Part 1 — Booleans

Start with the interpreter with function values, and extend the implementation to support boolean literals, an equality test, and a conditional form:

<Exp = ....
| true
| false
| {= <Exp <Exp}
| {if <Exp <Exp <Exp}
The = operator should only work on number values, and if should ony work when the value of the first subexpression is a boolean. Report a “not a number” error if a subexpression = produces a non-number value, and report a “not a boolean” error when the first subexpression of if does not produce a boolean. The if form should evaluate its second subexpression only when the first subexpression’s value is true, and it should evaluate its third subexpression only when the first subexpression’s value is false. True and false are always spelled true and false, not #t or #f.

Note that you not only need to extend Exp with new kinds of expressions, you will also need to add booleans to Value.

For example,

true
should produce a true value, while

{if true {+ 1 2} 5}
should produce 3, and

{if 1 2 3}
should report a “not a boolean” error.

As usual, update parse to support the extended language.

More examples:

(test (interp (parse `{if {= 2 {+ 1 1}} 7 8})
mt-env)
(interp (parse `7)
mt-env))
(test (interp (parse `{if false {+ 1 {lambda {x} x}} 9})
mt-env)
(interp (parse `9)
mt-env))
(test (interp (parse `{if true 10 {+ 1 {lambda {x} x}}})
mt-env)
(interp (parse `10)
mt-env))
(test/exn (interp (parse `{if 1 2 3})
mt-env)
"not a boolean")
Part 2 — Hiding Variables
Add an unlet form that hides the nearest visible binding (if any) of a specified variable, but lets other bindings through. For example,

{let {[x 1]}
{unlet x
x}}
should raise a “free variable” exception, but

{let {[x 1]}
{let {[x 2]}
{unlet x
x}}}
should produce 1.

As before, you must update the parse function.

Some examples:

(test/exn (interp (parse `{let {[x 1]}
{unlet x
x}})
mt-env)
"free variable")
(test (interp (parse `{let {[x 1]}
{+ x {unlet x 1}}})
mt-env)
(interp (parse `2) mt-env))
(test (interp (parse `{let {[x 1]}
{let {[x 2]}
{+ x {unlet x x}}}})
mt-env)
(interp (parse `3) mt-env))
(test (interp (parse `{let {[x 1]}
{let {[x 2]}
{let {[z 3]}
{+ x {unlet x {+ x z}}}}}})
mt-env)
(interp (parse `6) mt-env))
(test (interp (parse `{let {[f {lambda {z}
{let {[z 8]}
{unlet z
z}}}]}
{f 2}})
mt-env)
(interp (parse `2) mt-env))
Part 3 — Thunks
A thunk is like a function of zero arguments, whose purpose is to delay a computation. Extend your interpreter with a delay form that creates a thunk, and a force form that causes a thunk’s expression to be evaluated:

<Exp = ....
| {delay <Exp}
| {force <Exp}
A thunk is a new kind of value, like a number, function, or boolean.

For example,

{delay {+ 1 {lambda {x} x}}}
produces a thunk value without complaining that a function is not a number, while

{force {delay {+ 1 {lambda {x} x}}}}
triggers a “not a number” error. As another example,

{let {[ok {delay {+ 1 2}}]}
{let {[bad {delay {+ 1 false}}]}
{force ok}}}
produces 3, while

{let {[ok {delay {+ 1 2}}]}
{let {[bad {delay {+ 1 false}}]}
{force bad}}}
triggers a “not a number” error.

More examples:

(test/exn (interp (parse `{force 1})
mt-env)
"not a thunk")
(test (interp (parse `{force {if {= 8 8} {delay 7} {delay 9}}})
mt-env)
(interp (parse `7)
mt-env))
(test (interp (parse `{let {[d {let {[y 8]}
{delay {+ y 7}}}]}
{let {[y 9]}
{force d}}})
mt-env)
(interp (parse `15)
mt-env))
Note: In plain Racket, delay creates a promise. A promise is like a thunk, but when a promise is forced multiple times, the result from the first time is remembered and returned all the other times, so that the expression in a promise is evaluated at most once. We’re not concerned with that facet of delay and force, for now.