PLQ #9

Question 1 @ 2021-03-23 18:22

Consider this segment of code which was part of our work on turning eval into a compile:

(: compile : TOY -> ENV -> VAL)
;; compiles TOY expressions to Racket functions.
(define (compile expr)
(cases expr
...
[(If cond-expr then-expr else-expr)
(lambda ([env : ENV])
(: compile* : TOY -> VAL)
(define (compile* expr) ((compile expr) env))
(compile* (if (cases (compile* cond-expr)
[(RktV v) v]
[else #t])
then-expr
else-expr)))]
...))

Which of the following is FALSE?

a

The environment is a place to only hold values at run-time.
b

We used currying to split the two arguments into the compile-time and run-time information.
c

This is a proper compiler because no compiler calls happen at runtime.
d

With this definition, work is done at compile-time because the code identifies and deconstructs the If structure, then creates a runtime closure.
e

To turn this into a proper compiler, the cond-expr should be pre-compiled.
f

To turn this into a proper compiler, the then-expr and else-expr should be pre-compiled.

Question 2 @ 2021-03-23 18:27

What is the result of the following:

#lang pl lazy
(define a (cons a null))
(length a)

1

Nothing: it gets stuck in an infinite loop.
2

Nothing: it will run for a while and abort with a stack overflow.
3

It will result in a syntax error, since the value of a is not initialized so it cannot refer to itself.
4

There is no way for this to work as-is, but if we applied the Y combinator, we would get 1.
5

1
6

0
7

An infinite list of ones: #0=(1 #0#)

PLQ #9Done on: Tuesday, March 23rd

Question 1 @ 2021-03-23 18:22

Question 2 @ 2021-03-23 18:27