# Homework 5: Real World Haskell

CIS 194: Homework 5
Due Tuesday, October 4

The general remarks about style and submission from the first week still apply.

## Exercise 0: Install Haskell locally

From this week on, we will work locally. Install Haskell (with GHC version 7.10.3), and make sure you can open files in the editor.

## Exercise 1: Lists, lists, lists

This exercise is mostly about discovering the functions provided by the Prelude and other modules such as Data.List, Data.Char, Data.Ord, Data.Function. Try to implement the following functions while making best use of the provided library functions. None of these should require you to write a recursive function! Use the given examples to understand the function better, if required.

1. halveEvens :: [Integer] -> [Integer]

From a list of integers, remove any odd entry and halve every even entry.

ex_halveEvens =
[ halveEvens [] == []
, halveEvens [1,2,3,4,5] == [1,2]
, halveEvens [6,6,6,3,3,3,2,2,2] == [3,3,3,1,1,1]
]
2. safeString :: String -> String

In a string, replace every character that is a control character or not an ASCII character by an underscore. Use the Data.Char module.

ex_safeString =
[ safeString [] == []
, safeString "Hello World!" == "Hello World!"
, safeString "That’s your line:\n" == "That_s your line:_"
, safeString "🙋.o(“Me Me Me”)" == "_.o(_Me Me Me_)"
]
3. holes :: [a] -> [[a]]

Given a list, return the a list of lists that contains every list that is obtained by the original list by removing one element, in order. (The examples might be more helpful).

ex_holes =
[ holes "" == []
, holes "Hello" == ["ello", "Hllo", "Helo", "Helo", "Hell"]
]
4. longestText :: Show a => [a] -> a

Given a non-empty list, find the entry for which show results the longest text shown. If there are ties, prefer the last one.

ex_longestText =
[ longestText [True,False] == False
, longestText [2,4,16,32] == (32::Int)
, longestText (words "Hello World") == "World"
, longestText (words "Olá mundo") ==  "Olá"
5. adjacents :: [a] -> [(a,a)]

Pair each element with the next one in the list.

ex_adjacents =
[ adjacents "" == []
, adjacents [True] == []
, adjacents "Hello" == [('H','e'),('e','l'),('l','l'),('l','o')]
]
6. commas :: [String] -> String

Add commas between strings.

ex_commas =
[ commas [] == ""
, commas ["Hello"] == "Hello"
, commas ["Hello", "World"] == "Hello, World"
, commas ["Hello", "", "World"] == "Hello, , World"
, commas ["Hello", "new", "World"] == "Hello, new, World"
]
7. addPolynomials :: [[Integer]] -> [Integer]

Given coefficients to polynomial equations as lists of the same length, output the coefficients for the sum of these equations.

You may assume that at least one polynomial is given.

ex_addPolynomials =
[ addPolynomials [[]] == []
, addPolynomials [[0, 1], [1, 1]] == [1, 2]
, addPolynomials [[0, 1, 5], [7, 0, 0], [-2, -1, 5]] == [5, 0, 10]
]
8. sumNumbers :: String -> Integer

Output the sum of all natural numbers contained in the given string. A natural number in this sense is any maximal subsequence of digits, i.e. one that is neither preceded nor followed by an integer. (The examples should provide more clarification.)

ex_sumNumbers =
[ sumNumbers "" == 0
, sumNumbers "Hello world!" == 0
, sumNumbers "a1bc222d3f44" == 270
, sumNumbers "words0are1234separated12by3integers45678" == 46927
, sumNumbers "000a." == 0
, sumNumbers "0.00a." == 0
]

## Exercise 2: Word count

Write a function

wordCount :: String -> String

that returns a few statistics on the input string. Some of the functions above might be useful.

Use the following example output as specification:

Number of lines: 23
Number of empty lines: 10
Number of words: 40
Number of unique words: 25
Number of words followed by themselves: 3
Length of the longest line: 5

A line and a word is what lines respectively words return.

Note that if you define main = interact wordCount and compile that, you have create a generally useful program!

## Exercise 3: Test suite

Exercise 1 defines test cases for each function, which are conveniently all of type [Bool]. Copy them into your file. Also add and complete the following definition:

testResults :: [(String, [Bool])]
testResults = [ ("halveEvens",      ex_halveEvens)
, ("safeString",      ex_safeString)
, ("holes",           ex_holes)
…
]

Write a function

formatTests :: [(String, [Bool])] -> String

which presents the data nicely. Here is a possible output (but feel free to be more creative):

halveEvens: 3/3 successful tests
safeString: 1/3 successful tests. Failing tests: 1, 3 and 4
holes: All 2 tests failed.

Define main to print the string returned by formatTests applied to testResults.

(Naturally, all your tests are failing. You can add some bogus data to testResults to test your formatTests function.)