> X | Y isn't so different from 'a' A | 'b' B Except for the fact that the union of a type with itself (X | X) doesn't add any extra values, while a tagged uni

> X | Y isn't so different from 'a' A | 'b' B Except for the fact that the union of a type with itself (X | X) doesn't add any extra values, while a tagged union can have distinct tags with the same data type ('a' A | 'b' A) and actually sums the possible values for each tag. They are equivalent only so long as the unions are disjoint with no overlap between the members.