Question

I want to include String as part of my constructor:

> data Car = Wheel | Trunk | String deriving (Show)

> test::Car->Car->Car
> test Wheel Wheel = "Wheel"
> test _ _ = ""

it says: Couldn’t match Car with [Char]

if I change the constructor to

> data Car = Wheel | Trunk | [Char] deriving (Show)

it says: error in constructor in data/newtype declaration: [Char]

So how do I create a data type where one of the constructors is also a string?

Answer 1

Haskell does not provide unnamed unions as you have used in your Car data type. Each element in the definition of a data type must have a constructor. For example, you could define the following data type.

data Car = Wheel | Trunk | Constr String

In this definition Constr denotes a constructor and String is the type of its argument. For example, you can construct a value of type Car by using Constr "Wheel". If you omit the String in the example above, it is a constructor called Constr without an argument. Similarly, in your example, String is a constructor, even if there exists a type with the same name.

With the definition of Car above, you can define test as follows.

test :: Car -> Car -> Car
test Wheel Wheel = Constr "Wheel"
test _     _     = Constr ""

By adding the constructor String on the right hand side of the definitions, the string is lifted into the Car data type. However, as mentioned in the comments, a definition like this seems a little odd as test always yields a value of type String. Therefore, another possible definition would be the following.

test :: Car -> Car -> String
test Wheel Wheel = "Wheel"
test _     _     = ""