Custom Types
Custom data types are named types that you can define in your program to refer, via an alias, to a
primitive type, a compound type, or a union. Unions are another special class of custom types that
allow you to define a type by enumerating its possible variants.
Both type aliases and union declarations must be made at the top level of a module, and are not
allowed inside a predicate declaration.
This chapter covers type aliases and unions. It also covers match expressions and match
statements which help reason about unions and their variants.