A union type, also called a sum or variant type, models a value that may take one of several named forms. It is a building block of algebraic data types, which combine alternative forms with product types such as tuples or records to represent a domain. section 2.1.2
Use variants when one domain concept has distinct cases with different data. This makes the permitted actions explicit instead of forcing unrelated fields into a single broad structure. section 2.1.2
Different kinds of actions
Actions can send money, invite people to a party, or schedule a meeting. Each variant carries only the information it needs: a recipient and amount, attendees, or a person and time. section 2.1.2
data Actions
= SendMoney { to :: Person, amount :: Money }
| InviteToParty { attendees :: List Person }
| ScheduleMeeting { with :: Person, when :: DateTime }
Inspect a declaration with three alternatives and variant-specific record fields. section 2.1.2
