CS61A Homework 7-Scheme Data Abstractions, Programs as Data Solution

Instructions
Download hw07.zip. Inside the archive, you will find a file called hw07.scm, along with a copy of the ok autograder.
Using Ok: If you have any questions about using Ok, please refer to this guide.
Readings: You might find the following references useful:
Scheme Specification
Scheme Built-in Procedure Reference
Grading: Homework is graded based on correctness. Each incorrect problem will decrease the total score by one point. There is a homework recovery policy as stated in the syllabus. This homework is out of 2 points.
Scheme is a famous functional programming language from the 1970s. It is a dialect of Lisp (which stands for LISt Processing). The first observation most people make is the unique syntax, which uses a prefix notation and (often many) nested parentheses (see http://xkcd.com/297/). Scheme features first-class functions and optimized tail-recursion, which were relatively new features at the time.
Scheme Editor
As you're writing your code, you can debug using the Scheme Editor. In your scheme folder you will find a new editor. To run this editor, run python3 editor. This should pop up a window in your browser; if it does not, please navigate to localhost:31415 and you should see it.
Make sure to run python3 ok in a separate tab or window so that the editor keeps running.
If you find that your code works in the online editor but not in your own interpreter, it's possible you have a bug in code from an earlier part that you'll have to track down. Every once in a while there's a bug that our tests don't catch, and if you find one you should let us know!
Required Questions Getting Started Videos
YouTube link
Keyword Lists
In the following problems, you will explore creating two separate implementations for the same abstraction.
A keyword list is the Scheme analogue of a dict in Python, with a few key differences:
It allows for repeating keys
It functions as a list as well, which allows for ordering.
The kwlist abstraction keeps a mapping of keys and values. To create a kwlist, call the constructor (make-kwlist keys values) where keys is a Scheme list of symbols and values is a Scheme list of any type. This returns some abstracted item lst that we can call the following methods to either retrieve or add items:
py scm> (define lst (make-kwlist '(x y z) '(7 8 9)) ; create the keyword list lst scm> (get-first-from-kwlist lst 'x) ; get an item 7 scm> (define lst (add-to-kwlist lst 'a 10)) ; add a new item lst scm> (get-first-fromkwlist lst 'a) ; get the new item. 10
Q1: Keyword List: Construct
First, implement abstractions for kwlist in two ways, with the following example: (kwlist '(x y z) '(7 8 9))
1. kwlist1, which stores a keyword list in the following manner: ((key1 key2 key3 ...) (value1 value2 value3 ...). With the example above, this should look like ((x y z) (7 8 9)).
2. kwlist2, which stores a keyword list in the following manner: ((key1 value1) (key2 value2) ...). With the example above, this should look like ((x 7) (y 8) (z 9)).
Specifically, implement constructors and selectors for kwlist1 and kwlist2.
The constructors, make-kwlist1 and make-kwlist2, should take in Scheme lists for both keys and values, and construct the abstraction as above.
The selectors, get-keys-kwlist1, get-keys-kwlist2, get-values-kwlist1, and get-values-kwlist1, should take in a kwlist1 or kwlist2 and return their keys and values respectively. Note that because you are currently creating the implementation, you are "under the abstraction barrier;" feel free to refer to specific details of the structure of kwlist1 and kwlist2.
py scm> (define ex-lst1 (make-kwlist1 '(a b c) '(1 2 3))) ex-list scm> (get-keys-kwlist1 ex-lst1) (a b c) scm> (get-values-kwlist1 ex-lst1) (1 2 3) scm> (define ex-lst2 (make-kwlist2 '(a b c) '(1 2 3))) ex-list scm> (getkeys-kwlist2 ex-lst) (a b c) scm> (get-values-kwlist2 ex-lst) (1 2 3)
```py (define (make-kwlist1 keys values) 'YOUR-CODE-HERE )
(define (get-keys-kwlist1 kwlist) 'YOUR-CODE-HERE )
(define (get-values-kwlist1 kwlist) 'YOUR-CODE-HERE ) (define (make-kwlist2 keys values) 'YOUR-CODE-HERE )
(define (get-keys-kwlist2 kwlist) 'YOUR-CODE-HERE ) (define (get-values-kwlist2 kwlist) 'YOUR-CODE-HERE ) ```
Use Ok to test your code:
py python3 ok -q kwlist_construct
Important: For the following questions, your implementations should be invariant with respect to the abstraction used; that is, it should work regardless of whether kwlist1 or kwlist2 is used. Specifically, in the tests, we will define the abstraction kwlist as either kwlist1 or kwlist2:
py scm> (define make-kwlist make-kwlist1) scm> (define get-keys-kwlist get-keys-kwlist1) scm> (define get-values-kwlist get-values-kwlist1) ; tests here... scm> (define make-kwlist make-kwlist2) scm> (define get-keys-kwlist get-keys-kwlist2) scm> (define get-values-kwlist get-values-kwlist2) ; tests here...
You should refer to the above kwlist procedures, not kwlist1 or kwlist2's procedures in your implementation.
Q2: Keyword List: Add
Now, implement add-to-kwlist, which implements support for adding a new (key, value) pair to any implementation of a kwlist. Specifically, add-to-kwlist takes in a kwlist, a key, and a value as input, and returns a new kwlist with updated keys and values. Note that kwlists are ordered; that is, a pair p1 that was added to a kwlist before a different pair p2 should appear earlier in the kwlist.
py scm> (define ex-lst (make-kwlist '(a b c) '(1 2 3))) ex-lst scm> (get-keys-kwlist ex-lst) (a b c) scm> (get-values-kwlist ex-lst) (1 2 3) scm> (define ex-lst (add-to-kwlist ex-lst 'd '4)) ex-lst scm> (get-keyskwlist ex-lst) ; note that new items are at the end of the list! (a b c d) scm> (get-values-kwlist ex-lst) ; here too! (1 2 3 4) py (define (add-to-kwlist kwlist key value) 'YOUR-CODE-HERE )
Use Ok to test your code:
py python3 ok -q kwlist_add
Q3: (Optional) Keyword List: Get
Now, implement get-first-from-kwlist, which implements support for getting the first value bound to a key in kwlist. If key is not present in the list, the function should return nil to indicate that there were no valid keys found.
Hint: Consider using let to temporarily bind names to values. To make your implementation work with both abstractions, be sure to use methods ending in kwlist, not kwlist1 or kwlist2.
py scm> (define ex-lst (make-kwlist '(a b c) '(1 2 3))) ex-lst scm> (get-first-from-kwlist ex-lst 'b) 2 scm> (get-first-from-kwlist ex-lst 'd) ; if not found, return nil () scm> (define ex-lst (add-to-kwlist ex-lst 'd '4)) ex-lst scm> (get-first-from-kwlist ex-lst 'b) 2 scm> (get-first-from-kwlist ex-lst 'd) 4 scm> (define exlst (add-to-kwlist ex-lst 'd '5)) ex-lst scm> (get-first-from-kwlist ex-lst 'b) 2 scm> (get-first-from-kwlist ex-lst 'd) ; return the *first* occurrence 4 py (define (get-first-from-kwlist kwlist key) 'YOUR-CODE-HERE )
Use Ok to test your code:
py python3 ok -q kwlist_get
Programs as Data
Note that the following question is separate from the previous questions.
Q4: Prune
Implement prune-expr, a procedure that takes in an expression, which is represented as a list, and returns the same expression with every other argument included. The operator should not be modified.
The behavior of prune-expr is specified by the following doctests:
py scm> (prune-expr '(+ 10 20)) (+ 10) scm> (prune-expr '(+ 10 20 30)) (+ 10 30) scm> (eval (prune-expr '(+ 10
20 30))) 40 py (define (prune-expr expr) (define (prune-helper lst) 'YOUR-CODE-HERE ) 'YOUR-CODE-HERE )
Use Ok to test your code:
py python3 ok -q prune-expr
Chef Curry
Recall that currying transforms a multiple argument function into a series of higher-order, one argument functions. In the next set of questions, you will be creating functions that can automatically curry a function of any length using the notion that programs are data!
Q5: Cooking Curry
For example, if you wanted to curry the function (lambda (x y) (+ x y)), you would set formals equal to '(x y), the body equal to '(+ x y), and make a call to curry-cook: (curry-cook '(x y) '(+ x y)).
py scm> (curry-cook '(a) 'a) (lambda (a) a) scm> (curry-cook '(x y) '(+ x y)) (lambda (x) (lambda (y) (+ x y))) py (define (curry-cook formals body) 'YOUR-CODE-HERE )
Use Ok to test your code:
py python3 ok -q curry_cook
Q6: Consuming Curry
1. If curries is an n-curried function, then there will be at most n arguments in args.
Note that there can be fewer args than formals for the corresponding lambda function curries! In the case that there are fewer arguments, curry-consume should return a curried lambda function, which is the result of partially applying curries up to the number of args provdied.
py scm> (define three-curry (curry-cook '(x y z) '(+ x (* y z)))) three-curry scm> three-curry (lambda (x) (lambda (y) (lambda (z) (+ x (* y z))))) scm> (define three-curry-fn (eval three-curry)) ; three-curry-fn is a lambda function derived from the program three-curry-fn scm> (define eat-two (curry-consume three-curry-fn '(1
2))) ; pass in only two arguments, return should be a one-arg lambda function! eat-two scm> eat-two (lambda (z) (+ x (* y z))) scm> (eat-two 3) ; pass in the last argument; 1 + (2 * 3) 7 scm> (curry-consume threecurry-fn '(1 2 3)) ; all three arguments at once 7 py (define (curry-consume curries args) 'YOUR-CODE-HERE )
Use Ok to test your code:
py python3 ok -q curry_consume
Optional Questions
Homework assignments will also contain prior exam-level questions for you to take a look at. These questions have no submission component; feel free to attempt them if you'd like a challenge!