Homework Assignment 1

In this assignment, you will practice the basics of functional programming in Haskell. You

will earn total 100 points. Here are some general instructions.

1. Read the problem descriptions and requirements carefully! There may be significant

penalties for not fulfilling the requirements.

2. Some problems ask you to explain the working of your function with the given input.

Your explanation must be consistent with your definition of the function. Your work

will be graded not only on the correctness of your answer, but also on the consistency

and clarity with which you express it.

3. This homework set is an individual homework, not a team-based effort. Discussion

of the concept is encouraged, but actual write-up of the solutions must be done individually and your final product – the code as well as the comments and explanations

– should never be shared.

4. Submit electronically exactly one file named YourLastName-YourFirstName-hw1.hs,

and nothing else, on eCampus.tamu.edu.

5. Make sure that the Haskell script (the .hs file) you submit compiles without any error

when compiled using the Glasgow Haskell Compilation System (ghc or ghci) version

7 and above1

.

If your program does not compile, you will receive very few points (more likely zero)

for this assignment. To avoid receiving zero for the entire assignment, if you cannot

complete defining a function correctly without compile error, you can set the function

definition undefined, see the skeleton code provided.

6. Remember to put the head comment in your file, including your name, UIN, and

acknowledgements of any help received in doing this assignment.

1Version 7 is installed in the departmental servers (linux.cse.tamu.edu and compute.cse.tamu.edu), and

version 8 is what you will get if you install the Haskell system in your computer.

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Below, the exercise problems are from the Haskell Textbook: “Programming in Haskell,

2nd Ed.” by Graham Hutton. Some problems are modified (with additional requirements)

by the instructor. Please read textbook Chapters 1 thru 6 and the problem statements

carefully. Keep the name and type of each function exactly the same as given in the

problem statement and the skeleton code.

Problem 1. (5 points) Put your full name, UIN, and acknowledgements of any help

received in the head comment in your .hs file.

Problem 2. (10 points) The n-th Lucas number `n is recursively defined as follows: `0 = 2

and `1 = 1 and `n = `n−1+`n−2 for n ≥ 2. Write a recursive function lucas that computes

the n-th Lucas number.

lucas :: Int – Int

Problem 3. (5 + 10 = 15 points) Chapter 1, Exercise 4 (modified).

Study the definition of the function qsort given in the text carefully, and try it out with

an integer list, for example, [3,2,3,1,4]. You will notice that it sorts a list of elements

in an ascending order.

3.1 (5 points) Write a recursive function qsort1 that sorts a list of elements in a descending

order.

qsort1 :: Ord a = [a] – [a]

3.2 (10 points) Write your answer for this question in a block comment following the definition of function qsort1. Suppose that qsort1 is invoked with the input [3,2,3,1,4].

How many times is qsort1 called recursively (i.e., without counting the first invocation of qsort1 [3,2,3,1,4])? Explain step-by-step, in particular, at each level of

recursive call, what are the values of x, smaller, and larger? [Hint: Think using a

binary tree structure.]

Problem 4. (10 + 10 = 20 points) Chapter 5, Exercise 9 (modified).

The scalar product function should be able to take not only two lists of the same length n

but also those of different lengths.

4.1 (10 points) Using the list comprehension with the zip prelude function, write the scalar

product function.

scalarproduct :: [Int] – [Int] – Int

4.2 (10 points) Explain carefully step-by-step the working of your scalarproduct function

when it is invoked as follows.

*Main scalarproduct [1,2,3] [4..]

Problem 5. (10 points) Chapter 6, Exercise 7.

Like in the previous problem, the merge function should be able to take two sorted lists of

different lengths as well as those of the same length.

merge :: Ord a = [a] – [a] – [a]

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Problem 6. (8 + 7 = 15 points) Chapter 6, Exercise 8.

Defining function halve is required (hint: use splitAt prelude function).

(8 points) msort :: Ord a = [a] – [a]

(7 points) halve :: [a] – ([a], [a])

Problem 7. (10 points) Write a recursive function isElem that returns True if a given

value is an element of a list or False otherwise.

isElem :: Eq a = a – [a] – Bool

Sets are the most fundamental discrete structure on which all other discrete structures are

built. In the following problem (and continuing in the next homework set), we are going

to implement mathematical sets and their operations using Haskell lists.

A set is an unordered collection of elements (objects) without duplicates, whereas a list is

an ordered collection of elements in which multiplicity of the same element is allowed. We

define Set as a type synonym for lists as follows:

type Set a = [a]

Even though the two types – Set a and [a] – are the same to the Haskell compiler, they

communicate to the programmer that values of the former are sets and those of the latter

are lists.

Problem 8. (15 points) Write a recursive function that constructs a set from a list.

Constructing a set from a list simply means removing all duplicate values. Use isElem

from the previous problem in the definition of toSet.

toSet :: Eq a = [a] – Set a

Skeleton code and modes of running your code: The file hw1-skeleton.hs contains “stubs” for all the functions you are going to implement and placeholders for your

explanations. The Haskell function bodies are initially undefined, a special Haskell value

that has all possible types (thus, the skeleton file at least compiles).

In the skeleton file you find a test suite that test-evaluates the functions. Initially, all

tests fail, until you provide correct implementation for the Haskell function. The tests are

written using the HUnit library. Feel free to add more tests to the test suite.

The skeleton code can be loaded to the interpreter ( ghci hw1-skeleton.hs). In the interpreter mode, you can test individual functions one at a time while you are implementing

them. Evaluating the function main (by main in the interpreter mode) runs all of the

tests in the test suite.

Alternatively, you can compile the code and execute it in the terminal mode:

ghc hw1-skeleton.hs and then

./hw1-skeleton

which has the same effect as when you do main in the interpreter mode.

Have fun!

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