Tute Exercises Week 8

User-defined Data Types

A Store for Books, Videos, and CDs

Online book sellers these days usually also sell video tapes and CDs. So, they need to maintain a product database that is flexible enough to deal with all three types of goods. In the following, we will define a module BookVideoCD that contains the necessary data types to deal with such a database.

First, define a data type Product that can represent any of the three types of goods. For each good, the following additional information has to be maintained:

• Book: the title and author
• Video: the title
• CD: the title, artist, and number of tracks

Define appropriate type synonyms for titles, authors, and artists.

Extract the title information from a product

Implement a function getTitle that, given a product, extracts its title. (Note that all three types of products contain title information.)

Extract titles from a product list

Now, use getTitle to implement getTitles, which, given a list of products, produces a list with all titles. There are two ways to implement this function: with an explicit recursive definition or by using the higher-order function map. Discuss both variants and compare them.

Unix permissions

Make sure that everybody understands file permissions in Unix. What is the meaning of the triplets? What are the owner and group of a file? What does execute permission imply for directories?

Simple shell scripts

In the lecture, we briefly discussed the following simple shell script, hslines, to printing the sum of the numbers of lines of all Haskell files in the current directory:

#!/bin/sh

wc *.hs | tail -1 | cut -d ' ' -f2-2

Discuss how it works and what the individual tools wc, tail, and cut do.

Partial functions revisited

As a preparation, recall the term partial function that we discussed earlier. So far, we raised an error whenever a partial function was applied to an argument on which it wasn't defined. An alternative would be to return a special value (let's call it Nothing) whenever we previously raised an error. The advantage of that is that we can actually test for the error in the calling function, rather than aborting the whole program.

Define a data type (Maybe a) that can be used for this purpose.

Safe div

What's the result of evaluating 10 `div` 0? Implement a safe devision function

safeDiv :: Int -> Int -> Maybe Int

So that 10 `safeDiv` 0 returns Nothing, rather than aborting with an error message.