Constructor

From HaskellWiki

Constructor can mean:

• Type constructor
• Data constructor

Contents 1 Type constructor 2 Data constructor 2.1 Note 3 Deconstructing data constructors 4 Notes

1 Type constructor

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

illustrates how to define a data type with type constructors (and data constructors at the same time). The type constructor is named , but a tree of what? Of any specific type , be it , , or even , in which case it will be a tree of tree of . The data type is polymorphic (and is a type variable that is to be substituted by a specific type). So when used, the values will have types like or .

2 Data constructor

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

where there are two data constructors, and . Any value that belongs to the type (I'm happy leaving the type parameter unspecified) will be a constructed by either or . is a constructor alright, but it groups no value whatsoever, that is, it's a nullary constructor. There can be only one value that will have this constructor, also conveniently denoted . So nullary constructors contain no data apart from its name! For example, the data type is defined to be

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

On the other hand, contains other data. The types of those data are its parameters. The first one has type , so it's just a value of the parameter type . This one is the value the tree node holds in it. The remaining two are the branches. Each of them have type , naturally.

2.1 Note

Data constructors are not types! They denote values. It would be illegal to write there, because the type is , not .

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,

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 ). The left branch is named , the right , allowing us to use these values in the right hand side.

4 Notes

• 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,
  (4, True)
  is really a value of the form
  (,) 4 True
  having the type
  (,) Int Bool
  , which, too, is written conveniently as
  (Int, Bool)
  to make it more readable. Incidentally, the empty tuple type
  ()
  with its only value
  ()
  is used throughout, and is called unit.
• You can, in fact, name the values grouped together, using the record syntax,

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

so that for a person , you can say to select his/her age, without resorting to pattern matching.