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CSI 3350
Homework 05
Suggested reading:
EOPL 2.4

This homework introduced you to “define-datatype”, a tool available
in the “eopl” library (meaning it comes with your textbook, and its
not part of the standard scheme language). We discussed it to length
in our regular lecture time on Nov 4.
Essentially, define-datatype is a more sophisticated way that
allows you to easily define recursive datatypes, it automatically
generates:
– a constructor for each variant
– one predicate for the top level datatype, e.g. Env?, it doesn’t
generate one predicate per variant.

> (#%require (lib “eopl.ss” “eopl”))
;define-datatype takes two top level parameters, its detailed
structure is explained below:
> (define-datatype name-of-datatype name-of-predicate
;each variant is described by a list of field-name predicate
;pairs
;here f1, f2 have to be numbers
;f3 is a list-of numbers, please refer to the “list-of” function
;from the “eopl” library for more information (you can look it
;up in the online racket index)
(variant1 (f1 number?)
(f2 number?)
(f3 (list-of number?))
)

;as you can notice, the fields may be named as you please, and you
;can use custom predicates to describe them
(variant2 (i-can-give-the-fields-whatever-name-I-want string?)
(n-or-s number-of-string?)
;it is possible to use the predicate of the data-type we
;are currently defining
(field-of-this-datatype name-of-predicate)
)
(empty-variant)
)

(define (number-of-string? x)
(or (number? x) (string? x))
)
The role of extractors and predicates is taken over by the “cases”,
syntactically it resembles a “cond” expression:
(define (fun adt)
(cases name-of-datatype adt
(variant1 (f1 f2 f3) #| expression that can use f1 f2 f3, each of
which are bound to the values of the three
fields of this variant |#
(+ f1 f2))

(variant2 (dont-have-to-use-same-name-as-in-the-definition
but-it-is recommended) 42)
(else (raise “If you leave out any variants from the cases
expression then the else clause has to be present”))
)
)
Please read section 2.4. very carefully as define-datatype is specific
to our textbook (EOPL).
NOTE: Please experiment with “define-datatype” and
“cases” thoroughly because we will be building our interpreter using
these two expressions extensively.
IMPORTANT:
The test cases will not compile until you implement your datatype
using “define-datatype”; currently all the test cases are commented
out.
====================================================================
1. [40p] Step revisited, again
Recall the <step> data-type:
<step> ::= <step> <step> “seq-step”
| “up” number “up-step”
| “down” number “down-step”
| “left” number “left-step”
| “right” number “right-step”
1.a
Re-implement the interface using define-datatype.
1.b
Implement the function:
(move starting-point step)
Input:
– starting-point: a point in the x,y coordinate system, represented
as a 2-element list
– step: a step, as defined in 1.a

Output:
– the end-point after moving the specified number of steps

For this exercise you *are required* to use the “cases” expression in
the implementation of move. From here on out, when using
define-datatype, we will no longer write entire interfaces unless
necessary. The cases expression will do the job that was previously
assigned to predicates and extractors.
=====================================================================
2. [60p] Define-datatype implementation of environment
Recall the <environment> data type described in the lecture slides and
in your textbook:
<environment> ::=
“empty-env”
| “extend-env” symbol value <environment>
(empty-env)
initializes an environment

(extend-env)
adds a new mapping from “symbol” to “value” in the old environment.
If the same symbol is added twice, then the previous mapping is not
replaced, it is only shadowed.
This data-type is accompanied by the function:
(apply-env env sym)
this function will return the “value” mapped to “sym” in the given
environment; it raises an exception if no such mapping exists.
2.a [20p]
Implement the environment using define-datatype.

2.b [40p]
Implement a procedure (extend-env-wrapper sym val old-env final?)
Compared to the constructor “extend-env” it has a boolean parameter
“final?” indicating whether or not the new mapping should be
shadow-able.
Whenever an environment is extended using the “final” variant, the
symbol from its corresponding mapping cannot be shadowed, any attempt
to shadow the value of a “final” symbol will result in an error.

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