PLQ #5

Question 1 @ 2021-02-23 18:21

What should I name bar?

(define (foo x)
(lambda (y) (+ x y)))
(define bar (foo 5))
(bar 12)

We run this program in the FLANG interpreter that uses environments and closures:

;; Reminder:
(define-type ENV
[EmptyEnv]
[Extend Symbol VAL ENV])

(run "{with {x 3}
{with {f {fun {y} {+ x y}}}
{with {x 5}
{call f 4}}}}")

When we execute the body of f, what does env look like?

(Technically, the values should be in NumVs, but it’s simplified away.)

1
(EmptyEnv)
2
(Extend x 3 (EmptyEnv))
3
(Extend x 5 (EmptyEnv))
4
(Extend y 4 (EmptyEnv))
5
(Extend y 4 (Extend x 3 (EmptyEnv)))
6
(Extend x 3 (Extend y 4 (EmptyEnv)))
7
(Extend y 4 (Extend x 5 (EmptyEnv)))
8
The code will error since we don’t have functions as values, and call is clearly using f as a value.
9
Functions bodies will have their free identifiers substituted away.

What do we get if we run the following code:

(define (foobar)
(+ x y z))

(define (fun z)
(foobar))

(let* ([x 2] [y 4]) ; same as two nested `let`s
(fun 3))

(Choose the best answer.)

What happens when we run the following in Racket?

(define x (make-parameter 10))

(define (div-by y)
(/ (x) y))

(let ([x 50])
(let ([f (lambda (y) (parameterize ([x x]) (div-by y)))])
(f 5)))

Which one of the following statements is correct?