difference between 2.2.2 and 2.2.3

[Andy]

Hi,

I am a novice computer enthusiast studying eopl 3rd edition these days. Could anyone please point out the difference between procedural and data structure representation of the environment interfaces, explained in the sections mentioned in the subject? I am having a hard time trying to come up with an solution as I have experience only in Ocaml and am new to Scheme. Please help.

[Dan Friedman]

You can represent an environment, a procedure that maps symbols to values

as a function for example, the roman environment is

(define roman-fun

  (lambda (y)

    (cond

      ((eq? y 'I) 1)

      ((eq? y 'V) 5)

      ((eq? y 'X) 10)

      ((eq? y 'L) 50)

      ((eq? y 'C) 100)

      ((eq? y 'D) 500)

      ((eq? y 'M) 1000)

      (else (error)))))

And then define lookup like this

   (define lookup

      (lambda (y env)

         (env y))))

> (lookup 'X roman-fun)  ==> 10

> (lookup 'Z roman-fun)  ==> error.

or as a data structure:

(define roman-data

  '((I . 1) (V . 5) (X . 10) (L . 50) (C . 100) (D . 500) (M . 1000))

(define lookup

  (lambda (y env)

    (cond

      ((null? env) (error))

      ((eq? (car (car env)) y) (cdr (car env)))

      (else (lookup y (cdr env)))))  

> (lookup 'X roman-data) ==> 10

> (lookup 'Z roman-data) ==> error.

... Dan

On Wed, Aug 14, 2013 at 8:35 AM, Andy <nuv...@gmail.com> wrote:

Hi,

I am a novice computer enthusiast studying eopl 3rd edition these days. Could anyone please point out the difference between procedural and data structure representation of the environment interfaces, explained in the sections mentioned in the subject? I am having a hard time trying to come up with an solution as I have experience only in Ocaml and am new to Scheme. Please help.

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