Constructors in Haskell #

In haskell, you shall provide a type constructor and one or more data constructors when you define a datatype.

{- File: Shape.hs
   `Shape` is a type constructor.
   `Circle` and `Rectangle` are data constructors.
-}
data Shape = Circle Float 
            | Rectangle Float Float
            deriving (Show)

• Shape is a type constructor, NOT a type itself. It is a function that takes other types to produce a type at the type level.
• Circle and Rectangle are data constructors. They are effectively functions that produce a value of the type Shape.

When defining a datatype in Haskell, the pattern is:

data TypeName = DataConstructor1 Type1 Type2 | DataConstructor2 Type 3 | ... deriving Show

• TypeName defines the new type name
• DataConstructor1, DataConstructor2, etc. are data constructors. They are used to create values of the TypeName type. These data constructors can take parameters of other types (Type1, Type2, Type3, etc.) to create a value of the new type.
• The | symbol separates different data constructors within the same type definition.
• The deriving Show part automatically creates an instance of the Show type class for TypeName, enabling it to be represented as a string for printing.