy-lecithin))
  (define praline-paste (list 'hazelnuts 'sugar milk-chocolate
  	bittersweet-chocolate))
  (define powdered-sugar (list 'sugar 'corn-starch))
  (define dark-chocolate (list 'sugar 'chocolate 'cocoa-butter 'butter-oil
  	'natural-flavor 'soy-lecithin))
  (define hazelnut-swirl (list praline-paste 'corn-oil powdered-sugar
  	dark-chocolate 'corn-starch 'cocoa-processed-with-alkali
  	'coconut-oil (list 'monoglycerides 'diglycerides) 'salt
  	'soy-lecithin))
  (define semi-sweet-chocolate (list 'sugar 'chocolate 'cocoa-butter
  	'soy-lecithin))
  (define thiamine-mononitrate 'vitamin-B1)
  (define riboflavin 'vitamin-B2)
  (define enriched-flour (list 'flour 'niacin 'reduced-iron
  	thiamine-mononitrate riboflavin 'folic-acid))
  (define chocolate-cookie-crumbs (list enriched-flour 'sugar
  	(list 'partially-hydrogenated-soybean-oil
  	      'partially-hydrogenated-cottonseed-oil
  	      'partially-hydrogenated-canola-oil)
  	'cocoa-processed-with-alkali 'high-fructose-corn-syrup
  	'yellow-corn-flour 'chocolate 'salt 'dextrose 'baking-soda
  	'soy-lecithin))
  (define chocolate-truffle-cake (list semi-sweet-chocolate 'cream
  	chocolate-cookie-crumbs 'corn-syrup 'butter 'chocolate 'sugar
  	'natural-flavor))
  (define godiva-chocolate-hazelnut-praline
  	(list 'milk 'skim-milk 'cream hazelnut-swirl 'sugar
  	chocolate-truffle-cake bittersweet-chocolate
  	'cocoa-processed-with-alkali 'egg-yolks 'natural-flavor
  	'guar-gum 'carob-bean-gum 'carrageenan 'dextrose))