Creating Custom Data Types

Creating a simple data type

The easiest way to create a custom data type in Haskell is to use the data keyword:

data Foo = Bar | Biz

The name of the type is specified between data and =, and is called a type constructor. After = we specify all value constructors of our data type, delimited by the | sign. There is a rule in Haskell that all type and value constructors must begin with a capital letter. The above declaration can be read as follows:

Define a type called Foo, which has two possible values: Bar and Biz.

Creating variables of our custom type

let x = Bar

The above statement creates a variable named x of type Foo. Let’s verify this by checking its type.

:t x

prints

x :: Foo

Creating a data type with value constructor parameters

Value constructors are functions that return a value of a data type. Because of this, just like any other function, they can take one or more parameters:

data Foo = Bar String Int | Biz String

Let’s check the type of the Bar value constructor.

:t Bar

prints

Bar :: String -> Int -> Foo

which proves that Bar is indeed a function.

Creating variables of our custom type

let x = Bar "Hello" 10
let y = Biz "Goodbye"

Creating a data type with type parameters

Type constructors can take one or more type parameters:

data Foo a b = Bar a b | Biz a b

Type parameters in Haskell must begin with a lowercase letter. Our custom data type is not a real type yet. In order to create values of our type, we must substitute all type parameters with actual types. Because a and b can be of any type, our value constructors are polymorphic functions.

Creating variables of our custom type

let x = Bar "Hello" 10      -- x :: Foo [Char] Integer
let y = Biz "Goodbye" 6.0   -- y :: Fractional b => Foo [Char] b
let z = Biz True False      -- z :: Foo Bool Bool

Custom data type with record parameters

Assume we want to create a data type Person, which has a first and last name, an age, a phone number, a street, a zip code and a town.

We could write

data Person = Person String String Int Int String String String

If we want now to get the phone number, we need to make a function

getPhone :: Person -> Int
getPhone (Person _ _ _ phone _ _ _) = phone

Well, this is no fun. We can do better using parameters:

data Person' = Person' { firstName     :: String
                       , lastName      :: String
                       , age           :: Int
                       , phone         :: Int
                       , street        :: String
                       , code          :: String
                       , town          :: String }

Now we get the function phone where

:t phone
phone :: Person' -> Int

We can now do whatever we want, eg:

printPhone :: Person' -> IO ()
printPhone = putStrLn . show . phone

We can also bind the phone number by Pattern Matching:

getPhone' :: Person' -> Int
getPhone' (Person {phone = p}) = p

For easy use of the parameters see RecordWildCards