Data Declarations

With the keyword Data, we can define new types rather than just synonyms for already existing ones. Types declared with data are called algebraic, referencing "sum" and "product". In data types, "sum" means alteration (A | B, meaning A or B but not both), and "product" means combination (A B, meaning A and B together).


For example, we can create a type that represents a card suit:

data Suit = Hearts
          | Diamonds
          | Spades
          | Clubs

Suit is the type constructor in this definition, and the values (Hearts, Diamonds, Spades or Clubs) are data constructors. It states that the new type Suit can have one of the four values (the | symbol stands for "or"). This means that Suit is a sum type, which is any type that has multiple possible representations. Another example of a sum type would be Bool which can be either True or False:

data Bool = True | False

Data constructors can have zero or more arguments. Hearts | Diamonds and True | False are examples of data constructors that have zero arguments - nullary data constructors. Likewise, type constructors can have zero or more arguments. Suit and Bool are examples of nullary type constructors.

Let's take a look at some type- and data- constructors that have more than zero arguments. Here is an example of an error type from the cardano-node repository:

data ConfigError =
    ConfigErrorFileNotFound FilePath
  | ConfigErrorNoEKG
    deriving Show

ConfigError is a nullary type constructor, and its data constructors are ConfigErrorFileNotFound FilePath, and ConfigErrorNoEKG. ConfigErrorFileNotFound FilePath is a unary data constructor meaning that it actually holds some data, in this case of type FilePath. Nullary type constructors we have seen so far contain no data aside from their names.

Both the type name and its constructors must begin with a capital letter, and constructors must be unique to that type, i.e. the same constructor cannot be defined in more than one type. Once a new type is defined, it can be used in functions just like any other data type in Haskell. To illustrate, a very simple function to get the card suit in a string format using pattern matching would be:

suitStr :: Suit -> String
suitStr Hearts = "... of hearts."
suitStr Diamonds = "... of diamonds."
suitStr Spades = "... of spades."
suitStr Clubs = "... of clubs."

ghci> suitStr Hearts
"... of hearts."


The Suit type is an example of a sum-type. Now let's look at an example of a product-type that combines multiple fields together. We can imagine a rectangle, which has two sides a and b, and to construct a valid rectangle both of them must be provided. We can define the type as:

data Rect = Rect Double Double
    deriving Show

In this case, the Rect on the left side is the type constructor (nullary), and the Rect on the right side is the data constructor (binary as it takes two arguments). To create a valid Rect type, both Double arguments must be joined together:

ghci> a = Rect 2.0 3.0
ghci> a
ghci> Rect 2.0 3.0

It is interesting to note that the Rect on the right side is a function:

ghci> :type Rect
ghci> Rect :: Double -> Double -> Rect

This means that even here we can take advantage of partial application:

ghci> a = Rect 2.0
ghci> :type a
ghci> a :: Double -> Rect

And we can create functions based on the Rect type, for example, calculating the rectangle area:

area :: Rect -> Double
area (Rect a b) = a * b

ghci> area Rect 2.0 3.0
ghci> 6.0

Field labels (records)

As you can imagine, with product-types that represent more complex things, such as people or any large set of parameters, the definitions might become unclear from just the list of the arguments in the type. For example, imagine a type to define a company. We would need fields for e.g. the company name, its address, year of foundation, number of employees...

data Company = Company String String Int Int

From the above definition alone, it is hard to intuitively understand which String or Integer stands for what. A more intuitive way to define complex types is by using the field labels:

data Company = Company {
    cName :: String,
    cAddress :: String,
    cYear :: Int,
    cNre :: Int
} deriving Show

This is much more user-friendly. In addition, by using field labels, we get automatic getter functions for individual fields:

ghci> c = Company "MyCompany" "25th Street" 1999 26
ghci> cAddress c
"25th Street"
ghci> cYear c

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