This is another introductory homework, and again it is for individual work and submission. In this homework you will be introduced to the course language (which is based on Typed Racket) and some of the additional extensions in this language.
In this homework (and in all future homework) you should be working in
the “Racket language” — in the language selection dialog
(Ctrl+L/Command+L) choose the topmost option. In this mode (which is
the default for most Racket work), the actual language is determined by
#lang line that names the language. In our case, this will
#lang pl for course work, and more specifically, it will be
#lang pl 02 for a particular homework.
You should also click the “Show Details” button at the bottom of the
language selection dialog, and check the “Syntactic test suite coverage”
option to see parts of your code that are not covered by tests. With
this option on, parts of the code that were not covered by tests will be
highlighted after you click “run”, and if you have complete coverage,
then the colors will stay the same. Note that you can also set the
default language that is inserted into new programs to
#lang pl (also
in the details of the language dialog), to make things more convenient.
There are some other variants of the
pl language for various purposes
— in particular,
#lang pl untyped will ignore all type declarations,
and will essentially run your code in plain untyped Racket.
The language for this homework is:
Note that this language is basically a restriction of the
language that we’re using in class. Such languages would be available
for future homework too. They will usually be similarly restricted
subsets of the course language, but some will be very different (you
will be told about it, of course). Note that the server will require a
specific language, so if you’re developing your solution with
or any other dialect, you should switch to the appropriate language
submission to avoid server barfs.
As shown in class, this language has a new special form for tests:
test. It can be used to test that an expression is true, that an
expression evaluates to some given value, or that an expressions raises
an error with some expected text message. For example, the three kinds
of tests are used in the following:
Note the second test: when you’re testing predicates, don’t use
=> #f since you can just use either the expression or it’s
In case of an expected error, the string specifies a pattern to match
against the error message. (Most text stands for itself, “
a single character and “
*” matches any sequence of characters.)
Note that the
=error> facility checks only errors that your code
throws, not Racket errors. For example, the following test will not
The server will require all bindings to be present; if you don’t know how to solve a problem, write some bogus definition and indicate this in a clear comment.
Warning: the submission server will be less restrictive this time, and will not complain if you submit buggy code; however, the fact that the language is statically typed will compensate for many bugs. Still, you should use proper tests to avoid bugs. Remember to test all code, and to write proper types, otherwise your code will be rejected by the server.
Reminder: code quality will still be graded (now and in the future). Write clean and tidy code. Consult the Style Guide, and if something is unclear, ask questions on Piazza.
Write a function called
sequence that receives a function f, and
two values first and last. The function should return a list of
values that starts with first and ends with last, and for each
consecutive pair of values a1, a2 in the list,
results in a2 — that is,
(equal? (f a1) a2) should
A few test cases will clarify further how this function works:
You can also use these tests in your code (but do add more tests).
Side note: you cannot avoid bad uses of this function, where the final
value is one that you will not reach, such as
(sequence add1 1 0).
This is essentially the halting problem: there is no way to forbid it in
the type declaration, no way to test for it at runtime, and no good way
to test for it.
Note also that Typed Racket cannot properly infer types for uses of
polymorphic higher order functions, so if you want to try this function
with a list function, you need to use
inst to “instantiate” a
polymorphic function with a specific type. For example:
(It is also possible by providing a redundant function just for its type, for example
inst is more convenient.)
Pay attention to the type you declare for
sequence — if you use
(Number -> Number) for the first argument you will
reduce the utility of this function, since it can only work with numeric
functions. Your solution must also be usable with other types too, as
shown in the above tests.
Note, however, that Typed Racket has subtypes — and this can lead to a different extreme where the type declaration is too general:
This is also not a good type declaration, because it provides very little information about the function. Specifically, it will not allow you to use even apply the first argument, since it is not known that it is a function.
A proper type will need to be a polymorphic function: the type of
sequence should refer to “some type
A” in the following way:
In this question you will implement a type that is often useful: a possibly sparse set of integers. For the purpose of this question, we represent such sets as one of:
Note that for simplicity there are no empty sets.
Begin by a type definition. Fill in the following template (
a valid Racket identifier):
define-typeis something that is specific to the
#lang pllanguage, do not confuse it with the same syntax from Typed Racket. See how we used it in the type definition for the
AElanguage implementation that we went over in class.
An integer set is said to be in normal form if no numbers are repeated
(singleton numbers and ranges have no overlap), if all ranges are formed
from two numbers where the first is smaller than the second, when
2Sets unions are also in order (the set on the left side is completely
below the set on the right), and there is some gap between the two
subsets in a
2Set value. In other words, a normalized integer set is
one that is completely ordered and has the shortest number of elements.
intset-normalized? predicate that checks this. Some
examples of intsets that are not normalized:
It is relatively easy to define this function given two utility
intset-max, that return the smallest and
biggest members of an
INTSET respectively. But these two functions
are quite similar, so implement a third function,
can return either result, based on an comparator function. Your
intset-min/max should be implemented such that the other two utility
functions are defined as follows:
Note that using this is not going to be too efficient; specifically,
each side of a
2Sets construct is scanned twice (which makes the
run-time asymptotically worse than a single scan — not just double!).
Finding an implementation that performs a single scan is more difficult,
however, so it is not necessary. But this does not mean that you should
neglect common sense — for example, don’t do more scans than needed.
Remember: correct type, purpose statement, and tests. Note that since
the main functionality in the above is
intset-normalized?, then that’s
what should be tested — do not test the helper functions directly.
Note that the only thing you can do with a type is pattern-match over it with
casesis very similar to
match, only restricted to types that we define using
define-type, and in addition you cannot do anything with values of these types other than using them in
cases— you cannot use them in
matchpatterns. Read about it more in the class notes. Again, refer to the
AEexample in the class notes to see how we used it in class.
casesfunctionality would be folded into
match, but achieving this has some technical limitations that we could never resolve.]
INTSET type is useful, for example, in representing a range of
pages to print: a comma separated sequence of integers and integer
ranges (two integers separated by a dash). For example,
Write a BNF for such specification. Write the grammar in a
comment. Don’t call it
<INTSET> since that name relates to the
implementation, call it
<PAGES>. Also, make sure that it is
For simplicity, you can use
<int> as a known terminal, so there’s no
need to specify the syntax for integers. Note that you can use “
as shown in class, but it won’t make the BNF shorter, so you can just as
well avoid using it.
minutes-spent as the number of minutes you spent on your