Table of Contents
As you read the early chapters of this book, keep in mind that we will sometimes introduce ideas in restricted, simplified form. Haskell is a deep language, and presenting every aspect of a given subject all at once is likely to prove overwhelming. As we build a solid foundation in Haskell, we will expand upon these initial explanations.
Haskell is a language with many implementations, of which two are in wide use. Hugs is an interpreter that is primarily used for teaching. For real applications, the Glasgow Haskell Compiler (GHC) is much more popular. Compared to Hugs, GHC is more suited to “real work”: it compiles to native code, supports parallel execution, and provides useful performance analysis and debugging tools. For these reasons, GHC is the Haskell implementation that we will be using throughout this book.
GHC has three main components.
How we refer to the components of GHC  

When we discuss the GHC system as a whole, we will refer to it as GHC. If we are talking about a specific command, we will mention ghc, ghci, or runghc by name. 
In this book, we assume that you're using at least version 6.8.2 of GHC, which was released in 2007. Many of our examples will work unmodified with older versions. However, we recommend using the newest version available for your platform. If you're using Windows or Mac OS X, you can get started easily and quickly using a prebuilt installer. To obtain a copy of GHC for these platforms, visit the GHC download page, and look for the list of binary packages and installers.
Many Linux distributors, and providers of BSD and other Unix variants, make custom binary packages of GHC available. Because these are built specifically for each environment, they are much easier to install and use than the generic binary packages that are available from the GHC download page. You can find a list of distributions that custombuild GHC at the GHC distribution packages page.
For more detailed information about how to install GHC on a variety of popular platforms, we've provided some instructions in Appendix A, Installing GHC and Haskell libraries.
The interactive interpreter for GHC is a program
named ghci. It lets us enter and evaluate Haskell
expressions, explore modules, and debug our code. If you are
familiar with Python or Ruby, ghci is somewhat similar to
python
and irb
, the
interactive Python and Ruby interpreters.
On Unixlike systems, we run ghci as a command in a shell window. On Windows, it's available via the Start Menu. For example, if you installed using the GHC installer on Windows XP, you should go to “All Programs”, then “GHC”; you will then see ghci in the list. (See the section called “Windows” for a screenshot.)
When we run ghci, it displays a startup banner, followed
by a Prelude>
prompt. Here, we're showing
version 6.8.3 on a Linux box.
$
ghci
GHCi, version 6.8.3: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done.Prelude>
The word Prelude
in the prompt
indicates that Prelude
, a
standard library of useful functions, is loaded and ready to
use. When we load other modules or source files, they will show
up in the prompt, too.
The Prelude
module is sometimes referred to as “the standard
prelude”, because its contents are defined by the
Haskell 98 standard. Usually, it's simply shortened to
“the prelude”.
The prelude is always implicitly available; we don't need to take any actions to use the types, values, or functions it defines. To use definitions from other modules, we must load them into ghci, using the :module command.
ghci>
:module + Data.Ratio
We can now use the functionality of the
Data.Ratio
module, which lets us work with rational
numbers (fractions).
In addition to providing a convenient interface for testing code fragments, ghci can function as a readily accessible desktop calculator. We can easily express any calculator operation in ghci and, as an added bonus, we can add more complex operations as we become more familiar with Haskell. Even using the interpreter in this simple way can help us to become more comfortable with how Haskell works.
We can immediately start entering expressions, to see what ghci will do with them. Basic arithmetic works similarly to languages like C and Python: we write expressions in infix form, where an operator appears between its operands.
ghci>
2 + 2
4ghci>
31337 * 101
3165037ghci>
7.0 / 2.0
3.5
The infix style of writing an expression is just a convenience: we can also write an expression in prefix form, where the operator precedes its arguments. To do this, we must enclose the operator in parentheses.
ghci>
2 + 2
4ghci>
(+) 2 2
4
As the expressions above imply, Haskell has a
notion of integers and floating point numbers. Integers can
be arbitrarily large. Here, (^)
provides integer exponentiation.
ghci>
313 ^ 15
27112218957718876716220410905036741257
Haskell presents us with one peculiarity in how we must write numbers: it's often necessary to enclose a negative number in parentheses. This affects us as soon as we move beyond the simplest expressions.
We'll start by writing a negative number.
ghci>
3
3
The 
above is a unary operator. In other
words, we didn't write the single number “3”; we
wrote the number “3”, and applied the operator

to it. The 
operator is
Haskell's only unary operator, and we cannot mix it with infix
operators.
ghci>
2 + 3
<interactive>:1:0: precedence parsing error cannot mix `(+)' [infixl 6] and prefix `' [infixl 6] in the same infix expression
If we want to use the unary minus near an infix operator, we must wrap the expression it applies to in parentheses.
ghci>
2 + (3)
1ghci>
3 + ((13 * 37))
478
This avoids a parsing ambiguity. When we apply a function
in Haskell, we write the name of the function, followed by its
argument, for example f 3
. If we did not need to
wrap a negative number in parentheses, we would have two
profoundly different ways to read f3
: it could
be either “apply the function f
to
the number 3
”, or “subtract the
number 3
from the variable
f
”.
Most of the time, we can omit white space (“blank” characters such as space and tab) from expressions, and Haskell will parse them as we intended. But not always. Here is an expression that works:
ghci>
2*3
6
And here is one that seems similar to the problematic negative number example above, but results in a different error message.
ghci>
2*3
<interactive>:1:1: Not in scope: `*'
Here, the Haskell implementation is reading
*
as a single operator. Haskell lets us
define new operators (a subject that we will return to later),
but we haven't defined *
. Once again, a
few parentheses get us and ghci looking at the expression in
the same way.
ghci>
2*(3)
6
Compared to other languages, this unusual treatment of negative numbers might seem annoying, but it represents a reasoned tradeoff. Haskell lets us define new operators at any time. This is not some kind of esoteric language feature; we will see quite a few userdefined operators in the chapters ahead. The language designers chose to accept a slightly cumbersome syntax for negative numbers in exchange for this expressive power.
The values of Boolean logic in Haskell are
True
and False
. The capitalization of these names is
important. The language uses Cinfluenced operators for
working with Boolean values: (&&)
is logical “and”, and ()
is logical “or”.
ghci>
True && False
Falseghci>
False  True
True
While some programming languages treat the
number zero as synonymous with False
,
Haskell does not, nor does it consider a nonzero value to be
True
.
ghci>
True && 1
<interactive>:1:8: No instance for (Num Bool) arising from the literal `1' at <interactive>:1:8 Possible fix: add an instance declaration for (Num Bool) In the second argument of `(&&)', namely `1' In the expression: True && 1 In the definition of `it': it = True && 1
Once again, we are faced with a
substantiallooking error message. In brief, it tells us that
the Boolean type, Bool, is not a member of the
family of numeric types, Num
. The error message
is rather long because ghci is pointing out the location of
the problem, and hinting at a possible change we could make
that might fix the problem.
Here is a more detailed breakdown of the error message.
“No instance for (Num Bool)
”
tells us that ghci is trying to treat the numeric value
1 as having a Bool type, but it
cannot.
“arising from the literal
`1'
” indicates that it was our use of
the number 1
that caused the problem.
“In the definition of `it'
”
refers to a ghci short cut that we will revisit in a few
pages.
Most of Haskell's comparison operators are similar to those used in C and the many languages it has influenced.
ghci>
1 == 1
Trueghci>
2 < 3
Trueghci>
4 >= 3.99
True
One operator that differs from its C counterpart
is “is not equal to”. In C, this is written as
!=
. In Haskell, we write
(/=)
, which resembles the ≠
notation used in mathematics.
ghci>
2 /= 3
True
Also, where Clike languages often use
!
for logical negation, Haskell uses the
not
function.
ghci>
not True
False
Like written algebra and other programming languages that use infix operators, Haskell has a notion of operator precedence. We can use parentheses to explicitly group parts of an expression, and precedence allows us to omit a few parentheses. For example, the multiplication operator has a higher precedence than the addition operator, so Haskell treats the following two expressions as equivalent.
ghci>
1 + (4 * 4)
17ghci>
1 + 4 * 4
17
Haskell assigns numeric precedence values to operators, with 1 being the lowest precedence and 9 the highest. A higherprecedence operator is applied before a lowerprecedence operator. We can use ghci to inspect the precedence levels of individual operators, using its :info command.
ghci>
:info (+)
class (Eq a, Show a) => Num a where (+) :: a > a > a ...  Defined in GHC.Num infixl 6 +ghci>
:info (*)
class (Eq a, Show a) => Num a where ... (*) :: a > a > a ...  Defined in GHC.Num infixl 7 *
The information we seek is in the line
“infixl 6 +
”, which indicates that
the (+)
operator has a precedence of 6.
(We will explain the other output in a later chapter.)
The “infixl 7 *
” tells us that the
(*)
operator has a precedence of 7. Since
(*)
has a higher precedence than
(+)
, we can now see why 1 + 4 *
4
is evaluated as 1 + (4 * 4)
, and not
(1 + 4) * 4
.
Haskell also defines associativity of
operators. This determines whether an expression containing
multiple uses of an operator is evaluated from left to right, or
right to left. The (+)
and
(*)
operators are left associative, which
is represented as infixl
in the ghci output
above. A right associative operator is displayed with
infixr
.
ghci>
:info (^)
(^) :: (Num a, Integral b) => a > b > a  Defined in GHC.Real infixr 8 ^
The combination of precedence and associativity rules are usually referred to as fixity rules.
Haskell's prelude, the standard library we mentioned earlier, defines at least one wellknown mathematical constant for us.
ghci>
pi
3.141592653589793
But its coverage of mathematical constants is
not comprehensive, as we can quickly see. Let us look for
Euler's number, e
.
ghci>
e
<interactive>:1:0: Not in scope: `e'
Oh well. We have to define it ourselves.
Don't worry about the error message  

If the above “not in scope” error
message seems a little daunting, do not worry. All it means
is that there is no variable defined with the name

Using ghci's let
construct, we can make a
temporary definition of e
ourselves.
ghci>
let e = exp 1
This is an application of the exponential
function, exp
, and our first example of
applying a function in Haskell. While languages like Python
require parentheses around the arguments to a function,
Haskell does not.
With e
defined, we can now
use it in arithmetic expressions. The
(^)
exponentiation operator that we
introduced earlier can only raise a number to an integer
power. To use a floating point number as the exponent, we use
the (**)
exponentiation operator.
ghci>
(e ** pi)  pi
19.99909997918947
This syntax is ghcispecific  

The syntax for 
It is sometimes better to leave at least some parentheses in place, even when Haskell allows us to omit them. Their presence can help future readers (including ourselves) to understand what we intended.
Even more importantly, complex expressions that rely completely on operator precedence are notorious sources of bugs. A compiler and a human can easily end up with different notions of what even a short, parenthesisfree expression is supposed to do.
There is no need to remember all of the precedence and associativity rules numbers: it is simpler to add parentheses if you are unsure.
On most systems, ghci has some amount of command line editing ability. In case you are not familiar with command line editing, it's a huge time saver. The basics are common to both Unixlike and Windows systems. Pressing the up arrow key on your keyboard recalls the last line of input you entered; pressing up repeatedly cycles through earlier lines of input. You can use the left and right arrow keys to move around inside a line of input. On Unix (but not Windows, unfortunately), the tab key completes partially entered identifiers.
Where to look for more information  

We've barely scratched the surface of command line editing here. Since you can work more effectively if you're more familiar with the capabilities of your command line editing system, you might find it useful to do some further reading. On Unixlike systems, ghci uses the GNU readline library, which is powerful and customisable. On Windows, ghci's command line editing capabilities are provided by the doskey command. 
A list is surrounded by square brackets; the elements are separated by commas.
ghci>
[1, 2, 3]
[1,2,3]
A list can be of any length. The empty list is
written []
.
ghci>
[]
[]ghci>
["foo", "bar", "baz", "quux", "fnord", "xyzzy"]
["foo","bar","baz","quux","fnord","xyzzy"]
All elements of a list must be of the same type. Here, we violate this rule: our list starts with two Bool values, but ends with a string.
ghci>
[True, False, "testing"]
<interactive>:1:14: Couldn't match expected type `Bool' against inferred type `[Char]' Expected type: Bool Inferred type: [Char] In the expression: "testing" In the expression: [True, False, "testing"]
Once again, ghci's error message is verbose, but it's simply telling us that there is no way to turn the string into a Boolean value, so the list expression isn't properly typed.
If we write a series of elements using enumeration notation, Haskell will fill in the contents of the list for us.
ghci>
[1..10]
[1,2,3,4,5,6,7,8,9,10]
Here, the ..
characters denote
an enumeration. We can only use this
notation for types whose elements we can enumerate. It makes no
sense for text strings, for instance: there is not any sensible,
general way to enumerate ["foo".."quux"]
.
By the way, notice that the above use of range notation gives us a closed interval; the list contains both endpoints.
When we write an enumeration, we can optionally specify the size of the step to use by providing the first two elements, followed by the value at which to stop generating the enumeration.
ghci>
[1.0,1.25..2.0]
[1.0,1.25,1.5,1.75,2.0]ghci>
[1,4..15]
[1,4,7,10,13]ghci>
[10,9..1]
[10,9,8,7,6,5,4,3,2,1]
In the latter case above, the list is quite sensibly missing the end point of the enumeration, because it isn't an element of the series we defined.
We can omit the end point of an enumeration. If a
type doesn't have a natural “upper bound”, this
will produce values indefinitely. For example, if you type
[1..]
at the ghci prompt, you'll have to
interrupt or kill ghci to stop it from printing an infinite
succession of everlarger numbers. If you are tempted to do
this, type C to
halt the enumeration. We will find later on that infinite lists
are often useful in Haskell.
There are two ubiquitous operators for working with
lists. We concatenate two lists using the
(++)
operator.
ghci>
[3,1,3] ++ [3,7]
[3,1,3,3,7]ghci>
[] ++ [False,True] ++ [True]
[False,True,True]
More basic is the (:)
operator, which adds an element to the front of a list. This
is pronounced “cons” (short for
“construct”).
ghci>
1 : [2,3]
[1,2,3]ghci>
1 : []
[1]
You might be tempted to try writing [1,2]:3
to add an element to the end of a list, but ghci will reject
this with an error message, because the first argument of
(:)
must be an element, and the second
must be a list.
If you know a language like Perl or C, you'll find Haskell's notations for strings familiar.
A text string is surrounded by double quotes.
ghci>
"This is a string."
"This is a string."
As in many languages, we can represent hardtosee
characters by “escaping” them. Haskell's escape
characters and escaping rules follow the widely used conventions
established by the C language. For example,
'\n'
denotes a newline character, and
'\t'
is a tab character. For complete
details, see Appendix B, Characters, strings, and escaping rules.
ghci>
putStrLn "Here's a newline >\n< See?"
Here's a newline > < See?
The putStrLn
function prints a
string.
Haskell makes a distinction between single characters and text strings. A single character is enclosed in single quotes.
ghci>
'a'
'a'
In fact, a text string is simply a list of individual characters. Here's a painful way to write a short string, which ghci gives back to us in a more familiar form.
ghci>
let a = ['l', 'o', 't', 's', ' ', 'o', 'f', ' ', 'w', 'o', 'r', 'k']
ghci>
a
"lots of work"ghci>
a == "lots of work"
True
The empty string is written ""
, and is a
synonym for []
.
ghci>
"" == []
True
Since a string is a list of characters, we can use the regular list operators to construct new strings.
ghci>
'a':"bc"
"abc"ghci>
"foo" ++ "bar"
"foobar"
While we've talked a little about types already, our interactions with ghci have so far been free of much typerelated thinking. We haven't told ghci what types we've been using, and it's mostly been willing to accept our input.
Haskell requires type names to start with an uppercase letter, and variable names must start with a lowercase letter. Bear this in mind as you read on; it makes it much easier to follow the names.
The first thing we can do to start exploring the world of types is to get ghci to tell us more about what it's doing. ghci has a command, :set, that lets us change a few of its default behaviours. We can tell it to print more type information as follows.
ghci>
:set +t
ghci>
'c'
'c' it :: Charghci>
"foo"
"foo" it :: [Char]
What the +t
does is tell ghci to
print the type of an expression after the expression. That
cryptic it
in the output can be very useful:
it's actually the name of a special variable, in which ghci
stores the result of the last expression we evaluated. (This
isn't a Haskell language feature; it's specific to ghci
alone.) Let's break down the meaning of the last line of ghci
output.
Here are a few more of Haskell's names for types, from expressions of the sort we've already seen.
ghci>
7 ^ 80
40536215597144386832065866109016673800875222251012083746192454448001 it :: Integer
Haskell's integer type is named Integer. The size of an Integer value is bounded only by your system's memory capacity.
Rational numbers don't look quite the same as
integers. To construct a rational number, we use the
(%)
operator. The numerator is on the
left, the denominator on the right.
ghci>
:m +Data.Ratio
ghci>
11 % 29
11%29 it :: Ratio Integer
For convenience, ghci lets us abbreviate many commands, so we can write :m instead of :module to load a module.
Notice two words on the right
hand side of the ::
above. We can read this as a
“Ratio of Integer”. We
might guess that a Ratio must have values of type
Integer as both numerator and denominator. Sure
enough, if we try to construct a Ratio where the
numerator and denominator are of different types, or of the same
nonintegral type, ghci complains.
ghci>
3.14 % 8
<interactive>:1:0: Ambiguous type variable `t' in the constraints: `Integral t' arising from a use of `%' at <interactive>:1:07 `Fractional t' arising from the literal `3.14' at <interactive>:1:03 Probable fix: add a type signature that fixes these type variable(s)ghci>
1.2 % 3.4
<interactive>:1:0: Ambiguous type variable `t' in the constraints: `Integral t' arising from a use of `%' at <interactive>:1:08 `Fractional t' arising from the literal `3.4' at <interactive>:1:68 Probable fix: add a type signature that fixes these type variable(s)
Although it is initially useful to have
:set +t
giving us type information for
every expression we enter, this is a facility we will quickly
outgrow. After a while, we will often know what type we expect an
expression to have. We can turn off the extra type information
at any time, using the :unset command.
ghci>
:unset +t
ghci>
2
2
Even with this facility turned off, we can still get that type information easily when we need it, using another ghci command.
ghci>
:type 'a'
'a' :: Charghci>
"foo"
"foo"ghci>
:type it
it :: [Char]
The :type command will print type
information for any expression we give it (including
it
, as we see above). It won't actually
evaluate the expression; it only checks its type and prints
that.
Why are the types reported for these two expressions different?
ghci>
3 + 2
5ghci>
:type it
it :: Integerghci>
:type 3 + 2
3 + 2 :: (Num t) => t
Haskell has several numeric types. For example, a literal
number such as 1
could, depending on the
context in which it appears, be an integer or a floating point
value. When we force ghci to evaluate the expression 3
+ 2
, it has to choose a type so that it can print the
value, and it defaults to Integer. In the second
case, we ask ghci to print the type of the expression without
actually evaluating it, so it does not have to be so specific.
It answers, in effect, “its type is numeric”. We
will see more of this style of type annotation in Chapter 6, Using Typeclasses.
Let's take a small leap ahead, and write a small program
that counts the number of lines in its input. Don't expect to
understand this yet; it's just fun to get our hands dirty. In a
text editor, enter the following code into a file, and save it
as WC.hs
.
 file: ch01/WC.hs  lines beginning with "" are comments. main = interact wordCount where wordCount input = show (length (lines input)) ++ "\n"
Find or create a text file; let's call it
quux.txt
^{[1]}.
$
cat quux.txt
Teignmouth, England Paris, France Ulm, Germany Auxerre, France Brunswick, Germany BeaumontenAuge, France Ryazan, Russia
From a shell or command prompt, run the following command.
$
runghc WC < quux.txt
7
We have successfully written a simple program that interacts with the real world! In the chapters that follow, we will successively fill the gaps in our understanding until we can write programs of our own.
^{[1] }Incidentally, what do these cities have in common?