Question

#lang eopl

(define-datatype env env?
  (empty-env)
  (extended-env (var symbol?)
                (val scheme-val?)
                (envi env?)))

(define (scheme-val? x) #t)

; examples
(define e-env (empty-env))
(define e1 (extended-env 'x 1 (extended-env 'y #f e-env)))

I don’t get how scheme-val? is used. Why is there an x, and why are we returning #t?
Environment is a function associating a variable with a value.

So in the example, we are associating x = 1, and y = #f, right?

Thanks.

Answer 1

In scheme, #t is means boolean true, #f being false. I don’t fully understand the rest, but I suggest DrRacket’s debugger to see what’s going on.

(define e-env (empty-env))

assigns e-env to be an empty-env

(define (scheme-val? x) #t)

defines an anonymous function that takes a parameter x, and if it is a scheme-val (in this case, anything), returns #t.

The result of e1 after this code runs is:

#(struct:extended-env x 1 #(struct:extended-env y #f #(struct:empty-env)))

so e1 is an extended-env with var='x, val=1, envi=(extended-env with var='y, val=#f, envi=e-env)

(I think)