Haskell Cheat Sheet (Page 3 of 13) in pdf

Data

So-called algebraic data types can be declared as fol-

Notice that the declaration only gives the type

lows:

Guards

Guards, or conditional matches, can be

signature of the function—no implementation is

used in cases just like function deﬁnitions. The only

given here (with some exceptions, see

“Defaults”

difference is the use of the

instead of . Here

on page 3). Continuing, we can deﬁne several in-

is the type’s name.

and

is a simple function which does a case-insensitive

stances:

are values of the type and are called con-

string match:

structors. Multiple constructors are separated with

the ‘ ’ character. Note that type and constructor

names must start with a capital letter. It is a syntax

error otherwise.

Constructors with Arguments

The type above

Evaluating

gives:

is not very interesting except as an enumeration.

Constructors that take arguments can be declared,

allowing more information to be stored:

Class

While

gives:

A Haskell function is deﬁned to work on a certain

type or set of types and cannot be deﬁned more

Notice that the arguments for each constructor are

than once. Most languages support the idea of

type names, not constructors. That means this kind

“overloading”, where a function can have different

of declaration is illegal:

behavior depending on the type of its arguments.

Defaults

Default implementations can be given

for functions in a class. These are useful when cer-

Haskell accomplishes overloading through

and

declarations. A

deﬁnes one

tain functions can be deﬁned in terms of others in

instead, the

type must be used:

or more functions that can be applied to any types

the class. A default is deﬁned by giving a body to

which are members (i.e., instances) of that class. A

one of the member functions. The canonical exam-

class is analogous to an interface in Java or C#, and

ple is

, which deﬁnes

(not equal) in terms of

. :

instances to a concrete implementation of the inter-

Type and Constructor Names

Type and con-

face.

structor names can be the same, because they will

A class must be declared with one or more type

never be used in a place that would cause confu-

variables. Technically, Haskell 98 only allows one

sion. For example:

type variable, but most implementations of Haskell

support so-called multi-parameter type classes, which

allow more than one type variable.

Recursive deﬁnitions can be created, but an

which declares a type named

with two con-

We can deﬁne a class which supplies a ﬂavor for

declaration must always implement at

structors,

and

. Using this type in a

a given type:

least one class member.

function makes the difference clear:

c 2009 Justin Bailey.

Haskell Cheat Sheet Page 3

Related Articles

Related forms

Related Categories