Haskell Cheat Sheet (Page 4 of 13) in pdf

same accessor function for values of the same type,

Because seven of these operations are so com-

but that can be dangerous if the accessor is not used

mon, Haskell provides the

keyword

by all constructors. Consider this rather contrived

which will automatically implement the typeclass

Some literature refers to this practice as type pun-

example:

on the associated type. The seven supported type-

ning.

classes are:

, and

Type Variables

Declaring so-called polymorphic

Two forms of

are possible. The ﬁrst is

data types is as easy as adding type variables in

used when a type only derives one class:

the declaration:

This declares a type

with two constructors,

is called with a

value, a runtime

and

. The

constructor can take

error will occur.

an argument of any type, which is represented by

The second is used when multiple classes are de-

Finally, as explained elsewhere, these names

the type variable

above.

rived:

can be used for pattern matching, argument cap-

We can also mix type variables and speciﬁc

ture and “updating.”

types in constructors:

Class Constraints

Data types can be declared

with class constraints on the type variables, but

Above, the

constructor can take a value of

this practice is generally discouraged. It is gener-

It is a syntax error to specify

for any other

any type and an

value.

ally better to hide the “raw” data constructors us-

classes besides the six given above.

ing the module system and instead export “smart”

Record Syntax

Constructor arguments can be

constructors which apply appropriate constraints.

declared either positionally, as above, or using

In any case, the syntax used is:

record syntax, which gives a name to each argu-

Deriving

ment. For example, here we declare a

type

See the section on

under the

key-

with names for appropriate arguments:

word on page 4.

This declares a type

which has one

type variable argument. Valid types are those in

the

class.

These names are referred to as selector or accessor

functions and are just that, functions. They must

Deriving

Many types have common operations

The

keyword indicates that the code to follow

start with a lowercase letter or underscore and can-

which are tedious to deﬁne yet necessary, such as

will be in a monadic context. Statements are sepa-

not have the same name as another function in

the ability to convert to and from strings, compare

rated by newlines, assignment is indicated by

scope. Thus the “ ” preﬁx on each above. Mul-

for equality, or order in a sequence. These capabil-

and a

form is introduce which does not require

tiple constructors (of the same type) can use the

ities are deﬁned as typeclasses in Haskell.

the

keyword.

c 2009 Justin Bailey.

Haskell Cheat Sheet Page 4

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