Description
For this homework, you will practice writing functions by recursion and pattern-matching in Haskell.
You are free to write helper functions as needed for any problems.
How to Turn In Your Solution
You should create a directory called hw1 (exactly this!) in your personal repository, and add
your Haskell files to this directory. Then push the directory (and all your Haskell files) to your
github.uiowa.edu repop. You should copy the files from the hw1 directory of the course repo to
your own hw1 directory, before you start modifying them. We may release changes to the original
homework files if a bug is reported, for example, so you do not want to modify the original files,
just your copies in the hw1 directory of your personal repo.
As for hw0, you can check that you have submitted correctly by going to github.uiowa.edu in a
web browser and checking that you can see your submitted files in your repo there.
Partners Allowed
You may work by yourself or with one partner (no more). Only one partner should submit the
solution by adding the Haskell files to his/her personal repository. Both partners should submit, in
their hw1 directories, files called partner.txt. Each file contains the hawkid of the other partner.
So if jan and ben are partners, jan should include a partner.txt file that contains just the word
ben. Similarly, ben’s partner.txt file should contain just the word jan. This protocol is to ensure
that both partners agree that they are submitting the solution together. You are free to divide up
the problems and tackle them separately, or to work on problems together.
How To Get Help
You can post questions in the hw1 section on Piazza.
You are also welcome to come to our office hours. See the course’s Google Calendar, linked from
the README.md file of the github.uiowa.edu page for the class, for the locations and times for
office hours.
1
1 Reading
Read Chapters 3, 4, 5, and 6 of the required book, Programming in Haskell, by Graham Hutton.
2 Basic Problems [42 points]
In the file TableTags.hs is the start of some code dealing with the HTML tags for tables (you can
search online for “html table tags” to find basic background information – but the problems below
do not require this). There are apparently a bunch more tags than the ones we will include below,
but these are the basics.
1. Fill in the definition of the TableTag datatype to give a single constructor for each of the
table tags table, tr, th, and td. The constructor’s name should be the same as the tag
except starting with a capital letter: Table, Tr, Th, Td (because Haskell requires constructor
names to start with a capital letter). [6 points]
2. Fill in the definition of showTableTag to turn TableTags into strings for the tag name. Table
should turn into “Table”, etc. You will notice that the code I am providing you already makes
TableTag an instance of the Show class, so TableTags can be printed by ghci, once you define
showTableTage. [6 points]
3. Fill in the definition of equalTableTags to return True when the two input TableTags are
exactly the same (like Tr and Tr), and False otherwise (like Table and Td). [6 points]
4. Fill in the definition of directElt to say which tag is for an entity that can be a direct
component of which other tag [6 points]:
• a tr (table row) can be a direct element of a table
• a td (table data) can be a direct element of a tr
• a th (table header) can be a direct element of a tr
• a table can be a direct element of a td or th (this is actually allowed, it seems)
• that is it: your code should return False for all other pairs
5. Now skip ahead in TableTags.hs just a little for the following, which concern the recursive
datatype TableHtml, intended to represent the HTML for a table:
• Implement functions getSubelts, hasTag, and rowLength, according to the descriptions
in the comments above those functions. [6 points each]
3 Intermediate Problems for TableHtml [25 points]
1. Fill in the definition of showTable to turn a TableHtml object into a String. Do not add
any extra spaces or newlines to your output (this is actually easier to code, though it makes
the output pretty hard to read). So for the testTable in Tests.hs, the output should be
2
| hi | bye | ||
| open | shut | ||
2. Fill in the definition of tableOk to check that the given TableHtml is really a valid table.
The things you should check are:
• The input has Table as its table tag
• As you descend into the structure of the table, the pairs of tags you see are allowed by
the directElt function you wrote above.
• The rows of the table all have the same length
I found I had to write a couple helper functions for this. The problem is worth 15 points.
You can find some test tables in Tests.hs.
4 Intermediate Problems for SnocLists [21 points]
In SnocLists.hs, you will find a definition of a recursive datatype SList, representing lists where
the head is the second argument and the tail is the first to the Scons constructor (corresponding
to the usual (:) constructor for Haskell’s built-in lists).
1. Fill in the definitions of sappend, slength, and smap, corresponding to (++) (append),
length, and map on Haskell’s built-in lists. Each function is worth 7 points.
5 Challenge Problems [12 points]
1. Fill in the definitions of sfilter, sintersperse, and sconcat in SnocLists.hs, so that
they behave in corresponding manner to the Haskell list operations filter, intersperse,
and concat (see the documentation for these functions on Hoogle). These are worth 2 points
each.
2. In a file called More.hs (which you must create), define a function called descendings which,
given a list of elements of type a where a is in the Ord type class, return a list of lists of a
elements, consisting of the maximal descending subsequences of the input list. For example,
given [100,1,4,2,3,2,1], your function should return [[100,1],[4,2],[3,2,1]]. Getting
the correct type for this (which you should write as part of your code) is worth 2 points, and
the correct code is then 4 more points.
3
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