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" is alteration (A | B, meaning A or B but not both), and "product" is 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 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 / True | False are all 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 or 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. Just to illustrate, a very simple function to get the card suit in a string format 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."
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