# Constructor

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== Data constructor == | == Data constructor == | ||

− | A data constructor groups values together and tags alternatives in an algebraic data type, | + | A data constructor groups values together and tags alternatives in an [[algebraic data type]], |

<haskell> | <haskell> | ||

data Tree a = Tip | Node a (Tree a) (Tree a) | data Tree a = Tip | Node a (Tree a) (Tree a) | ||

Line 32: | Line 32: | ||

− | === | + | === Data constructors are not types=== |

− | Data constructors are not types | + | As discussed above, they denote values. It is illegal to write <hask>Node a (Node a) (Node a)</hask> there, because the type is <hask>Tree</hask>, not <hask>Node</hask>. |

== Deconstructing data constructors == | == Deconstructing data constructors == | ||

Line 43: | Line 43: | ||

So, the depth of a tip is zero. The depth of a node depends on its branches, but not it's content. See how the constructor in the left hand side names its parts? we don't need the content so we don't name it (using <hask>_</hask>). The left branch is named <hask>l</hask>, the right <hask>r</hask>, allowing us to use these values in the right hand side. | So, the depth of a tip is zero. The depth of a node depends on its branches, but not it's content. See how the constructor in the left hand side names its parts? we don't need the content so we don't name it (using <hask>_</hask>). The left branch is named <hask>l</hask>, the right <hask>r</hask>, allowing us to use these values in the right hand side. | ||

− | == Notes == | + | == Notes and tips== |

* You can declare a constructor (for both type and data) to be infix, and this can make your code a lot more readable. | * You can declare a constructor (for both type and data) to be infix, and this can make your code a lot more readable. | ||

− | * Tuples are a built in feature of the syntax but are plain old algebraic data types! They have only one constructor though. Having the same name as their types (don't freak out, it's just a matter of convenience, as the type constructors and the data constructors have separate namespaces). So, <hask>(4, True)</hask> is really a value of the form <hask>(,) 4 True</hask> having the type <hask>(,) Int Bool</hask>, which, too, is written conveniently as <hask>(Int, Bool)</hask> to make it more readable. Incidentally, the empty tuple type <hask>()</hask> with its only value <hask>()</hask> is used throughout, and is called ''unit''. | + | * [[Tuples]] are a built in feature of the syntax but are plain old algebraic data types! They have only one constructor though. Having the same name as their types (don't freak out, it's just a matter of convenience, as the type constructors and the data constructors have separate namespaces). So, <hask>(4, True)</hask> is really a value of the form <hask>(,) 4 True</hask> having the type <hask>(,) Int Bool</hask>, which, too, is written conveniently as <hask>(Int, Bool)</hask> to make it more readable. Incidentally, the empty tuple type <hask>()</hask> with its only value <hask>()</hask> is used throughout, and is called ''unit''. |

* You can, in fact, name the values grouped together, using the [[record]] syntax, <haskell> | * You can, in fact, name the values grouped together, using the [[record]] syntax, <haskell> | ||

data Person = Person { name :: String, age :: Int, address :: String } | data Person = Person { name :: String, age :: Int, address :: String } | ||

− | </haskell> | + | </haskell> so that for a person <hask>p</hask>, you can say <hask>age p</hask> to select his/her age, without resorting to pattern matching. |

− | so that for a person <hask>p</hask>, you can say <hask>age p</hask> to select his/her age, without resorting to pattern matching. | + | * Sometimes you need a little editting or checking when constructing your data. If you do, check [[smart constructors]] |

## Revision as of 04:06, 20 December 2006

**Constructor** can mean:

- Type constructor
- Data constructor

## Contents |

## 1 Type constructor

A type constructor is used to construct new types from given ones.

data Tree a = Tip | Node a (Tree a) (Tree a)

Tree

a

Integer

Maybe String

Tree b

b

a

Tree Int

Tree (Tree Boolean)

## 2 Data constructor

A data constructor groups values together and tags alternatives in an algebraic data type,

data Tree a = Tip | Node a (Tree a) (Tree a)

Tip

Node

Tree a

Tip

Node

Tip

Tip

Bool

data Bool = True | False

and for all practical purposes you can just think of them as *constants* belonging to a type.

Node

a

a

Tree a

### 2.1 Data constructors as first class values

Data constructors are first class values in Haskell and actually have a type. For instance, the type of theLeft

Either

Left :: forall b a. a -> Either a b

As first class values, they may be passed to functions, held in a list, be data elements of other algebraic data types and so forth.

### 2.2 Data constructors are not types

As discussed above, they denote values. It is illegal to writeNode a (Node a) (Node a)

Tree

Node

## 3 Deconstructing data constructors

All a data constructor does is holding values together. But you want to separate them if you want to use them. This is done via pattern matching,

depth Tip = 0 depth (Node _ l r) = 1 + max (depth l) (depth r)

_

l

r

## 4 Notes and tips

- You can declare a constructor (for both type and data) to be infix, and this can make your code a lot more readable.
- Tuples are a built in feature of the syntax but are plain old algebraic data types! They have only one constructor though. Having the same name as their types (don't freak out, it's just a matter of convenience, as the type constructors and the data constructors have separate namespaces). So, is really a value of the form(4, True)having the type(,) 4 True, which, too, is written conveniently as(,) Int Boolto make it more readable. Incidentally, the empty tuple type(Int, Bool)with its only value()is used throughout, and is called()
*unit*. - You can, in fact, name the values grouped together, using the record syntax, so that for a person
data Person = Person { name :: String, age :: Int, address :: String }

, you can saypto select his/her age, without resorting to pattern matching.age p - Sometimes you need a little editting or checking when constructing your data. If you do, check smart constructors