Sum type

A sum type is a type of algebraic data type. It is a user-defined data type that stores a fixed set of possible values. In other words, it is a choice of possibilities as to the data it contains.

Example

Suppose we are in a zoo and want to keep track of animals in the zoo.

An example of a sum type could be an Animal data type. Variables of this data type can only contain one of these 4 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.

Types of sum-types

Other algebraic data types

See: Algebraic data type