The problem here is not the absence of pattern matching, it is the absence of the "tag" part of tagged unions. Union types like the one you presented (union result = user | Errors) are different from sum types, as they don't have a tag. Without the tag you need a way to know the type of the result at runtime. For example in TypeScript, you can use a type assertion and control-flow analysis to emulate sum types and pattern matching: https://www.typescriptlang.org/docs/handbook/2/everyday-type....

While sum types and union types are close, they are not exactly the same. Let's take an example:

    union bools = bool | bool

    enum Bools = A(bool) | B(bool)

The example is a bit weird but shows an important point: bools has 2 values, true and false. Bools has 4 values, A(true), A(false), B(true), B(false). In bools (the union), true and true is always the same. In Bools (the sum/enum), A(true) is not the same as B(true).

I don't know enough about type theory to know if union types have a relationship with sum types. They can be used the same for most cases but they have differences around the edges.