Exercises for Week 1: Getting Started with Haskell
This document is designed as introduction to Haskell and the interactive Haskell interpreter GHCi. You are supposed to go through this document using a computer. It's meant to be pretty simple stuff – feel free to skip over some exercises.
You are supposed to work through this document in an afternoon. The
most important tasks are to answer questions that look like this:
GHCi as a simple calculator
GHCi is an Haskell interpreter, i.e. it is a program that evaluates Haskell expressions.
You can start GHCi by typing ghci
in a terminal.
ghci
We will use this style
for the text that you as user of GHCi have to enter.
When you start GHCi you will see this welcome message:
GHCi, version 8.0.1: http://www.haskell.org/ghc/ :? for help
Right now, we don't care about the precise meaning of the text in the
welcome
message, but focus on the last line with the >
symbol.
The >
symbol indicates that GHCi is ready to receive a command. We will
explain later on what the word Prelude
means.
The simplest command one can give to GHCi is an Haskell expression, given an Haskell expression GHCi will evaluate it, and write the result on the screen. Try:
4+5*6
34
After that, as indicated by the >
symbol, GHCi is ready to take a new command. We can repeat this interaction as many times as we want. Try:
4*(85)+3
153.5/(3+4)*4.5
2.25sin 1.57
0.9999996829318346
We don't always get a value as result when we give GHCi an expression, it can be the case that our expression contains an error... Try:
The first expression contains a syntax error; there is no right operand of the multiplication operator (3+5*
<interactive>:1:5: error: parse error (possibly incorrect indentation or mismatched brackets)1 `div` 0
*** Exception: divide by zero
*
). In the second expression we try to divide by zero.
Recognizing different kinds of errors and knowing their causes will
take some time to learn!More GHCi commands
We use the :q command to finish a GHCi session.:q
Leaving GHCi.
All GHCi commands except evaluating
an expressions start with a colon :
.
Restart GHCi and try to answer the following questions by inspecting the welcome message.
Two simple examples
A programmable pocket calculator
We have seen how we can use GHCi to do simple calculations. But this is just the beginning. To go further we need to define our own functions. As a first example we look at the currency conversion problem described above. If we need to repeatedly convert pounds to SEK we can define a function that does this for us. Defining a function consists of a number of steps.
 Decide upon a name for the function. A name should be chosen that
is suggestive of it's purpose, and
does not shadow a function currently in scope. For this example we chose
the name
pound.
 Decide which arguments (input) the function has, and what the
result (output) of the function is. In this case this is simple:
 Input: a floating point number representing an amount of crowns.
 Output: a floating point number representing an an amount of pounds.
 Decide upon a name for the parameters of the function that
indicates the usage of the argument. In this case we use the name
kr
as parameter.  Express the result in terms of the parameters. In this case
the result is:
kr/12.7775
pound.  Decide upon the type of the function. In this case:
pounds:: Double > Double
 Write the complete Haskell function definition. In this case:
pounds :: Double > Double
pounds kr = kr/12.7775

Use an editor (for example Emacs or nedit) to write the function
definition in a file. Do this, start Emacs with the command
emacs ex1.hs &
in a terminal, and write the definition above. Save this to a file with name ex1.hs 
Give GHCi a command to load the file
:l ex1.hs
Compiling Main ( ex1.hs, interpreted ) Ok, modules loaded: Main.
pounds 1000
78.2626pounds 12345
966.151pounds 127775
10000.0
pount 10
(note the spelling error)?
What happens if one writes:
pounds kr
?
Comparison and more complex functions
Up till now we have defined functions by equations of the form:name(parameter) = an expressions containing the parameter, numbers and arithmetic operations.
It is not always this easy, consider the following example:
A shop sells potatoes for 3.50 SEK/kg. To stimulates large sales, the shop offers a reduced price of 3 SEK/kg for the quantity exceeding 10 kg.Let's call this functionWe want to define the function that calculates the price.
price
and the parameter v
.
 When
v
is at most 10, then the price is3.50*v
.  If
v
is more than 10, the price is35+3*(v10)
(check this!) which is equal to5+3*v
.
price(v) = 3.5*v
, if v<=10
price(v) = 5+3*v
, if v>10
How does one write this in Haskell? First, notice that it is possible to compare numbers.Try:
The result of such a comparison is either3 < 6
True3.5 <= 3
Falsepounds 4000 > 3*100
True
True
or False
. Notice, that smaller than or equal to is written
as <=
.
Now, we can write the definition of price as follows:
price :: Double > Double price v  v <= 10 = 3.5*v  v > 10 = 5+3*vThe Boolean expressions between

and =
are called guards. When this function is
applied to a particular argument, GHCi checks the guards from
top to bottom until it finds one that evaluates to True
,
after that the righthand side of the expression is used to calculate
the result.
If all guards evaluate to False
GHCi gives an error message.
Load this function into GHCi, and test the function price on different
arguments.
v > 11 = 5+3*vWhat is the result of applying the function to 9.5, 10.5 respective 11.5?
price vWhat is the now result of applying the function to 9.5, 10.5 respective 11.5?
v <= 11 = 3.5*v
v > 10 = 5+3*v
otherwise
as a guard that is always
true. So, if otherwise
is used as the last guard, the
expression at the righthand side will be used if none of the other
guards evaluates to True
.So, we can write:
price :: Double > Double price v v <= 10 = 3.5*v otherwise = 5+3*v
Is otherwise
a Haskell keyword? What does Hoogle say?
When a function is applied to a simple argument (like 5) we don't need parentheses. But if the argument is composed (like 4 + 8) parentheses are needed.
price 7.5
26.25price 11
38.5price (4+8)
41.0
Functions with more than one argument
Functions can have more than one argument. As an example we define a function that defines the average of two numbers: The name of the function:
average
.  Input: Two floating point numbers.
 Output: A floating point number giving the average of the two arguments.
 Names of the parameters:
x
andy
.  The result in terms of the parameters:
(x+y)/2
 The type:
average :: Double > Double > Double

The complete definition:
average :: Double > Double > Double average x y = (x+y)/2
average 5 8
6.5
The Prelude
Some functions are very frequently used and therefore predefined in a module (file) called the Prelude, which is loaded automatically every time GHCi is started. An overview of the functions defined can be found in e.g., A Tour of the Haskell Prelude.To be able to program efficiently in Haskell it is necessary to know which functions are available in the Prelude. In the coming weeks you will become more familiar with them.
Operators and functions with two arguments
Operators like *,,+ are written infix, which means that the operator should be in between its arguments. Functions are usually written prefix, meaning that they are written before their arguments. However, one can use a function infix by writing it between ` `. An operator can be used prefix by writing it in between parentheses. Try:5 `average` 4
4.54 `max` (5 `max` 2)
5(+) 3 4
7
Integers
The prelude provide the functiondiv
for integer division.
Try:
The functiondiv 17 5
3div 34 8
4div 5 9
04 `div` 2
2
mod
gives the remainder for integer division:
17 `mod` 5
234 `mod` 8
25 `mod` 9
5mod 4 2
0
You will have to use relational operator
==
which evaluates to
True
if its arguments are equal.
A bigger example
Consider the following game:
Think of an whole number greater than one. If its even, divide it by two, otherwise multiply it by three and add one. Stop if the resulting number is one, otherwise repeat the procedure.As example, we start width 10.
 10 is even, so the next number is: 10/2 = 5
 5 is odd so the next number is: 3*5+1=16
 16 is even, so the next number is: 16/2 = 8
 8 is even, so the next number is: 8/2 = 4
 4 is even, so the next number is: 4/2 = 2
 2 is even, so the next number is: 1.
If we start with 7 we get: (Check this!)
7, 22, 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1.
We are interested in the question: Given a number n, how many numbers are there in the sequence? For n=10, we get 7 numbers (10, 5, 16, 8, 4, 2, 1). For n=7 we get 17 numbers (See above).
Note that we include both the number n we start with and the final
number 1.
How can GHCi help us to answer the question? We start with defining
a function next
that given a number computes the next number
in the sequence.
steps
that takes a number n as argument
and calculates the length of the generated sequence. What is steps
20
? And steps 3
?
steps :: Int > Int steps n  n == 1 = 1  otherwise = steps(next n)+1We call this a recursive definition. (You should have met this concept before  otherwise ask for a refund on your undergraduate education!) Write the definition in your file. Load it in to GHCi, and calculate
steps
n for several different values of n.
It all seems to work without any problem. But at the same time one can wonder:
Finally, we define the functionnumbers
that calculates the
actual list of numbers generated in the game:
numbers :: Int > [Int] numbers n  n==1 = [1]  otherwise = n : numbers(next n)Load the function into GHCi and try:
Study the function definition: When n=1 the result is the listnumbers 10
[10, 5, 16, 8, 4, 2, 1]numbers 17
[17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]numbers 1
[1]
[1]
. Otherwise we use the operator :
,
named cons, which constructs a list from an element and a list by placing
the element at the front of the list.
The result of numbers n
is n
followed by the list numbers(next n)
.
Now we have defined the function numbers we don't need to define steps anymore because there is a function length in the prelude that calculates the length of a list. Try:
length [1,6,33,8,7,14]
6length (numbers 10)
7