Details for Thursday Morning

  Scheme vs DrScheme vs TeachScheme! 
    what's difficult? what's programming? 
    problem solving w/o system is an accident 
    the more stringent the constraints, the higher the degree of creativity 
    creativity w/o technique doesn't exist 
      "style is personal implementation/interpretation of technique"

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  The Design Recipe Revisited: data-driven; template oriented

  Data Analysis: 
    what form of information must be represented? 
    do any "object descriptions" require compounding of any kind? 
    formulate data definitions
  Contract 
    what data goes in? what comes out? 
  Examples 
    make up examples of possible inputs
    what are the matching outputs? 
  Header
  Template 
    how many different kinds of data? 
      if more than one, probably use COND
    how do we distinguish them? 
    how do we access the fields? 
    Do we need to recur? 
  Body
    what is the purpose of the function? 
    formulate expressions for atomic inputs
    formulate expressions for non-recursive cases
    formulate expressions for recursive cases: 
      what does the recursion compute? 
      how can we use the result of the recursion and the remaining pieces
      to compute the final result? 
  Test

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   Web pages: 
     wp = empty | (cons sym wp) | (cons wp wp)
     examples of web pages
     ;; word-count : wp -> num
     how to distinguish among the last two alternatives 

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   Evaluate expresions:
     exp = num | (exp + exp) | (exp - exp)
     information to data: make-plus, make-minus
     let's count num's, op's, +'s ...

   ==> continue in LAB 

RECAP: Data-driven design

ASSIGNMENT: 

    ;; count-num : exp -> num

    ;; value-of : exp -> num

  NOW: extend exp: 
    (+ exp exp exp) : what needs to change? --> the variant for +
    (* exp exp) : what needs to change? --> one new cond-line in every
      function that consumes exp 
    symbol : a representation for variables --> one new cond-line again


   Add the following programs: 
      ;; subst : exp var number -> exp
      ;; closed: exp -> boolean
      ;; value-of: exp -> num