# Lecture 2: Algebraic data types

## Enumeration types

	data Thing = Shoe
        	   | Ship
		   | SealingWax
		   | Cabbage
		   | King
	derivating Show

The example above is a **type declaration**. The new type is called `Thing` and it has 5 **data constructors** (`Shoe`, `Ship`, etc.). A type's constructors are the only values for that type.

The line that says `deriving Show` is a special instruction that tells the GHC to automatically generate default code for converting a `Thing` to `String`.

	Shoe :: Thing
	Shoe = Shoe

	listO'Things :: [Thing]
	listO'Things = [Show, SealingWax, King, Cabbage, King]

Functions on enumeration types can be written using pattern-matching.

	isSmall :: Thing -> Bool
	isSmall Show		= True
	isSmall Ship		= False
	isSmall SealingWax	= True
	isSmall Cabbage		= True
	isSmall King		= False

Since function clauses are checked from top to bottom, `isSmall` can be rewritten like this:

	isSmall2 :: Thing -> Bool
	isSmall2 Ship	= False
	isSmall2 King	= False
	isSmall2 _	= True

## Beyond enumeration