Haskell Cheat Sheet (Page 6 of 13) in pdf

silly example which gives the sum of a list of num-

A module can only be deﬁned in one ﬁle, though

bers, their average, and their median:

its exports may come from multiple sources. To

make a Haskell ﬁle a module, just add a module

declaration at the top:

Import

See the section on

on page 6.

Module names must start with a capital letter but

otherwise can include periods, numbers and un-

derscores. Periods are used to give sense of struc-

See

on page 6.

ture, and Haskell compilers will use them as indi-

cations of the directory a particular source ﬁle is,

Deconstruction

The left-hand side of a

def-

Inﬁx, inﬁxl and inﬁxr

but otherwise they have no meaning.

inition can also destructure its argument, in case

The Haskell community has standardized a set

See the section on

operators

on page 11.

sub-components are to be accessed. This deﬁnition

of top-level module names such as

would extract the ﬁrst three characters from a string

, etc. Be sure to consult them for an appro-

Instance

priate place for your own module if you plan on

See the section on

on page 3.

releasing it to the public.

Let

Imports

The Haskell standard libraries are di-

vided into a number of modules. The functionality

Local functions can be deﬁned within a function us-

provided by those libraries is accessed by import-

ing

. The

keyword must always be followed

ing into your source ﬁle. To import all everything

Note that this is different than the following, which

. The

must appear in the same column as

exported by a library, just use the module name:

only works if the string has three characters:

the

keyword. Functions deﬁned have access to

all other functions and variables within the same

scope (including those deﬁned by

). In this ex-

Everything means everything: functions, data types

ample,

multiplies its argument

by , which

and constructors, class declarations, and even other

was passed to the original

is used

modules imported and then exported by the that

by map to give the multiples of x up to 10:

module. Importing selectively is accomplished by

giving a list of names to import. For example, here

we import some functions from

See the section on

on page 2.

“functions” with no arguments are actually

Module

Data types can imported in a number of ways. We

constants and, once evaluated, will not evaluate

can just import the type and no constructors:

again. This is useful for capturing common por-

A module is a compilation unit which exports func-

tions of your function and re-using them. Here is a

tions, types, classes, instances, and other modules.

c 2009 Justin Bailey.

Haskell Cheat Sheet Page 6

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