Part 1 — Boxes
Start with 5.rkt and add box and unbox:
<Exp = ...
| {box <Exp}
| {unbox <Exp}
Your interpreter can assume that the argument to unbox is always a box.
Examples:
(reset!)
(ntest (interpx (compile
(parse `{unbox {unbox {box {box 3}}}})
mt-env)
empty-env
(init-k))
3)
(reset!)
(ntest (interpx (compile
(parse
`{{lambda {mkrec}
{{{lambda {chain}
{lambda {unchain}
;; Make a chain of boxes, then traverse
;; them:
{{unchain 13} {chain 13}}}}
;; Create recursive chain function:
{mkrec
{lambda {chain}
{lambda {n}
{if0 n
1
{box {chain {+ n -1}}}}}}}}
;; Create recursive unchain function:
{mkrec
{lambda {unchain}
{lambda {n}
{lambda {b}
{if0 n
b
{unbox {{unchain {+ n -1}} b}}}}}}}}}
;; mkrec:
{lambda {body-proc}
{{lambda {fX}
{fX fX}}
{lambda {fX}
{body-proc {lambda {x} {{fX fX} x}}}}}}})
mt-env)
empty-env
(init-k))
1)
Part 2 — Extra Credit: Box Assignment
Add set-box!:
<Exp = ...
| {set-box! <Exp <Exp}
You can make set-box! return whatever you like, and your interpreter can assume that the first argument to set-box! is always a box.
(reset!)
(ntest (interpx (compile
(parse `{{lambda {b}
{{lambda {z}
{unbox b}}
{set-box! b {+ {unbox b} 1}}}}
{box 3}})
mt-env)
empty-env
(init-k))
4)
