Haskell Cheat Sheet (Page 7 of 13) in pdf

have a preﬁx attached through qualiﬁed imports.

This is particularly useful for modules which have

a large number of functions having the same name

functions.

is a good example:

As opposed to

, the

and

“values” on

The same syntax as used for importing can be used

are not just

values. The typechecker

here to specify which functions, types, construc-

tors, and classes are exported, with a few differ-

treats them as entirely new types. That means our

This form requires any function, type, constructor

function from above would not compile.

ences. If a module imports another module, it can

or other name exported by

to now be pre-

The following produces a type error:

also export that module:

ﬁxed with the alias (i.e.,

) given. Here is one way

to remove all duplicates from a list:

Instead, we must use pattern-matching to get to the

“values” to which we apply

A second form does not create an alias. Instead,

the preﬁx becomes the module name. We can write

a simple function to check if a string is all upper

A module can even re-export itself, which can be

case:

useful when all local deﬁnitions and a given im-

The key observation is that this keyword does not

ported module are to be exported. Below we export

introduce a new value; instead it introduces a new

ourselves and

, but not

type. This gives us two very useful properties:

No runtime cost is associated with the new

type, since it does not actually produce new

Except for the preﬁx speciﬁed, qualiﬁed imports

values. In other words, newtypes are abso-

support the same syntax as normal imports. The

lutely free!

name imported can be limited in the same ways as

described above.

The type-checker is able to enforce that com-

mon types such as

are used in

Exports

If an export list is not provided, then all

restricted ways, speciﬁed by the programmer.

functions, types, constructors, etc. will be available

Newtype

to anyone importing the module. Note that any im-

While

introduces new values and

just

Finally, it should be noted that any

ported modules are not exported in this case. Limit-

creates synonyms,

falls somewhere be-

clause which can be attached to a

declaration

ing the names exported is accomplished by adding

tween. The syntax for

is quite restricted –

can also be used when declaring a

a parenthesized list of names before the

key-

only one constructor can be deﬁned, and that con-

word:

structor can only take one argument. Continuing

Return

the example above, we can deﬁne a

type like

the following:

See

above.

c 2008 Justin Bailey.

Haskell Cheat Sheet Page 7

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