Com S 342 meeting -*- Outline -*-

* Conditional Evaluation (5.2)
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            CONDITIONALS (5.2)

What should be the concrete syntax for
if-expressions in the defined language?






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        ... <exp> ::= if <exp> then <exp> else <exp>

        give examples:
                if less(2,3) then 5 else 4

        Q: What would this be like if we had a distinction between statements
                and expressions?

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	     FOR YOU TO DO
	       IN GROUPS

Close your books.

1. What should the abstract syntax be?


2. What type should the test have?

3. Write the part you need to add to
   the interpreter, and any needed code.




4. Do any primitive procedures need to be
   built-in to make this useful?
   Write the necessary changes for them.




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        ... (define-record if (test-exp then-exp else-exp))
        ... (variant-case exp
             (if (test-exp then-exp else-exp)
                (if (true-value? (expressed->number (eval-exp test-exp)))
                    (eval-exp then-exp)
                    (eval-exp else-exp)))
             ...)

        (define true-value?  ; TYPE: (-> (number) boolean)
           (lambda (v)
              (not (zero? v))))

	Be sure to clarify what the actual syntax is that will be used
		from here on in the defined language.
	But it isn't necessary to use that.

        ... need primitives for equal, less, greater
                Can we have "not" as a primitive?