Records

A second use case of ADTs is often called record or struct types. The purpose of records is to group or structure several related pieces of data.

People

We create the Person ADT as a first simple example of a record datatype:

data Person = MkPerson String Int

Just like in the previous section, a new ADT is announced by the data keyword, followed by the name of the new type, which is Person in this case. Whereas the enumeration examples were defined in terms of a number of alternatives separated by | characters, a record type has only one alternative. This alternative is the MkPerson data constructor. What’s new is that this constructor takes two parameters, also called fields, of the String and Int types, respectively.

We create a new Person by calling the constructor on values of the appropriate type:

tom :: Person
tom = MkPerson "Tom" 45

In fact, the constructor behaves essentially like a function:

*Main> :t MkPerson
MkPerson :: String...