Sum
You can specify sum types using | operator, it is an explicit way of defining what the valid types of a value are.
For example dry-types defines the Bool type which is a sum consisting of the True and False types, expressed as Types::True | Types::False.
Another common case is defining that something can be either nil or something else:
nil_or_string = Types::Nil | Types::String
nil_or_string[nil] # => nil
nil_or_string["hello"] # => "hello"
nil_or_string[123] # raises Dry::Types::ConstraintError