Solution Files

You can find the solutions in hw09.scm.

Macros are a method of programming that allow programmers to treat expressions as data and create procedures of a language using the language itself. Macros open the door to many clever tricks of creating "shortcuts", and with Scheme, allow us to build our own special forms other than the ones that are built in.

Required Questions

Interpreters

Q1: WWSD: Eval and Apply

How many calls to scheme_eval and scheme_apply would it take to evaluate each of these Scheme expressions?

You may find the Interpreters Study Guide helpful.

Use Ok to test your knowledge by writing the number of calls needed to evaluate each expression:

python3 ok -q wwsd-eval_apply -u

scm> (+ 2 4 6 8) ; number of calls to scheme_eval
______
6
scm> (+ 2 4 6 8) ; number of calls to scheme_apply
______
1
scm> (+ 2 (* 4 (- 6 8))) ; number of calls to scheme_eval
______
10
scm> (+ 2 (* 4 (- 6 8))) ; number of calls to scheme_apply
______
3
scm> (if #f (+ 2 3) (+ 1 2)) ; number of calls to scheme_eval
______
5
scm> (if #f (+ 2 3) (+ 1 2)) ; number of calls to scheme_apply
______
1
scm> (define (cube a) (* a a a)) ; number of calls to scheme_eval
______
1
scm> (define (cube a) (* a a a)) ; number of calls to scheme_apply
______
0
scm> (cube 3) ; number of calls to scheme_eval
______
8
scm> (cube 3) ; number of calls to scheme_apply
______
2

Macros

Q2: When Macro

Using macros, define a new special form, when, that has the following structure:

(when <condition>
      (<expr1> <expr2> <expr3> ...))

If the condition is not false (a truthy expression), all the subsequent operands are evaluated in order and the value of the last expression is returned. Otherwise, the entire when expression evaluates to okay.

Hint: you may find the begin form useful.

scm> (when (= 1 0) ((/ 1 0) 'error))
okay

scm> (when (= 1 1) ((print 6) (print 1) 'a))
6
1
a

(define-macro (when condition exprs)
  `(if ,condition ,(cons 'begin exprs) 'okay)
  )

Use Ok to test your code:

python3 ok -q when-macro
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✂️

Q3: Switch

Define the macro switch, which takes in an expression expr and a list of pairs, cases, where the first element of the pair is some value and the second element is a single expression. switch will evaluate the expression contained in the list of cases that corresponds to the value that expr evaluates to.

scm> (switch (+ 1 1) ((1 (print 'a))
                      (2 (print 'b))
                      (3 (print 'c))))
b

You may assume that the value expr evaluates to is always the first element of one of the pairs in cases. You can also assume that the first value of each pair in cases is a value.

(define-macro (switch expr cases)
	(cons 'cond
		(map (lambda (case) (cons `(equal? ,expr (quote ,(car case))) (cdr case)))
    			cases))
)

Use Ok to test your code:

python3 ok -q switch
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✂️

Exam Practice

Homework assignments will also contain prior exam questions for you to try. These questions have no submission component; feel free to attempt them if you'd like some practice!

Macros

1. Fall 2019 Final Q9: Macro Lens
2. Summer 2019 Final Q10c: Slice
3. Spring 2019 Final Q8: Macros