TeachScheme! 2002

  Review the grammar so far; introduce values.

  Finished yesterday working with posns, compound data.  Compound data
  arises in many contexts:
  -- phone books
  -- library catalogs
  -- store inventory
  -- periodic table
  How can we make our own forms of compound data (structures)?

  Recall separation between domain knowledge and computing knowledge.
  Consider a non-CS-like example:

  Example: zoo animals
  -- (define-struct boa (name weight food))
     (make-boa 'Slinky 30 'pets)
     (make-boa 'Slim 50 'bushes)

     ; portable? : boa num -> boolean
     ; determine whether boa no heavier than given weight limit

     RECAP format of examples: some forget that they are code

     Re-motivate need for selectors

  What does define-struct provide? [in contracts, use different colors 
	for checked and unchecked types]
  -- make-boa : symbol num symbol -> boa
  -- boa-name : boa -> symbol
  -- boa-weight : boa -> num
  -- boa-food : boa -> food

  make-boa is the [constrictor] constructor
  the others are selectors
  show box diagram; emphasize the label ("boa", not "posn")

  PVD 2001:
  To deal with the "(make-...) looks like a process, not an answer"
  confusion, one teacher pointed out that it was no different from
  claiming that sqrt(3) [in math notation] is a number.  Nobody denies
  that; yet it too is clearly a "process".  We thought this was an
  excellent response.

  PVD 2001:
  On Kyle's suggestion (thanks, Matthew!), from the second data def using
  structs onward, we switched from the book style of enumerating all the
  fields to just writing 
    A boa is a structure
      (make-boa symbol number symbol)
  We liked this a lot, and used it the rest of the week.  It also saves
  precious time and board space.

  -- (define-struct armadillo (weight dead?))
     ["In Texas, there a'int nothin' in the middle of the road 'cept a
       yellow line and a dead armadillah"]

     ; run-over-by-truck : armadillo -> armadillo

  >> DATA DEFINITIONS FOR MIXED DATA <<

  -- An animal is either
       - a boa, or
       - an armadillo

  Your friend the zoo-keeper wants a program to check whether animals
  are above their recommended minimum weights.  Min weight for boa is
  25 and min weight for dillo is 5.

  ; too-thin? animal -> boolean

  PVD 2002:
  We introduced this instead of portable? on animals.
  On portable? on animals, some teachers always want a generic weight  
  selector, since the answer formula is the same for both animals
  sans the weight selector.  Nobody even raised the issue with this
  example, since the conds clearly needed to do different things.
  This also reinforces the domain-knowledge point.

   --> define-struct also introduces predicates
   CONDITIONAL, each line in the conditional: selectors

  >> DESIGN RECIPE (p88) <<
  specify, make examples, define, test

  PVD 2002: At the end of the week, teachers mentioned that summaries
  of the data definition formats and templates would be useful.  We
  recommend handing those summaries out as we cover each style of data
  definition (our samples are linked to the main Wonk page).