Sum data

Sum data is data that can be one of several distinct, fixed types, but only one at a time. It’s also known as a discriminated union, disjoint union, tagged union, or variant type — the name in each case pointing at the same idea, that a single value is tagged as belonging to exactly one of a closed set of alternative shapes.

A geometric shape is a typical example. A value of that type is a square, a circle, or a triangle, each carrying its own data — a side length, a radius, or three vertices — but never more than one of them at once, and the type system records which alternative a given value actually is.

Sum data is the counterpart to product data, where a value combines several fixed attributes together rather than choosing between alternatives. Together the two give a type system the vocabulary to model "and" (product data) and "or" (sum data) relationships between values precisely.