Enum

An enum is a sum type (a type of algebraic data type) in which its value represents one of a set number of possible states.

This makes invalid states unrepresentable, as the variable of the enum data type can only store certain values, nothing else.

Example

Say you are writing a program to track animals in a zoo.

You could have an enum type called Animal to store the different animals in the zoo. Variables of this type could then only contain one of four values:

• Parrot
• Monkey
• Tiger
• Giraffe

This has the benefit of making invalid state impossible: we are guaranteed that the value of an Animal variable will always be one of the 4 above.

Other sum types

See: Sum type

Other algebraic data types

See: Algebraic data type