Chapter 8 - Creating Our Own Types and Classes of Types

Chapter 8 - Creating Our Own Types and Classes of Types

1. To create our own data types you can use the word data. data means that we are going to define a new data type. .

   • data(type of date) =data constructors.
     • =
       • The part to the right are the data constructors
       • The part to the left of the = denotes the type.

2. record syntax

   • It is used as an alternative to define data types.
   • e.g.
     • data Person = Person { firstName :: String , lastName :: String}

3. type parameters

   • Type parameters are useful because they allow us to create different types depending on the type we want to store in our data types.
   • e.g
     • data Maybe a = Nothing | Just a

4. derived instances

   • Deriving an instance consists of placing the word deriving ( class ) at the end of the declaration of the types of data that the constructor will contain.
   • e.g
     • data Person = Person { ...} deriving (Eq, Show, Read)

5. Type synonyms

   • simply give some type a different name.
   • e.g
     • type PhoneBook = [(String,String)]
     • type PhoneNumber = String
     • type Name = String
     • type PhoneBook = [(Name,PhoneNumber)]

6. Recursive data structures

   • we can create types whose builder fields are the type itself. In this way, we can create recursive data structures, in which a value of a certain type contains values of that same type, which will continue to contain values of the same type and so on.
   • e.g
     • data List a = Empty | Cons a (List a)

7. Yes/No type class

   • This function takes a value from any type that can express some value of truth and tells us if it is true or not.
   • ghci> yesno $ length [] False

8. Functor type class

   • the Functor type class, which is basically for things that can be mapped. It only works with lists.