Data declarations

[klequis]

Data declarations are how datatypes are defined.

Here is the data declaration for Bool which states that Bool can be True or False

--               [C]
data Bool = False | True
--   [A]     [B]      [D]

A. The type constructor for Bool, which is the datatype's name. B. Data constructor for the value False C. | (pipe) indicates "or". It is a sum type or logical disjunction D. Data constructor for the value True

• Data declarations can also use && ("and") which is logical conjunction
• A data declaration's type constructor and/or data constructors may or may not have arguments.
• The type constructor
  • is the name of the type which will show-up in the type-level.
  • is not a value and can only be used in type signatures (i.e., the type-level)
• The data constructors with will show-up in the term-level.
• Some data declarations do not have any data constructors.