CS217 Day 11 Wednesday, April 19, 2000

Environment Passing Interpreters II Conditional Evaluation

• interp3-1.scm implements the simplest environment passing interpreter, using code in 3-1 and 3-2. The language has variables as well as numbers and primitive applications.

  Character syntax grammar:

  [program] ::= [expression]
  [expression] ::= [number]
         | [symbol]
         | [primitive]([expression]{, [expression]}*)
  [primitive] ::= + | - | * | add1 | sub1
  

  It contains definitions with some examples of
  • environment
  • program
  • expression
  • primitive
  • eval-program
  • eval-expression
  • parse-program
  • parse-expression
  • unparse-program
  • unparse-expression
  • run-list-syntax
  • lex-spec
  • grammar-3-1
  • run for character syntax
  • read-eval-print for character syntax

  List syntax expressions can be interpreted with run-list-syntax, which exercises unparse-program as well as parse-program.

  (define run-list-syntax
    (lambda (datum)
      (pretty-print
        (unparse-program (parse-program datum)))
      (display "  => ")
      (eval-program (parse-program datum))))
  

  We would like to maintain the ability to parse and run list syntax as well as character syntax in our interpreters.

• Adding conditional evaluation necessitates adding an if-exp expression and a definition of boolean values.

  The text uses 0, 1 for boolean values, but we will introduce a bool type and corresponding expression.

  (define-datatype bool bool?
    (true-value)
    (false-value))
  

  In interpreting boolean values, we can use

  (define true-value?
    (lambda (x)
      (cases bool x
        (true-value () #t)
        (false-value () #f)
        (else
  	(error 'bool "not a bool" x)))))
  

  The clause for if-exp in eval-expression then becomes

  (if-exp (test-exp true-exp false-exp)
    (if (true-value? (eval-expression test-exp env))
        (eval-expression true-exp env)
        (eval-expression false-exp env)))
  

  An example primitive equal? which has bool value is also added, to give an example of adding a primitive.

  Modifications in the grammar are needed to parse character syntax conditional expressions and boolean constants true and false, say to grammar3-3.

  We also need to modify

  • eval-expression
  • eval-expresssion
  • expression
  • parse-expression
  • unparse-expression

• The result is interp3-3.scm
  You can use it as a guide for extending interp3-1, but use it only to help correct errors.

Continue working on Assignment 4.
Maintain separate interpreters, interp3-1, interp3-3, interp3-4, etc